[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › Biochemistry

# Biochemistry

Metabolic pathways as maps you can tap, every enzyme in 3D with its active site, reactions atom by atom, and the disease when a step fails. Open data (PDB, PubChem, Reactome), in our own words.

31 pathways · 104 molecules · 131 enzymes and proteins

## Metabolic pathways

[### Urea cycle The liver turns ammonia, which is toxic to the brain, into urea that leaves in the urine. It does so with a cycle of five enzymes. Open →](https://durgical.com/en/biochemistry/pathways/urea-cycle/) [### Phenylalanine and tyrosine metabolism Phenylalanine becomes tyrosine, which is then broken down to fumarate and acetoacetate. Each blocked enzyme gives a classic disease. Open →](https://durgical.com/en/biochemistry/pathways/phenylalanine-and-tyrosine-metabolism/) [### Glycolysis Glycolysis splits one glucose into two pyruvates in the cytosol, leaving a net 2 ATP and 2 NADH; it runs with or without oxygen. Open →](https://durgical.com/en/biochemistry/pathways/glycolysis/) [### Krebs cycle (citric acid cycle) The Krebs cycle burns the acetyl of acetyl-CoA in the mitochondria: it releases CO₂ and loads the NADH and FADH₂ that later yield most of the ATP. Open →](https://durgical.com/en/biochemistry/pathways/krebs-cycle/) [### Gluconeogenesis Gluconeogenesis makes new glucose from lactate, alanine or glycerol during a long fast; it happens mainly in the liver. Open →](https://durgical.com/en/biochemistry/pathways/gluconeogenesis/) [### Glycogen metabolism Liver and muscle store glucose as glycogen after a meal and release it when needed, between meals or during exercise. Open →](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/) [### Pentose phosphate pathway This pathway uses glucose 6-phosphate to make NADPH, which protects cells from oxidative damage, and ribose for DNA and RNA. Open →](https://durgical.com/en/biochemistry/pathways/pentose-phosphate-pathway/) [### Fructose metabolism The liver turns fructose from fruit, table sugar and syrups into trioses that enter glycolysis without going through PFK-1. Open →](https://durgical.com/en/biochemistry/pathways/fructose-metabolism/) [### Galactose metabolism Galactose from milk is turned into glucose in the liver by the Leloir pathway; if it fails, galactose builds up and harms liver, brain and lens. Open →](https://durgical.com/en/biochemistry/pathways/galactose-metabolism/) [### Cholesterol synthesis (mevalonate pathway) The liver and other cells build cholesterol from acetyl-CoA in the cytosol and ER; HMG-CoA reductase sets the pace of the pathway. Open →](https://durgical.com/en/biochemistry/pathways/cholesterol-synthesis/) [### Fatty acid synthesis (lipogenesis) With surplus sugar, the liver and fat tissue build fatty acids two carbons at a time, spending ATP and NADPH, up to palmitate. Open →](https://durgical.com/en/biochemistry/pathways/fatty-acid-synthesis/) [### Fatty acid β-oxidation When fasting, mitochondria cut fatty acids two carbons at a time: they give acetyl-CoA, NADH and FADH₂ and far more energy than glucose. Open →](https://durgical.com/en/biochemistry/pathways/fatty-acid-oxidation/) [### Ketogenesis (ketone body synthesis) During fasting or without insulin, the liver turns surplus acetyl-CoA into ketone bodies, a fuel the brain and heart can use. Open →](https://durgical.com/en/biochemistry/pathways/ketogenesis/) [### Heme synthesis Eight enzymes turn glycine and succinyl-CoA into heme, the iron ring of hemoglobin; when one of them fails, a porphyria appears. Open →](https://durgical.com/en/biochemistry/pathways/heme-synthesis/) [### Heme breakdown and bilirubin Heme from old red cells becomes bilirubin; the liver conjugates it and bile carries it to the gut, where it colours the stool. Open →](https://durgical.com/en/biochemistry/pathways/heme-breakdown-and-bilirubin/) [### Thyroid hormone synthesis The thyroid takes up iodine, attaches it to thyroglobulin and forms T4 and T3; tissues then turn T4 into T3, the active form. Open →](https://durgical.com/en/biochemistry/pathways/thyroid-hormone-synthesis/) [### De novo purine synthesis Cells build the purine ring piece by piece on a phosphorylated ribose until they reach IMP, which then becomes AMP and GMP. Open →](https://durgical.com/en/biochemistry/pathways/de-novo-purine-synthesis/) [### Purine degradation and salvage Spare purines are turned into uric acid, which leaves in the urine; many of the bases are recycled before they get that far. Open →](https://durgical.com/en/biochemistry/pathways/purine-degradation-and-salvage/) [### Pyrimidine synthesis Pyrimidines are built the other way round from purines: ring first (orotate), sugar second. UMP, CTP and dTMP come from there. Open →](https://durgical.com/en/biochemistry/pathways/pyrimidine-synthesis/) [### Electron transport chain and oxidative phosphorylation The respiratory chain passes electrons from NADH and FADH₂ to oxygen; the energy pumps protons, and ATP synthase uses them to make most of the cell's ATP. Open →](https://durgical.com/en/biochemistry/pathways/electron-transport-chain-and-oxidative-phosphorylation/) [### Branched-chain amino acid breakdown (leucine route) Muscle and liver break down leucine, isoleucine and valine. The map follows leucine, which ends as acetyl-CoA and acetoacetate (ketogenic). Open →](https://durgical.com/en/biochemistry/pathways/branched-chain-amino-acid-breakdown/) [### Catecholamine synthesis and breakdown The adrenal medulla and some neurons build dopamine, norepinephrine and epinephrine from tyrosine, then break them down to metanephrines and VMA. Open →](https://durgical.com/en/biochemistry/pathways/catecholamine-synthesis-and-breakdown/) [### Methionine cycle and transsulfuration Methionine is turned into SAM, the cell's main methyl donor. The leftover homocysteine is recycled to methionine or used to make cysteine. Open →](https://durgical.com/en/biochemistry/pathways/methionine-cycle-and-transsulfuration/) [### Propionyl-CoA to succinyl-CoA Odd-chain fatty acids and four amino acids leave propionyl-CoA. Three enzymes turn it into succinyl-CoA, which enters the Krebs cycle. Open →](https://durgical.com/en/biochemistry/pathways/propionyl-coa-to-succinyl-coa/) [### Serotonin and melatonin synthesis Tryptophan becomes serotonin in the gut and brain, and serotonin becomes melatonin in the pineal gland at night. Open →](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/) [### Adrenal cortisol synthesis In the adrenal cortex cholesterol is trimmed and hydroxylated step by step to make cortisol. A block at any step causes congenital adrenal hyperplasia. Open →](https://durgical.com/en/biochemistry/pathways/adrenal-cortisol-synthesis/) [### Classic bile acid synthesis pathway The liver turns cholesterol into bile acids, which help digest fat. It is also the main way the body gets rid of cholesterol. Open →](https://durgical.com/en/biochemistry/pathways/classic-bile-acid-synthesis-pathway/) [### Prostaglandin and thromboxane synthesis Cells free arachidonic acid from membranes and turn it into prostaglandins and thromboxane, local signals for pain, fever, clotting and the stomach. Open →](https://durgical.com/en/biochemistry/pathways/prostaglandin-and-thromboxane-synthesis/) [### Malate–aspartate shuttle NADH cannot cross the inner mitochondrial membrane. This shuttle carries its electrons inside on malate, so liver and heart get the full ATP yield. Open →](https://durgical.com/en/biochemistry/pathways/malate-aspartate-shuttle/) [### Lysosomal ganglioside breakdown Lysosomes take gangliosides apart one piece at a time, down to sphingosine. A missing enzyme at any step causes a storage disease. Open →](https://durgical.com/en/biochemistry/pathways/lysosomal-ganglioside-breakdown/) [### Ethanol metabolism The liver oxidises ethanol to acetaldehyde and then to acetate; along the way it makes a lot of NADH, which upsets the rest of metabolism. Open →](https://durgical.com/en/biochemistry/pathways/ethanol-metabolism/)

## Build a molecule

Join amino acids into a peptide and fold it into an α-helix, or build a DNA strand with its complement.

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## Molecules

- [**Ammonia (ammonium)**](https://durgical.com/en/biochemistry/molecules/ammonia/) Ammonia comes from breaking down amino acids. It is toxic to the brain, so the liver turns it into urea.
- [**Urea**](https://durgical.com/en/biochemistry/molecules/urea/) Urea is how the body gets rid of spare nitrogen: the liver makes it and the kidney excretes it.
- [**Glutamate (glutamic acid)**](https://durgical.com/en/biochemistry/molecules/glutamate/) Glutamate gathers the nitrogen of amino acids and hands it on for urea; in the brain it is the main excitatory neurotransmitter.
- [**Glutamine**](https://durgical.com/en/biochemistry/molecules/glutamine/) Glutamine is the most abundant amino acid in blood: it carries ammonia safely and feeds the gut, the kidney and immune cells.
- [**Alanine**](https://durgical.com/en/biochemistry/molecules/alanine/) Alanine carries nitrogen from muscle to the liver, where it becomes pyruvate again to make glucose (glucose–alanine cycle).
- [**Phenylalanine**](https://durgical.com/en/biochemistry/molecules/phenylalanine/) Phenylalanine is an essential aromatic amino acid. The liver turns it into tyrosine; if it cannot, it builds up and damages the brain.
- [**Tyrosine**](https://durgical.com/en/biochemistry/molecules/tyrosine/) Tyrosine comes from phenylalanine and is the starting point of dopamine, adrenaline, melanin and thyroid hormones.
- [**Tryptophan**](https://durgical.com/en/biochemistry/molecules/tryptophan/) Tryptophan is an essential amino acid that gives serotonin, melatonin and part of our niacin (vitamin B3).
- [**Homocysteine**](https://durgical.com/en/biochemistry/molecules/homocysteine/) Homocysteine forms when methionine donates methyl groups. If it builds up, it damages vessels and favours thrombosis.
- [**S-adenosylmethionine (SAM)**](https://durgical.com/en/biochemistry/molecules/s-adenosylmethionine/) SAM is the body's main methyl-group donor: it methylates DNA, neurotransmitters, phospholipids and creatine.
- [**Tetrahydrobiopterin (BH4)**](https://durgical.com/en/biochemistry/molecules/tetrahydrobiopterin/) BH4 is the cofactor of the phenylalanine, tyrosine and tryptophan hydroxylases and of nitric oxide synthase.
- [**Glucose**](https://durgical.com/en/biochemistry/molecules/glucose/) Glucose is the sugar carried in the blood and the main fuel of the brain and the red blood cells.
- [**Glucose 6-phosphate**](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/) Glucose 6-phosphate is glucose trapped in the cell: from here it can go to energy, to glycogen or to the pentose pathway.
- [**Fructose 1,6-bisphosphate**](https://durgical.com/en/biochemistry/molecules/fructose-1-6-bisphosphate/) Fructose 1,6-bisphosphate is the two-phosphate sugar made by the committed step of glycolysis; aldolase splits it in two.
- [**Pyruvate**](https://durgical.com/en/biochemistry/molecules/pyruvate/) Pyruvate is where glycolysis ends: with oxygen it enters the mitochondria and is burned; without oxygen it becomes lactate.
- [**Lactate**](https://durgical.com/en/biochemistry/molecules/lactate/) Lactate is made when cells work without enough oxygen; the liver turns it back into glucose. Measuring it helps spot shock.
- [**ATP (adenosine triphosphate)**](https://durgical.com/en/biochemistry/molecules/atp/) ATP is the cell's energy currency: giving up its last phosphate pays for movement, transport and building.
- [**NAD⁺ (nicotinamide adenine dinucleotide)**](https://durgical.com/en/biochemistry/molecules/nad/) NAD⁺ picks up electrons in glycolysis and the Krebs cycle and carries them to the mitochondria as NADH. It is made from niacin.
- [**Acetyl-CoA**](https://durgical.com/en/biochemistry/molecules/acetyl-coa/) Acetyl-CoA is where sugars, fats and proteins meet: it enters the Krebs cycle or is used to make fat, cholesterol and ketone bodies.
- [**Citrate**](https://durgical.com/en/biochemistry/molecules/citrate/) Citrate is the first molecule of the Krebs cycle; it also carries acetyl to the cytosol to make fat and slows glycolysis when energy is plentiful.
- [**Glycogen**](https://durgical.com/en/biochemistry/molecules/glycogen/) Glycogen is the body's glucose store: a highly branched polymer kept in liver and muscle.
- [**NADPH**](https://durgical.com/en/biochemistry/molecules/nadph/) NADPH is the electron currency for building molecules and fighting oxidative damage; it comes mainly from the pentose pathway.
- [**Ribose 5-phosphate**](https://durgical.com/en/biochemistry/molecules/ribose-5-phosphate/) Ribose 5-phosphate is the five-carbon sugar the cell uses to build DNA, RNA, ATP, NAD and coenzyme A.
- [**UDP-glucose**](https://durgical.com/en/biochemistry/molecules/udp-glucose/) UDP-glucose is activated glucose: the form enzymes use to build glycogen and to swap sugars with galactose.
- [**Fructose**](https://durgical.com/en/biochemistry/molecules/fructose/) Fructose is the sugar of fruit and honey, and half of table sugar; the liver handles almost all of it.
- [**Galactose**](https://durgical.com/en/biochemistry/molecules/galactose/) Galactose is half of lactose, the sugar of milk; the liver turns it into glucose through the Leloir pathway.
- [**Ceramide**](https://durgical.com/en/biochemistry/molecules/ceramide/) Ceramide is the core of every sphingolipid: sphingosine joined to a fatty acid. It also acts as a stress and cell-death signal.
- [**Sphingomyelin**](https://durgical.com/en/biochemistry/molecules/sphingomyelin/) Sphingomyelin is the sphingolipid with phosphate: rich in myelin and the outer leaflet of membranes. It builds up in Niemann–Pick disease.
- [**Phosphatidylcholine (lecithin)**](https://durgical.com/en/biochemistry/molecules/phosphatidylcholine/) Phosphatidylcholine, or lecithin, is the most abundant membrane phospholipid and the basis of the surfactant that keeps alveoli open.
- [**Glucosylceramide (glucocerebroside)**](https://durgical.com/en/biochemistry/molecules/glucosylceramide/) Glucosylceramide is a ceramide carrying one glucose. It is the lipid that builds up in macrophages in Gaucher disease.
- [**Galactosylceramide (galactocerebroside)**](https://durgical.com/en/biochemistry/molecules/galactosylceramide/) Galactosylceramide is a ceramide with one galactose and a main myelin lipid. Failure to break it down causes Krabbe disease.
- [**Sulfatide**](https://durgical.com/en/biochemistry/molecules/sulfatide/) Sulfatide is a galactosylceramide with a sulfate group. It is part of myelin and builds up in metachromatic leukodystrophy.
- [**GM2 ganglioside**](https://durgical.com/en/biochemistry/molecules/gm2-ganglioside/) GM2 ganglioside is a sialic acid-containing glycolipid of neuronal membranes. It builds up in neurons in Tay–Sachs disease.
- [**Hyaluronic acid (hyaluronan)**](https://durgical.com/en/biochemistry/molecules/hyaluronic-acid/) Hyaluronic acid is a giant, sulfate-free glycosaminoglycan that holds water: it lubricates joints and gives bulk to the vitreous and skin.
- [**Cholesterol**](https://durgical.com/en/biochemistry/molecules/cholesterol/) Cholesterol is a 27-carbon sterol that stiffens membranes and is the raw material for steroid hormones, bile acids and vitamin D.
- [**Mevalonate**](https://durgical.com/en/biochemistry/molecules/mevalonate/) Mevalonate is the product of HMG-CoA reductase: a six-carbon compound that commits carbon to cholesterol and isoprenoids.
- [**Isopentenyl pyrophosphate (IPP)**](https://durgical.com/en/biochemistry/molecules/isopentenyl-pyrophosphate/) Isopentenyl pyrophosphate is the five-carbon building block (active isoprene) used to build cholesterol and many other isoprenoids.
- [**Squalene**](https://durgical.com/en/biochemistry/molecules/squalene/) Squalene is a linear 30-carbon hydrocarbon, with no oxygen, that folds up to form the first sterol.
- [**Lanosterol**](https://durgical.com/en/biochemistry/molecules/lanosterol/) Lanosterol is the first sterol of the pathway: it already has the four rings but carries three extra methyl groups.
- [**Cholic acid**](https://durgical.com/en/biochemistry/molecules/cholic-acid/) Cholic acid is one of the two primary bile acids: the liver makes it from cholesterol to emulsify fats in the gut.
- [**Palmitate (palmitic acid)**](https://durgical.com/en/biochemistry/molecules/palmitate/) Palmitate is the 16-carbon saturated fatty acid the body makes and the most common one in human fat.
- [**Malonyl-CoA**](https://durgical.com/en/biochemistry/molecules/malonyl-coa/) Malonyl-CoA is both the building block of fat synthesis and the signal that stops fat being burned at the same time.
- [**Carnitine (L-carnitine)**](https://durgical.com/en/biochemistry/molecules/carnitine/) Carnitine is the carrier that takes long-chain fatty acids into the mitochondrion to be burned.
- [**Acetoacetate (acetoacetic acid)**](https://durgical.com/en/biochemistry/molecules/acetoacetate/) Acetoacetate is the first ketone body the liver makes; other tissues turn it back into acetyl-CoA.
- [**β-Hydroxybutyrate**](https://durgical.com/en/biochemistry/molecules/hydroxybutyrate/) β-Hydroxybutyrate is the most abundant ketone body in blood; it is measured to diagnose and follow ketoacidosis.
- [**Propionyl-CoA**](https://durgical.com/en/biochemistry/molecules/propionyl-coa/) Propionyl-CoA is the 3-carbon remnant left by odd-chain fatty acids and some amino acids; it ends up as succinyl-CoA.
- [**Creatine**](https://durgical.com/en/biochemistry/molecules/creatine/) Creatine is a small nitrogen compound that muscle and brain use as a quick energy reserve, stored as phosphocreatine.
- [**Creatinine**](https://durgical.com/en/biochemistry/molecules/creatinine/) Creatinine is the waste product of muscle creatine. It is made at a steady rate and cleared by the kidneys, so it is the main blood marker of filtration.
- [**Nitric oxide (NO)**](https://durgical.com/en/biochemistry/molecules/nitric-oxide/) Nitric oxide is a short-lived gas made in the body that widens blood vessels, acts as a nerve messenger and is used by immune cells to kill microbes.
- [**Globotriaosylceramide (Gb3)**](https://durgical.com/en/biochemistry/molecules/globotriaosylceramide/) Gb3 is a ceramide carrying three sugars. α-Galactosidase A breaks it down; in Fabry disease it builds up in vessels, kidney, heart and nerves.
- [**Heme**](https://durgical.com/en/biochemistry/molecules/heme/) Heme is a porphyrin ring with an iron at its centre; it lets hemoglobin carry oxygen and cytochromes move electrons.
- [**Bilirubin**](https://durgical.com/en/biochemistry/molecules/bilirubin/) Bilirubin is the yellow pigment left when heme is broken down; when it builds up, it turns skin and eyes yellow (jaundice).
- [**Protoporphyrin IX**](https://durgical.com/en/biochemistry/molecules/protoporphyrin-ix/) Protoporphyrin IX is the heme ring before it receives iron; it is red, fluorescent and builds up in protoporphyria.
- [**Porphobilinogen**](https://durgical.com/en/biochemistry/molecules/porphobilinogen/) Porphobilinogen is the first pyrrole ring of heme synthesis; high in urine, it points to an acute porphyria attack.
- [**δ-Aminolevulinic acid (ALA)**](https://durgical.com/en/biochemistry/molecules/aminolevulinic-acid/) ALA is the first committed molecule of heme synthesis; it rises in acute porphyrias and lead poisoning.
- [**Cyclic AMP (cAMP)**](https://durgical.com/en/biochemistry/molecules/cyclic-amp/) The classic second messenger: adenylyl cyclase makes it from ATP when a hormone arrives, and phosphodiesterase destroys it.
- [**Cortisol**](https://durgical.com/en/biochemistry/molecules/cortisol/) The main glucocorticoid: it raises glucose, mobilises reserves in fasting and stress, and damps down inflammation.
- [**Aldosterone**](https://durgical.com/en/biochemistry/molecules/aldosterone/) The zona glomerulosa mineralocorticoid: it makes the kidney keep sodium and water and excrete potassium and hydrogen ions.
- [**17-Hydroxyprogesterone (17-OHP)**](https://durgical.com/en/biochemistry/molecules/17-hydroxyprogesterone/) An intermediate of cortisol synthesis; it piles up when 21-hydroxylase is missing, which is why newborn screening measures it.
- [**Thyroxine (T4)**](https://durgical.com/en/biochemistry/molecules/thyroxine/) The hormone the thyroid makes most; it is mainly a prohormone that tissues turn into active T3.
- [**Triiodothyronine (T3)**](https://durgical.com/en/biochemistry/molecules/triiodothyronine/) The active form of thyroid hormone: it binds nuclear receptors and speeds up metabolism, the heart and brain development.
- [**Adrenaline**](https://durgical.com/en/biochemistry/molecules/adrenaline/) The adrenal medulla hormone for acute stress: it raises glucose and fatty acids and speeds up the heart.
- [**5-Methylcytosine**](https://durgical.com/en/biochemistry/molecules/5-methylcytosine/) 5-Methylcytosine is cytosine with an extra methyl group; it is the main epigenetic mark on DNA and usually switches genes off.
- [**α-Amanitin**](https://durgical.com/en/biochemistry/molecules/amanitin/) α-Amanitin is the toxin of mushrooms such as Amanita phalloides; it blocks RNA polymerase II and destroys liver cells.
- [**PRPP (phosphoribosyl pyrophosphate)**](https://durgical.com/en/biochemistry/molecules/prpp/) PRPP is activated ribose: the starting point for making and recycling every nucleotide.
- [**IMP (inosine monophosphate)**](https://durgical.com/en/biochemistry/molecules/imp/) IMP is the first complete purine the cell makes; AMP and GMP come from it.
- [**Uric acid**](https://durgical.com/en/biochemistry/molecules/uric-acid/) Uric acid is the final product of purines in humans; when it builds up, it forms crystals that cause gout and stones.
- [**Hypoxanthine**](https://durgical.com/en/biochemistry/molecules/hypoxanthine/) Hypoxanthine is a purine the cell can either recycle (HGPRT) or break down to uric acid (xanthine oxidase).
- [**Orotic acid**](https://durgical.com/en/biochemistry/molecules/orotic-acid/) Orotic acid is the newly made pyrimidine ring before it joins the sugar; in urine it signals a block in the pathway or the urea cycle.
- [**dTMP (thymidine monophosphate)**](https://durgical.com/en/biochemistry/molecules/dtmp/) dTMP is the thymine nucleotide, unique to DNA; many cancer drugs work by preventing its formation.
- [**Ubiquinone (coenzyme Q₁₀)**](https://durgical.com/en/biochemistry/molecules/ubiquinone/) Ubiquinone is a fatty molecule that moves within the mitochondrial membrane and carries electrons from complexes I and II to complex III.
- [**Glutathione (GSH)**](https://durgical.com/en/biochemistry/molecules/glutathione/) Glutathione is a tripeptide with a thiol group that neutralises oxidants and toxins; it is the main antioxidant inside the cell.
- [**FAD (flavin adenine dinucleotide)**](https://durgical.com/en/biochemistry/molecules/fad/) FAD is the riboflavin-derived coenzyme that collects electrons in the Krebs cycle and β-oxidation and hands them to ubiquinone.
- [**Arachidonic acid**](https://durgical.com/en/biochemistry/molecules/arachidonic-acid/) Arachidonic acid is a 20-carbon omega-6 fatty acid stored in cell membranes. Released on demand, it is the source of all eicosanoids.
- [**Dopamine**](https://durgical.com/en/biochemistry/molecules/dopamine/) Dopamine is a catecholamine neurotransmitter for movement, reward and prolactin control, and the precursor of norepinephrine.
- [**L-DOPA (levodopa)**](https://durgical.com/en/biochemistry/molecules/l-dopa/) L-DOPA is the first product of catecholamine synthesis. Given as levodopa, it crosses into the brain and replaces dopamine in Parkinson disease.
- [**Leucine**](https://durgical.com/en/biochemistry/molecules/leucine/) Leucine is an essential branched-chain amino acid. It is purely ketogenic and a strong signal for muscle protein synthesis.
- [**Melatonin**](https://durgical.com/en/biochemistry/molecules/melatonin/) Melatonin is the pineal hormone of darkness. It rises at night and tells the body's clock that it is time to sleep.
- [**Methionine**](https://durgical.com/en/biochemistry/molecules/methionine/) Methionine is an essential sulfur amino acid. It starts every protein and is the source of SAM, the body's main methyl donor.
- [**Norepinephrine (noradrenaline)**](https://durgical.com/en/biochemistry/molecules/norepinephrine/) Norepinephrine is the transmitter of sympathetic nerves and a hormone of the adrenal medulla. It raises blood pressure and alertness.
- [**Prostaglandin H₂ (PGH₂)**](https://durgical.com/en/biochemistry/molecules/prostaglandin-h/) PGH₂ is the unstable product of COX. Each cell type turns it into its own prostanoid: TXA₂, PGI₂, PGE₂, PGD₂ or PGF₂α.
- [**S-Adenosylhomocysteine (SAH)**](https://durgical.com/en/biochemistry/molecules/s-adenosylhomocysteine/) SAH is what is left of SAM after it gives away its methyl group. It must be cleared quickly because it blocks methylation.
- [**Serotonin (5-hydroxytryptamine)**](https://durgical.com/en/biochemistry/molecules/serotonin/) Serotonin is made from tryptophan. It is a brain neurotransmitter for mood and sleep, and in the gut it controls motility.
- [**Sphingosine**](https://durgical.com/en/biochemistry/molecules/sphingosine/) Sphingosine is the amino alcohol backbone of all sphingolipids. It is released when lysosomes break down ceramide.
- [**Thromboxane A₂ (TXA₂)**](https://durgical.com/en/biochemistry/molecules/thromboxane-a/) TXA₂ is the platelet signal that calls more platelets and narrows vessels. Aspirin blocks its synthesis for the platelet's whole life.
- [**Histidine**](https://durgical.com/en/biochemistry/molecules/histidine/) Histidine is an essential amino acid with an imidazole ring that gains or loses protons at physiological pH: it buffers and takes part in catalysis.
- [**Hydroxyproline**](https://durgical.com/en/biochemistry/molecules/hydroxyproline/) Hydroxyproline is a proline with an added OH group inside collagen; without vitamin C it is not formed and collagen falls apart.
- [**Ethanol**](https://durgical.com/en/biochemistry/molecules/ethanol/) The alcohol in drinks: a small molecule that crosses every membrane and that the liver oxidises to acetaldehyde and acetate.
- [**Acetaldehyde**](https://durgical.com/en/biochemistry/molecules/acetaldehyde/) The first product of alcohol in the liver: highly reactive, it causes hangover symptoms, flushing and part of the liver damage and cancer.
- [**NAPQI (N-acetyl-p-benzoquinone imine)**](https://durgical.com/en/biochemistry/molecules/napqi/) The toxic metabolite of acetaminophen: glutathione neutralises it, but when glutathione runs out it destroys liver cells.
- [**N-acetylcysteine (NAC)**](https://durgical.com/en/biochemistry/molecules/n-acetylcysteine/) A cysteine derivative that refills liver glutathione; it is the antidote for acetaminophen poisoning.
- [**Fomepizole (4-methylpyrazole)**](https://durgical.com/en/biochemistry/molecules/fomepizole/) An alcohol dehydrogenase inhibitor: it stops methanol and ethylene glycol from being turned into their toxic acids.
- [**Vitamin A (retinol)**](https://durgical.com/en/biochemistry/molecules/vitamin-a/) Vitamin A lets you see in dim light, keeps the linings of the eye, skin and gut healthy, and supports immunity.
- [**Vitamin D (calcitriol)**](https://durgical.com/en/biochemistry/molecules/vitamin-d/) Vitamin D is made in sunlit skin and activated in the liver and kidney; it raises calcium and phosphate to mineralise bone.
- [**Vitamin E (α-tocopherol)**](https://durgical.com/en/biochemistry/molecules/vitamin-e/) Vitamin E is the membrane antioxidant: it slows the oxidation of cell fats, especially in nerves and red blood cells.
- [**Vitamin K (phylloquinone)**](https://durgical.com/en/biochemistry/molecules/vitamin-k/) Vitamin K lets the liver activate clotting factors; without it blood takes too long to clot.
- [**Thiamine (vitamin B1)**](https://durgical.com/en/biochemistry/molecules/thiamine/) Thiamine, as thiamine pyrophosphate, is essential for drawing energy from glucose, especially in the brain and heart.
- [**Riboflavin (vitamin B2)**](https://durgical.com/en/biochemistry/molecules/riboflavin/) Riboflavin forms FAD and FMN, the electron carriers of many oxidation reactions in the cell.
- [**Niacin (vitamin B3)**](https://durgical.com/en/biochemistry/molecules/niacin/) Niacin forms NAD⁺ and NADP⁺, the coenzymes that move electrons in hundreds of reactions; its lack causes pellagra.
- [**Pantothenic acid (vitamin B5)**](https://durgical.com/en/biochemistry/molecules/pantothenic-acid/) Pantothenic acid forms coenzyme A, which carries acyl groups such as acetyl in fat, sugar and protein metabolism.
- [**Vitamin B6 (pyridoxal phosphate)**](https://durgical.com/en/biochemistry/molecules/vitamin-b6/) Vitamin B6, as pyridoxal phosphate, is the coenzyme of amino acid metabolism: transamination, neurotransmitters and heme.
- [**Biotin (vitamin B7)**](https://durgical.com/en/biochemistry/molecules/biotin/) Biotin is the coenzyme of the carboxylases, the enzymes that add CO₂ in gluconeogenesis and fatty acid synthesis.
- [**Folate (vitamin B9)**](https://durgical.com/en/biochemistry/molecules/folate/) Folate carries one-carbon groups to build DNA; its lack causes megaloblastic anaemia and neural tube defects.
- [**Vitamin C (ascorbic acid)**](https://durgical.com/en/biochemistry/molecules/vitamin-c/) Vitamin C is needed to make stable collagen, absorb iron and protect against oxidative damage; its lack causes scurvy.

## Enzymes and proteins

- [**Carbamoyl phosphate synthetase I (CPS-I)**](https://durgical.com/en/biochemistry/enzymes/carbamoyl-phosphate-synthetase-i/) CPS-I opens the urea cycle in liver mitochondria: it joins ammonia and CO₂ using 2 ATP to make carbamoyl phosphate.
- [**Ornithine transcarbamylase (OTC)**](https://durgical.com/en/biochemistry/enzymes/ornithine-transcarbamylase/) OTC joins carbamoyl phosphate to ornithine to make citrulline. Its X-linked deficiency is the most common urea cycle disorder.
- [**Argininosuccinate synthetase (ASS)**](https://durgical.com/en/biochemistry/enzymes/argininosuccinate-synthetase/) Argininosuccinate synthetase joins citrulline and aspartate in the cytosol; this is how the second nitrogen of urea enters.
- [**Argininosuccinate lyase (ASL)**](https://durgical.com/en/biochemistry/enzymes/argininosuccinate-lyase/) Argininosuccinate lyase splits argininosuccinate into arginine and fumarate; the fumarate links the urea cycle to the Krebs cycle.
- [**Arginase 1**](https://durgical.com/en/biochemistry/enzymes/arginase-1/) Arginase 1 closes the urea cycle in the liver cytosol: it releases urea and regenerates ornithine.
- [**Alanine aminotransferase (ALT, GPT)**](https://durgical.com/en/biochemistry/enzymes/alanine-aminotransferase/) ALT moves the amino group of alanine to α-ketoglutarate. Measured in blood, it is the most used marker of liver damage.
- [**Aspartate aminotransferase (AST, GOT)**](https://durgical.com/en/biochemistry/enzymes/aspartate-aminotransferase/) AST swaps aspartate and oxaloacetate with glutamate and α-ketoglutarate. It supplies urea cycle aspartate and marks liver injury.
- [**Glutamate dehydrogenase (GDH)**](https://durgical.com/en/biochemistry/enzymes/glutamate-dehydrogenase/) Glutamate dehydrogenase frees ammonia from glutamate in liver mitochondria and links amino acid breakdown to the Krebs cycle.
- [**Glutamine synthetase**](https://durgical.com/en/biochemistry/enzymes/glutamine-synthetase/) Glutamine synthetase traps free ammonia by joining it to glutamate. This protects the brain and carries nitrogen safely in the blood.
- [**Phenylalanine hydroxylase (PAH)**](https://durgical.com/en/biochemistry/enzymes/phenylalanine-hydroxylase/) Phenylalanine hydroxylase turns phenylalanine into tyrosine in the liver. If it fails, phenylalanine builds up: phenylketonuria.
- [**Branched-chain α-keto acid dehydrogenase (BCKDH)**](https://durgical.com/en/biochemistry/enzymes/branched-chain-keto-acid-dehydrogenase/) BCKDH breaks down leucine, isoleucine and valine after their transamination. If it fails, urine smells like maple syrup.
- [**Cystathionine β-synthase (CBS)**](https://durgical.com/en/biochemistry/enzymes/cystathionine-synthase/) Cystathionine β-synthase joins homocysteine and serine on the route to cysteine. Its deficiency causes classic homocystinuria.
- [**Methionine synthase**](https://durgical.com/en/biochemistry/enzymes/methionine-synthase/) Methionine synthase moves a methyl from 5-methyltetrahydrofolate to homocysteine. It links the folate cycle with vitamin B12.
- [**Tyrosine hydroxylase**](https://durgical.com/en/biochemistry/enzymes/tyrosine-hydroxylase/) Tyrosine hydroxylase makes L-DOPA from tyrosine: the rate-limiting step for making dopamine, noradrenaline and adrenaline.
- [**Hexokinase**](https://durgical.com/en/biochemistry/enzymes/hexokinase/) Hexokinase opens glycolysis: it puts a phosphate on glucose and traps it inside the cell.
- [**Phosphofructokinase-1 (PFK-1)**](https://durgical.com/en/biochemistry/enzymes/phosphofructokinase-1/) Phosphofructokinase-1 is the main switch of glycolysis: it decides whether glucose is burned according to the cell's energy.
- [**Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**](https://durgical.com/en/biochemistry/enzymes/glyceraldehyde-3-phosphate-dehydrogenase/) GAPDH is the glycolysis enzyme that makes NADH and builds a high-energy bond; arsenic fools it.
- [**Pyruvate kinase**](https://durgical.com/en/biochemistry/enzymes/pyruvate-kinase/) Pyruvate kinase makes the last step of glycolysis and its second ATP; if it fails in red cells, they are destroyed too early.
- [**Hemoglobin and 2,3-BPG**](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/) Hemoglobin carries oxygen in the blood; 2,3-BPG, made by a side branch of red-cell glycolysis, helps it let go in the tissues.
- [**Pyruvate dehydrogenase complex (PDH)**](https://durgical.com/en/biochemistry/enzymes/pyruvate-dehydrogenase-complex/) Pyruvate dehydrogenase is the gate between glycolysis and the Krebs cycle; it needs five coenzymes and four B vitamins.
- [**Citrate synthase**](https://durgical.com/en/biochemistry/enzymes/citrate-synthase/) Citrate synthase opens each turn of the Krebs cycle: it joins acetyl-CoA to oxaloacetate to make citrate.
- [**Isocitrate dehydrogenase**](https://durgical.com/en/biochemistry/enzymes/isocitrate-dehydrogenase/) Isocitrate dehydrogenase sets the pace of the Krebs cycle; its mutated relatives in some cancers make an oncometabolite.
- [**α-Ketoglutarate dehydrogenase**](https://durgical.com/en/biochemistry/enzymes/ketoglutarate-dehydrogenase/) α-Ketoglutarate dehydrogenase is PDH's twin inside the cycle: it releases the second CO₂ and uses the same vitamins.
- [**Succinate dehydrogenase (complex II)**](https://durgical.com/en/biochemistry/enzymes/succinate-dehydrogenase/) Succinate dehydrogenase is the Krebs cycle enzyme that is also part of the respiratory chain; if it fails, stored succinate favours tumours.
- [**Pyruvate carboxylase**](https://durgical.com/en/biochemistry/enzymes/pyruvate-carboxylase/) Pyruvate carboxylase turns pyruvate into oxaloacetate inside the mitochondrion: the first step in making glucose and in topping up the Krebs cycle.
- [**Phosphoenolpyruvate carboxykinase (PEPCK)**](https://durgical.com/en/biochemistry/enzymes/phosphoenolpyruvate-carboxykinase/) PEPCK turns oxaloacetate into phosphoenolpyruvate using GTP; it is a key gluconeogenic enzyme switched on by glucagon and off by insulin.
- [**Fructose-1,6-bisphosphatase (FBPase-1)**](https://durgical.com/en/biochemistry/enzymes/fructose-1-6-bisphosphatase/) Fructose-1,6-bisphosphatase removes a phosphate from fructose 1,6-bisphosphate and is the main control point of gluconeogenesis.
- [**Glucose-6-phosphatase**](https://durgical.com/en/biochemistry/enzymes/glucose-6-phosphatase/) Glucose-6-phosphatase releases glucose into the blood from liver and kidney; it is the last step of both gluconeogenesis and glycogenolysis.
- [**Glycogen synthase**](https://durgical.com/en/biochemistry/enzymes/glycogen-synthase/) Glycogen synthase lengthens glycogen chains with glucose taken from UDP-glucose; insulin activates it after a meal.
- [**Glycogen phosphorylase**](https://durgical.com/en/biochemistry/enzymes/glycogen-phosphorylase/) Glycogen phosphorylase releases glucose 1-phosphate from glycogen; it controls glycogen breakdown in liver and muscle.
- [**Glycogen debranching enzyme**](https://durgical.com/en/biochemistry/enzymes/glycogen-debranching-enzyme/) The debranching enzyme removes glycogen's branches so phosphorylase can keep going; it has two activities in a single protein.
- [**Glucose-6-phosphate dehydrogenase (G6PD)**](https://durgical.com/en/biochemistry/enzymes/glucose-6-phosphate-dehydrogenase/) G6PD makes NADPH in the pentose pathway; in the red blood cell it is the only defence against oxidative damage.
- [**Transketolase**](https://durgical.com/en/biochemistry/enzymes/transketolase/) Transketolase swaps two-carbon units between sugars in the pentose pathway; it needs thiamine (vitamin B1).
- [**Aldolase B**](https://durgical.com/en/biochemistry/enzymes/aldolase-b/) Liver aldolase B splits fructose 1-phosphate into two trioses; if it is missing, dietary fructose becomes toxic.
- [**Galactose-1-phosphate uridylyltransferase (GALT)**](https://durgical.com/en/biochemistry/enzymes/galactose-1-phosphate-uridylyltransferase/) GALT is the central enzyme of the Leloir pathway: it turns galactose 1-phosphate into a sugar the body can use. Its absence causes classic galactosaemia.
- [**Glucocerebrosidase (acid β-glucosidase)**](https://durgical.com/en/biochemistry/enzymes/glucocerebrosidase/) Glucocerebrosidase is the lysosomal enzyme that removes glucose from glucosylceramide. Its absence causes Gaucher disease.
- [**Hexosaminidase A (β-hexosaminidase A)**](https://durgical.com/en/biochemistry/enzymes/hexosaminidase-a/) Hexosaminidase A is the lysosomal enzyme that cleaves GM2 ganglioside. Without its α subunit, neurons fill with GM2: Tay–Sachs disease.
- [**α-Galactosidase A**](https://durgical.com/en/biochemistry/enzymes/galactosidase-a/) α-Galactosidase A removes the terminal galactose from globotriaosylceramide in the lysosome. Its X-linked loss causes Fabry disease.
- [**Acid sphingomyelinase**](https://durgical.com/en/biochemistry/enzymes/acid-sphingomyelinase/) Acid sphingomyelinase splits sphingomyelin into ceramide and phosphocholine inside the lysosome. Its absence causes Niemann–Pick types A and B.
- [**α-L-Iduronidase**](https://durgical.com/en/biochemistry/enzymes/l-iduronidase/) α-L-Iduronidase removes iduronic acid from the end of dermatan and heparan sulfate in the lysosome. Its absence causes mucopolysaccharidosis I (Hurler).
- [**LDL receptor**](https://durgical.com/en/biochemistry/enzymes/ldl-receptor/) The LDL receptor pulls LDL particles out of the blood into the cell; when it fails, LDL cholesterol is high from birth.
- [**Lipoprotein lipase (LPL)**](https://durgical.com/en/biochemistry/enzymes/lipoprotein-lipase/) Lipoprotein lipase, anchored in capillaries, cuts the triglycerides of chylomicrons and VLDL so muscle and fat can take up the fatty acids.
- [**LCAT (lecithin–cholesterol acyltransferase)**](https://durgical.com/en/biochemistry/enzymes/lcat/) LCAT esterifies free cholesterol on HDL; the ester sinks into the particle core, which matures from a disc into a sphere.
- [**CETP (cholesteryl ester transfer protein)**](https://durgical.com/en/biochemistry/enzymes/cetp/) CETP moves cholesteryl esters from HDL to ApoB lipoproteins and brings triglycerides back.
- [**PCSK9**](https://durgical.com/en/biochemistry/enzymes/pcsk9/) PCSK9 binds the LDL receptor and sends it to be destroyed; blocking it leaves more receptors and lowers LDL cholesterol sharply.
- [**Squalene epoxidase**](https://durgical.com/en/biochemistry/enzymes/squalene-epoxidase/) Squalene epoxidase activates squalene with oxygen so it can close into rings; it is the second control point of cholesterol synthesis.
- [**Cholesterol 7α-hydroxylase (CYP7A1)**](https://durgical.com/en/biochemistry/enzymes/cholesterol-7-hydroxylase/) Liver CYP7A1 takes the first step in turning cholesterol into bile acids and sets the pace of that pathway.
- [**Apolipoprotein B (ApoB-100 and ApoB-48)**](https://durgical.com/en/biochemistry/enzymes/apolipoprotein-b/) ApoB is the structural protein of chylomicrons, VLDL, IDL and LDL: each of these particles carries a single copy.
- [**Apolipoprotein E (ApoE)**](https://durgical.com/en/biochemistry/enzymes/apolipoprotein-e/) ApoE is the key that lets chylomicron remnants and IDL into the liver; its variants change cholesterol and Alzheimer risk.
- [**Acetyl-CoA carboxylase (ACC)**](https://durgical.com/en/biochemistry/enzymes/acetyl-coa-carboxylase/) Acetyl-CoA carboxylase decides whether the body makes fat: it turns acetyl-CoA into malonyl-CoA using biotin and ATP.
- [**Fatty acid synthase (FAS)**](https://durgical.com/en/biochemistry/enzymes/fatty-acid-synthase/) Fatty acid synthase is one giant protein with seven activities that builds palmitate two carbons at a time.
- [**Carnitine palmitoyltransferase I (CPT-I)**](https://durgical.com/en/biochemistry/enzymes/carnitine-palmitoyltransferase-i/) CPT-I is the gate for fat into the mitochondrion: it moves the fatty acid from CoA to carnitine so it can cross.
- [**Medium-chain acyl-CoA dehydrogenase (MCAD)**](https://durgical.com/en/biochemistry/enzymes/medium-chain-acyl-coa-dehydrogenase/) MCAD carries out the first β-oxidation step for fatty acids of 6 to 12 carbons; its lack is the most common defect in the group.
- [**Mitochondrial HMG-CoA synthase (HMGCS2)**](https://durgical.com/en/biochemistry/enzymes/mitochondrial-hmg-coa-synthase/) Mitochondrial HMG-CoA synthase is the enzyme that switches on ketone body production in the liver during fasting.
- [**β-Hydroxybutyrate dehydrogenase (BDH1)**](https://durgical.com/en/biochemistry/enzymes/hydroxybutyrate-dehydrogenase/) This enzyme turns acetoacetate into β-hydroxybutyrate in the liver, and back again in tissues that burn ketones.
- [**Succinyl-CoA:3-ketoacid CoA transferase (SCOT)**](https://durgical.com/en/biochemistry/enzymes/succinyl-coa-3-ketoacid-coa-transferase/) SCOT lets muscle, heart and brain use ketones: it moves CoA from succinyl-CoA onto acetoacetate.
- [**Hormone-sensitive lipase (HSL)**](https://durgical.com/en/biochemistry/enzymes/hormone-sensitive-lipase/) Hormone-sensitive lipase releases fatty acids from stored fat when the body needs energy (fasting, stress, exercise).
- [**Proteasome (26S, β5 subunit)**](https://durgical.com/en/biochemistry/enzymes/proteasome/) The proteasome is the cell's shredder for proteins tagged with ubiquitin. Myeloma drugs such as bortezomib block its cutting site.
- [**Nitric oxide synthase (NOS)**](https://durgical.com/en/biochemistry/enzymes/nitric-oxide-synthase/) Nitric oxide synthase turns arginine into citrulline and nitric oxide, a gas that relaxes blood vessels, carries nerve signals and helps kill microbes.
- [**Iduronate 2-sulfatase (I2S)**](https://durgical.com/en/biochemistry/enzymes/iduronate-2-sulfatase/) Iduronate 2-sulfatase strips the sulfate off iduronic acid in dermatan and heparan sulfate. Its loss causes Hunter syndrome (MPS II).
- [**Arylsulfatase A (ARSA)**](https://durgical.com/en/biochemistry/enzymes/arylsulfatase-a/) Arylsulfatase A removes the sulfate from sulfatide in the lysosome, helped by saposin B. Without it sulfatide destroys myelin: metachromatic leukodystrophy.
- [**Galactocerebrosidase (GALC)**](https://durgical.com/en/biochemistry/enzymes/galactocerebrosidase/) Galactocerebrosidase breaks down galactosylceramide and the toxic psychosine in lysosomes. Its deficiency kills myelin-forming cells: Krabbe disease.
- [**Hexosaminidase B (β-hexosaminidase B)**](https://durgical.com/en/biochemistry/enzymes/hexosaminidase-b/) Hexosaminidase B is the ββ form of β-hexosaminidase. It trims amino sugars from glycolipids and oligosaccharides; losing the β subunit causes Sandhoff disease.
- [**ALA synthase**](https://durgical.com/en/biochemistry/enzymes/ala-synthase/) ALA synthase opens and controls heme synthesis: it joins glycine and succinyl-CoA in the mitochondrion.
- [**ALA dehydratase**](https://durgical.com/en/biochemistry/enzymes/ala-dehydratase/) ALA dehydratase joins two ALA into the first pyrrole ring; it is the enzyme most sensitive to lead.
- [**Porphobilinogen deaminase**](https://durgical.com/en/biochemistry/enzymes/porphobilinogen-deaminase/) Porphobilinogen deaminase strings four pyrroles together; its deficiency causes acute intermittent porphyria.
- [**Uroporphyrinogen decarboxylase**](https://durgical.com/en/biochemistry/enzymes/uroporphyrinogen-decarboxylase/) Uroporphyrinogen decarboxylase removes four carboxyls from the ring; when it fails in the liver, porphyria cutanea tarda appears.
- [**Ferrochelatase**](https://durgical.com/en/biochemistry/enzymes/ferrochelatase/) Ferrochelatase puts iron inside protoporphyrin and finishes heme synthesis in the mitochondrion.
- [**Heme oxygenase**](https://durgical.com/en/biochemistry/enzymes/heme-oxygenase/) Heme oxygenase breaks the heme ring, releasing biliverdin, iron and carbon monoxide.
- [**Biliverdin reductase**](https://durgical.com/en/biochemistry/enzymes/biliverdin-reductase/) Biliverdin reductase turns green biliverdin into yellow bilirubin using NADPH.
- [**UGT1A1 (bilirubin UDP-glucuronosyltransferase)**](https://durgical.com/en/biochemistry/enzymes/ugt1a1/) Liver UGT1A1 attaches glucuronic acid to bilirubin so it dissolves in bile; its failure causes Gilbert and Crigler-Najjar.
- [**Myoglobin**](https://durgical.com/en/biochemistry/enzymes/myoglobin/) Myoglobin stores oxygen inside muscle; it has a single heme and gives it up only when oxygen falls very low.
- [**Adenylyl cyclase**](https://durgical.com/en/biochemistry/enzymes/adenylyl-cyclase/) The enzyme that turns ATP into cAMP when a hormone activates a Gs-coupled receptor; it is the signal's amplifier.
- [**Protein kinase A (PKA)**](https://durgical.com/en/biochemistry/enzymes/protein-kinase-a/) The kinase that turns cAMP into action: it phosphorylates glycogen, fat and glucose enzymes during fasting and stress.
- [**AMP-activated protein kinase (AMPK)**](https://durgical.com/en/biochemistry/enzymes/amp-activated-protein-kinase/) The cell's fuel gauge: when ATP falls and AMP rises, it switches off what spends energy and switches on what makes it.
- [**mTOR (mechanistic target of rapamycin)**](https://durgical.com/en/biochemistry/enzymes/mtor/) A kinase that decides whether the cell grows: with enough nutrients, insulin and energy it switches on protein synthesis and stops autophagy.
- [**Insulin receptor**](https://durgical.com/en/biochemistry/enzymes/insulin-receptor/) A receptor tyrosine kinase: insulin switches it on and it phosphorylates the proteins that move GLUT4 to the membrane and store glucose.
- [**Aromatase (CYP19A1)**](https://durgical.com/en/biochemistry/enzymes/aromatase/) The enzyme that turns androgens into oestrogens; it is in the ovary, placenta, fat, bone and brain.
- [**21-Hydroxylase (CYP21A2)**](https://durgical.com/en/biochemistry/enzymes/21-hydroxylase/) An adrenal enzyme needed to make cortisol and aldosterone; its deficiency causes 95 % of congenital adrenal hyperplasia.
- [**11β-Hydroxylase (CYP11B1)**](https://durgical.com/en/biochemistry/enzymes/11-hydroxylase/) It makes the last step of cortisol in fasciculata mitochondria; its deficiency causes adrenal hyperplasia with hypertension.
- [**17α-Hydroxylase/17,20-lyase (CYP17A1)**](https://durgical.com/en/biochemistry/enzymes/17-hydroxylase-17-20-lyase/) One enzyme with two activities that decide whether pregnenolone goes to cortisol, to androgens or only to aldosterone.
- [**Thyroid peroxidase (TPO)**](https://durgical.com/en/biochemistry/enzymes/thyroid-peroxidase/) The thyroid's key enzyme: it oxidises iodide, attaches it to thyroglobulin and joins iodinated tyrosines to form T4 and T3.
- [**Iodothyronine deiodinases (DIO1, DIO2, DIO3)**](https://durgical.com/en/biochemistry/enzymes/iodothyronine-deiodinases/) Three selenoenzymes that activate T4 to T3 or inactivate it; they decide how much active thyroid hormone each tissue gets.
- [**DNA polymerases**](https://durgical.com/en/biochemistry/enzymes/dna-polymerases/) DNA polymerases copy DNA by adding nucleotides one by one to the 3' end of the new strand, following the template strand.
- [**DNA helicases**](https://durgical.com/en/biochemistry/enzymes/dna-helicases/) Helicases are motors that separate the two DNA strands using ATP; they open the replication fork and help repair DNA.
- [**Topoisomerases (I and II)**](https://durgical.com/en/biochemistry/enzymes/topoisomerases/) Topoisomerases cut and rejoin DNA to remove the twists that build up when it opens; they are the target of several anticancer drugs.
- [**Bacterial DNA gyrase (target of fluoroquinolones)**](https://durgical.com/en/biochemistry/enzymes/bacterial-dna-gyrase/) DNA gyrase is a bacteria-only topoisomerase II that introduces negative supercoils; fluoroquinolones block it.
- [**Telomerase**](https://durgical.com/en/biochemistry/enzymes/telomerase/) Telomerase lengthens chromosome ends (telomeres) using its own RNA as a template; it is active in stem cells and in most cancers.
- [**RNA polymerases I, II and III**](https://durgical.com/en/biochemistry/enzymes/rna-polymerases-i-ii-and-iii/) RNA polymerases copy DNA into RNA; human cells have three in the nucleus, each dedicated to a type of RNA.
- [**Spliceosome**](https://durgical.com/en/biochemistry/enzymes/spliceosome/) The spliceosome is a large RNA–protein machine that cuts introns out of pre-mRNA and joins the exons; its catalytic core is RNA.
- [**Ribosome (and the antibiotics that target it)**](https://durgical.com/en/biochemistry/enzymes/ribosome/) The ribosome is the protein factory: it reads mRNA and joins amino acids. Bacterial ribosomes differ from human ones, so many antibiotics target them.
- [**PRPP synthetase**](https://durgical.com/en/biochemistry/enzymes/prpp-synthetase/) Makes PRPP, the activated ribose needed to build and recycle every nucleotide.
- [**Glutamine-PRPP amidotransferase**](https://durgical.com/en/biochemistry/enzymes/glutamine-prpp-amidotransferase/) The main switch of purine synthesis: the committed step that turns off when there are enough nucleotides.
- [**Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**](https://durgical.com/en/biochemistry/enzymes/hypoxanthine-guanine-phosphoribosyltransferase/) Recycles the bases hypoxanthine and guanine so they need not be built again; its loss causes Lesch-Nyhan syndrome.
- [**Adenine phosphoribosyltransferase (APRT)**](https://durgical.com/en/biochemistry/enzymes/adenine-phosphoribosyltransferase/) Recycles free adenine; when it is missing, adenine turns into an insoluble compound that forms kidney stones.
- [**Xanthine oxidase**](https://durgical.com/en/biochemistry/enzymes/xanthine-oxidase/) Performs the last two steps of purine breakdown and makes uric acid; it is the target of allopurinol and febuxostat.
- [**Adenosine deaminase (ADA)**](https://durgical.com/en/biochemistry/enzymes/adenosine-deaminase/) Clears adenosine and deoxyadenosine; without it lymphocytes die and a severe combined immunodeficiency appears.
- [**Ribonucleotide reductase**](https://durgical.com/en/biochemistry/enzymes/ribonucleotide-reductase/) Turns ribonucleotides into deoxyribonucleotides, the building blocks of DNA; it is the target of hydroxyurea.
- [**CAD (carbamoyl phosphate synthetase II)**](https://durgical.com/en/biochemistry/enzymes/cad/) A single protein performs the first three steps of pyrimidine synthesis; its CPS-II part is the pathway's control point.
- [**Dihydroorotate dehydrogenase (DHODH)**](https://durgical.com/en/biochemistry/enzymes/dihydroorotate-dehydrogenase/) The only mitochondrial enzyme of pyrimidine synthesis; leflunomide, used in rheumatoid arthritis, blocks it.
- [**UMP synthase**](https://durgical.com/en/biochemistry/enzymes/ump-synthase/) Finishes the first pyrimidine: joins orotate to ribose phosphate and turns it into UMP. Its loss causes orotic aciduria.
- [**IMP dehydrogenase (IMPDH)**](https://durgical.com/en/biochemistry/enzymes/imp-dehydrogenase/) Opens the route from IMP to GMP; lymphocytes rely on it heavily, so mycophenolate blocks it to prevent rejection.
- [**Complex I (NADH dehydrogenase)**](https://durgical.com/en/biochemistry/enzymes/complex-i/) Complex I is NADH's entry door to the respiratory chain: it passes NADH's electrons to ubiquinone and pumps protons.
- [**Complex III (cytochrome bc₁)**](https://durgical.com/en/biochemistry/enzymes/complex-iii/) Complex III passes electrons from ubiquinol to cytochrome c and, through the Q cycle, pumps protons.
- [**Cytochrome c oxidase (complex IV)**](https://durgical.com/en/biochemistry/enzymes/cytochrome-c-oxidase/) Cytochrome c oxidase is the last complex of the chain: it hands the electrons to oxygen, makes water and pumps protons.
- [**ATP synthase (complex V)**](https://durgical.com/en/biochemistry/enzymes/atp-synthase/) ATP synthase is a molecular motor: protons flowing back into the matrix spin it, and each turn makes ATP from ADP and phosphate.
- [**UCP1 (thermogenin)**](https://durgical.com/en/biochemistry/enzymes/ucp1/) UCP1 is a proton channel in brown fat: it lets the gradient discharge without making ATP, so the energy leaves as heat.
- [**Superoxide dismutase (SOD)**](https://durgical.com/en/biochemistry/enzymes/superoxide-dismutase/) Superoxide dismutase turns the superoxide radical into hydrogen peroxide and oxygen; it is the first antioxidant line.
- [**Catalase**](https://durgical.com/en/biochemistry/enzymes/catalase/) Catalase breaks hydrogen peroxide down into water and oxygen; it works mainly in peroxisomes and in red blood cells.
- [**Glutathione peroxidase (GPX)**](https://durgical.com/en/biochemistry/enzymes/glutathione-peroxidase/) Glutathione peroxidase removes hydrogen peroxide and lipid peroxides using glutathione; it contains selenium.
- [**Cytochrome c**](https://durgical.com/en/biochemistry/enzymes/cytochrome-c/) Cytochrome c is a small heme protein that carries one electron at a time from complex III to complex IV; out of place, it triggers apoptosis.
- [**Collagen**](https://durgical.com/en/biochemistry/enzymes/collagen/) Collagen is the body's most abundant protein: three braided chains that give strength to bone, skin, tendons and vessels.
- [**Elastin**](https://durgical.com/en/biochemistry/enzymes/elastin/) Elastin lets arteries, lungs and skin stretch and spring back; it is built on a scaffold of fibrillin.
- [**Fibrillin-1**](https://durgical.com/en/biochemistry/enzymes/fibrillin-1/) Fibrillin-1 forms microfibrils that support elastic fibres and store TGF-β; its variants cause Marfan syndrome.
- [**Albumin**](https://durgical.com/en/biochemistry/enzymes/albumin/) Albumin is the most abundant plasma protein: it keeps water inside vessels and carries fatty acids, bilirubin, calcium and drugs.
- [**Immunoglobulins (antibodies)**](https://durgical.com/en/biochemistry/enzymes/immunoglobulins/) Immunoglobulins are antibodies: Y-shaped proteins made by plasma cells to recognise and tag anything foreign.
- [**α1-antitrypsin**](https://durgical.com/en/biochemistry/enzymes/1-antitrypsin/) α1-antitrypsin, made in the liver, restrains neutrophil elastase and protects the lung; when it misfolds, it damages the liver too.
- [**Transthyretin (prealbumin)**](https://durgical.com/en/biochemistry/enzymes/transthyretin/) Transthyretin carries thyroxine and vitamin A in blood; if its tetramer falls apart, it forms amyloid in the heart and nerves.
- [**Prion protein (PrP)**](https://durgical.com/en/biochemistry/enzymes/prion-protein/) The normal prion protein sits on neurons; its misfolded form turns the others into copies of itself and destroys the brain.
- [**HMG-CoA reductase**](https://durgical.com/en/biochemistry/enzymes/hmg-coa-reductase/) HMG-CoA reductase makes mevalonate, the slow step that sets how much cholesterol the cell builds. Statins block it.
- [**Cyclooxygenase (COX-1 and COX-2)**](https://durgical.com/en/biochemistry/enzymes/cyclooxygenase/) Cyclooxygenase turns arachidonic acid into the parent of all prostaglandins and thromboxanes. Aspirin and NSAIDs block it.
- [**Angiotensin-converting enzyme (ACE)**](https://durgical.com/en/biochemistry/enzymes/angiotensin-converting-enzyme/) ACE makes angiotensin II, which narrows vessels, and breaks down bradykinin, which widens them. ACE inhibitors block it.
- [**Vitamin K epoxide reductase (VKORC1)**](https://durgical.com/en/biochemistry/enzymes/vitamin-k-epoxide-reductase/) VKORC1 recycles used vitamin K so the liver can keep activating clotting factors. Warfarin blocks it.
- [**Dihydrofolate reductase (DHFR)**](https://durgical.com/en/biochemistry/enzymes/dihydrofolate-reductase/) DHFR turns dihydrofolate back into tetrahydrofolate, the carrier of one-carbon units for DNA building blocks. Methotrexate blocks it.
- [**Thymidylate synthase**](https://durgical.com/en/biochemistry/enzymes/thymidylate-synthase/) Thymidylate synthase makes dTMP, the thymine building block that only DNA needs. Fluorouracil blocks it.
- [**Na⁺/K⁺-ATPase (sodium pump)**](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/) The sodium pump spends ATP to push Na⁺ out of the cell and bring K⁺ in. Digoxin blocks it in the heart.
- [**H⁺/K⁺-ATPase (gastric proton pump)**](https://durgical.com/en/biochemistry/enzymes/h-k-atpase/) The gastric proton pump spends ATP to swap H⁺ for K⁺ and makes stomach acid. Proton pump inhibitors block it.
- [**Alcohol dehydrogenase (ADH)**](https://durgical.com/en/biochemistry/enzymes/alcohol-dehydrogenase/) A liver enzyme that starts alcohol removal: it turns it into acetaldehyde using NAD⁺ and a zinc atom.
- [**Aldehyde dehydrogenase 2 (ALDH2)**](https://durgical.com/en/biochemistry/enzymes/aldehyde-dehydrogenase-2/) A mitochondrial enzyme that clears the acetaldehyde made from alcohol; when it fails, drinking causes flushing, nausea and palpitations.
- [**Cytochrome P450 2E1 (CYP2E1, MEOS)**](https://durgical.com/en/biochemistry/enzymes/cytochrome-p450-2e1/) A liver enzyme that oxidises ethanol in heavy drinkers and turns acetaminophen and other toxins into harmful products.
- [**Cytochrome P450 3A4 (CYP3A4)**](https://durgical.com/en/biochemistry/enzymes/cytochrome-p450-3a4/) The most abundant P450 enzyme in liver and gut; it transforms about half of all drugs and is the key to many interactions.

## Topics

- [**Transamination and deamination**](https://durgical.com/en/biochemistry/topics/transamination-and-deamination/) This is how amino acids lose their nitrogen: transaminases collect it on glutamate and glutamate dehydrogenase releases it as ammonia.
- [**One-carbon metabolism (folate, B12 and SAM)**](https://durgical.com/en/biochemistry/topics/one-carbon-metabolism/) Folate and vitamin B12 move one-carbon groups to make DNA and methylate molecules; when they fail, megaloblastic anaemia and high homocysteine appear.
- [**Molecules derived from amino acids**](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/) Besides building proteins, amino acids give hormones, neurotransmitters, pigments and key molecules such as nitric oxide, creatine and haem.
- [**Glucogenic and ketogenic amino acids**](https://durgical.com/en/biochemistry/topics/glucogenic-and-ketogenic-amino-acids/) Depending on where its carbon skeleton ends up, an amino acid can be used to make glucose, ketone bodies or both.
- [**Cori cycle**](https://durgical.com/en/biochemistry/topics/cori-cycle/) In the Cori cycle, lactate made by muscle and red cells travels to the liver, is turned back into glucose and returns to the blood.
- [**Glucose–alanine cycle**](https://durgical.com/en/biochemistry/topics/glucose-alanine-cycle/) Muscle sends its spare nitrogen to the liver as alanine; the liver turns it into urea and sends back glucose.
- [**Carbohydrate chemistry**](https://durgical.com/en/biochemistry/topics/carbohydrate-chemistry/) Sugars differ by their carbonyl group, their number of carbons and the direction of their OH groups; glycosidic bonds join them together.
- [**Lipid chemistry**](https://durgical.com/en/biochemistry/topics/lipid-chemistry/) Lipids are water-insoluble molecules: fatty acids, triacylglycerols for energy storage, and phospholipids, sphingolipids and steroids for membranes.
- [**Phospholipids**](https://durgical.com/en/biochemistry/topics/phospholipids/) Phospholipids have a phosphate head that attracts water and two fatty tails that avoid it, so they form the bilayers of every membrane.
- [**Sphingolipids and sphingolipidoses**](https://durgical.com/en/biochemistry/topics/sphingolipids-and-sphingolipidoses/) Sphingolipids are built on ceramide and are rich in myelin and neurons; when a lysosomal enzyme that breaks them down fails, they build up.
- [**Membrane structure**](https://durgical.com/en/biochemistry/topics/membrane-structure/) The cell membrane is a lipid bilayer with embedded proteins that move within it, like a fluid mosaic.
- [**Membrane transport**](https://durgical.com/en/biochemistry/topics/membrane-transport/) Molecules cross membranes by simple diffusion, through channels and carriers down a gradient, or with energy against it.
- [**Cell organelles and their metabolism**](https://durgical.com/en/biochemistry/topics/cell-organelles-and-their-metabolism/) Each organelle has its own metabolic job: mitochondria make ATP, lysosomes recycle, peroxisomes oxidise, and the ER and Golgi build.
- [**Lysosomal storage diseases**](https://durgical.com/en/biochemistry/topics/lysosomal-storage-diseases/) When a lysosomal enzyme is missing, the molecule it should break down stays inside, the lysosome swells and the cell eventually fails.
- [**Glycosaminoglycans (mucopolysaccharides)**](https://durgical.com/en/biochemistry/topics/glycosaminoglycans/) Glycosaminoglycans are long, negatively charged sugar chains that hold water and give bulk and toughness to cartilage and connective tissue.
- [**Mannose 6-phosphate tagging (sending enzymes to the lysosome)**](https://durgical.com/en/biochemistry/topics/mannose-6-phosphate-tagging/) In the Golgi, lysosomal enzymes get a mannose 6-phosphate tag; without it they leave the cell instead of reaching the lysosome.
- [**Lipoprotein metabolism**](https://durgical.com/en/biochemistry/topics/lipoprotein-metabolism/) Lipoproteins carry fat in the blood: the exogenous pathway delivers dietary fat, the endogenous pathway liver fat, and HDL returns cholesterol to the liver.
- [**Chylomicron**](https://durgical.com/en/biochemistry/topics/chylomicron/) Chylomicrons are the largest lipoproteins: they carry dietary fat from the gut, through the lymph, to the tissues.
- [**VLDL (very-low-density lipoprotein)**](https://durgical.com/en/biochemistry/topics/vldl/) VLDL carries the triglycerides made by the liver to the tissues; as it empties it becomes IDL and then LDL.
- [**LDL (low-density lipoprotein)**](https://durgical.com/en/biochemistry/topics/ldl/) LDL delivers cholesterol to the tissues; when there is too much in the blood it settles in the arteries, hence the name "bad" cholesterol.
- [**HDL (high-density lipoprotein)**](https://durgical.com/en/biochemistry/topics/hdl/) HDL collects cholesterol from tissues and returns it to the liver (reverse transport); it also stores apolipoproteins for other particles.
- [**Bile acid synthesis**](https://durgical.com/en/biochemistry/topics/bile-acid-synthesis/) The liver turns cholesterol into bile acids, which help digest fat and are the body's main way out for cholesterol.
- [**Steroid hormone synthesis (overview)**](https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/) The adrenals, ovaries, testes and placenta turn cholesterol into steroid hormones; everything starts with pregnenolone.
- [**Acid–base interpretation (step by step)**](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/) A fixed order for reading a blood gas: pH, primary cause, compensation, anion gap and mixed disorders.
- [**Bicarbonate–CO₂ buffer system**](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/) The bicarbonate–CO₂ pair buffers blood acids; the lungs control CO₂ and the kidneys control bicarbonate.
- [**Anion gap and delta ratio**](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/) The anion gap estimates unmeasured acids and splits metabolic acidoses into two broad groups.
- [**Serum osmolality and osmolar gap**](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/) The difference between measured and calculated osmolality reveals extra substances in the blood, such as methanol.
- [**Acid–base compensation rules**](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/) Formulas that tell whether the lungs or kidneys compensated as expected or a second disorder is hiding.
- [**Liver function tests: reading the pattern**](https://durgical.com/en/biochemistry/topics/liver-function-tests-reading-the-pattern/) The liver panel mixes markers of cell injury, cholestasis and true function; the pattern points to the cause.
- [**Aminotransferases (ALT and AST) as lab tests**](https://durgical.com/en/biochemistry/topics/aminotransferases-as-lab-tests/) ALT and AST leak into the blood when liver cells break; ALT is liver-specific, AST also comes from muscle.
- [**Alkaline phosphatase (ALP) as a lab test**](https://durgical.com/en/biochemistry/topics/alkaline-phosphatase-as-a-lab-test/) Alkaline phosphatase rises when bile flow is blocked or bone turns over fast; GGT tells which source it is.
- [**Gamma-glutamyl transferase (GGT) as a lab test**](https://durgical.com/en/biochemistry/topics/gamma-glutamyl-transferase-as-a-lab-test/) GGT is very sensitive to cholestasis and alcohol but not specific; its main use is confirming that a high ALP is from the liver.
- [**Creatine kinase (CK) and CK-MB as lab tests**](https://durgical.com/en/biochemistry/topics/creatine-kinase-and-ck-mb-as-lab-tests/) CK leaks from damaged muscle; it is the key marker of rhabdomyolysis, and its MB fraction was used for heart attack before troponin.
- [**Cardiac troponin as a lab test**](https://durgical.com/en/biochemistry/topics/cardiac-troponin-as-a-lab-test/) Cardiac troponin marks heart muscle injury; calling it a heart attack needs a rise or fall and a setting of ischaemia.
- [**Amylase as a lab test**](https://durgical.com/en/biochemistry/topics/amylase-as-a-lab-test/) Amylase rises in acute pancreatitis but also from the salivary glands and other causes; lipase is now preferred.
- [**Lipase as a lab test**](https://durgical.com/en/biochemistry/topics/lipase-as-a-lab-test/) Pancreatic lipase is the preferred marker of acute pancreatitis: more specific than amylase and high for more days.
- [**Creatinine and estimated glomerular filtration rate (eGFR)**](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/) Creatinine comes from muscle and is filtered by the kidney; it is used to estimate how much the glomeruli filter.
- [**Urea and blood urea nitrogen (BUN) as lab tests**](https://durgical.com/en/biochemistry/topics/urea-and-blood-urea-nitrogen-as-lab-tests/) Urea is the waste form of protein nitrogen; it rises when the kidney filters less, but also with dehydration or gut bleeding.
- [**Serum albumin as a lab test**](https://durgical.com/en/biochemistry/topics/serum-albumin-as-a-lab-test/) Albumin is the most abundant plasma protein; it falls in liver, kidney and inflammatory disease and in malnutrition.
- [**C-reactive protein (CRP) as a lab test**](https://durgical.com/en/biochemistry/topics/c-reactive-protein-as-a-lab-test/) CRP is a liver protein that rises within hours with inflammation or infection; it does not say where the problem is.
- [**Tumour markers: principles of use**](https://durgical.com/en/biochemistry/topics/tumour-markers-principles-of-use/) Tumour markers help follow a cancer already diagnosed; almost none can diagnose or screen for it on its own.
- [**Alpha-fetoprotein (AFP)**](https://durgical.com/en/biochemistry/topics/alpha-fetoprotein/) AFP is the 'fetal albumin'; in adults it rises in liver cancer and germ cell tumours, and in pregnancy it flags neural tube defects.
- [**Carcinoembryonic antigen (CEA)**](https://durgical.com/en/biochemistry/topics/carcinoembryonic-antigen/) CEA is a fetal gut glycoprotein; it is used to follow colorectal cancer after treatment, not to screen for it.
- [**Prostate-specific antigen (PSA)**](https://durgical.com/en/biochemistry/topics/prostate-specific-antigen/) PSA is a prostate enzyme; it rises in cancer but also with benign growth, infection and handling of the gland.
- [**CA-125**](https://durgical.com/en/biochemistry/topics/ca-125/) CA-125 is a mucin of coelomic linings; it follows epithelial ovarian cancer but rises in many benign conditions.
- [**Glycated haemoglobin (HbA1c)**](https://durgical.com/en/biochemistry/topics/glycated-haemoglobin/) HbA1c reflects average glucose over the past 2–3 months, because glucose sticks to haemoglobin over a red cell's life.
- [**Newborn screening**](https://durgical.com/en/biochemistry/topics/newborn-screening/) A few drops of heel blood detect treatable diseases before they cause harm, such as congenital hypothyroidism and PKU.
- [**Urinalysis**](https://durgical.com/en/biochemistry/topics/urinalysis/) Urinalysis combines appearance, dipstick and microscopy; it points to infection, kidney disease, diabetes and liver problems.
- [**Carnitine shuttle**](https://durgical.com/en/biochemistry/topics/carnitine-shuttle/) The inner mitochondrial membrane does not let long-chain acyl-CoA through: carnitine carries it across in four steps.
- [**Odd-chain fatty acids and propionate metabolism**](https://durgical.com/en/biochemistry/topics/odd-chain-fatty-acids-and-propionate-metabolism/) Odd-numbered fatty acids end as propionyl-CoA, which biotin and vitamin B12 turn into succinyl-CoA.
- [**Peroxisomal oxidation (β- and α-oxidation)**](https://durgical.com/en/biochemistry/topics/peroxisomal-oxidation/) The peroxisome shortens very-long-chain fatty acids and breaks down phytanic acid, which mitochondria cannot process.
- [**Eicosanoids: prostaglandins, thromboxanes and leukotrienes**](https://durgical.com/en/biochemistry/topics/eicosanoids-prostaglandins-thromboxanes-and-leukotrienes/) Eicosanoids are local messengers made from arachidonic acid: they control inflammation, pain, fever, clotting and airways.
- [**Energy requirements and basal metabolic rate (BMR)**](https://durgical.com/en/biochemistry/topics/energy-requirements-and-basal-metabolic-rate/) The body spends energy to stay alive at rest (BMR), to handle food and to move. A person's energy requirement is the sum of these three parts.
- [**Specific dynamic action (thermic effect of food)**](https://durgical.com/en/biochemistry/topics/specific-dynamic-action/) After a meal the body burns extra energy to digest, absorb, process and store the nutrients. Protein costs the most.
- [**Macronutrients and the energy value of food**](https://durgical.com/en/biochemistry/topics/macronutrients-and-the-energy-value-of-food/) Carbohydrates, fats and proteins supply energy: about 4, 9 and 4 kcal per gram. Alcohol gives 7 kcal per gram with no nutrients.
- [**Dietary fibre**](https://durgical.com/en/biochemistry/topics/dietary-fibre/) Fibre is the part of plant food that human enzymes cannot digest. It adds bulk to stools, slows sugar absorption and helps lower cholesterol.
- [**Protein quality and essential amino acids**](https://durgical.com/en/biochemistry/topics/protein-quality-and-essential-amino-acids/) A protein is good quality when it supplies all the essential amino acids in the right amounts and is easy to digest. Mixing cereals and legumes fixes the gaps.
- [**Nitrogen balance**](https://durgical.com/en/biochemistry/topics/nitrogen-balance/) Nitrogen balance compares the nitrogen eaten (as protein) with the nitrogen lost, mostly as urea. It shows whether the body is building or losing protein.
- [**Balanced diet**](https://durgical.com/en/biochemistry/topics/balanced-diet/) A balanced diet gives enough energy for a healthy weight and every nutrient the body needs, from varied foods, with little sugar, salt and unhealthy fat.
- [**pH, buffers and the Henderson–Hasselbalch equation**](https://durgical.com/en/biochemistry/topics/ph-buffers-and-the-henderson-hasselbalch-equation/) A buffer is a weak acid with its salt that soaks up added acid or base, keeping pH steady. Blood stays near pH 7.4 thanks to several buffers.
- [**Chromatography**](https://durgical.com/en/biochemistry/topics/chromatography/) Chromatography separates the parts of a mixture as it flows over a material that holds each part with a different strength.
- [**Spectrophotometry and colorimetry (Beer–Lambert law)**](https://durgical.com/en/biochemistry/topics/spectrophotometry-and-colorimetry/) A coloured solution absorbs light in proportion to how much substance it contains. Measuring that absorbed light tells the concentration.
- [**Centrifugation and ultracentrifugation**](https://durgical.com/en/biochemistry/topics/centrifugation-and-ultracentrifugation/) Spinning a sample very fast pushes heavier or denser particles to the bottom first. It separates blood cells, organelles and lipoproteins.
- [**Radioisotopes and radioimmunoassay (RIA)**](https://durgical.com/en/biochemistry/topics/radioisotopes-and-radioimmunoassay/) Radioisotopes are unstable atoms that give off radiation and can be traced. Radioimmunoassay used them to measure tiny amounts of hormones.
- [**Glycoproteins and protein glycosylation (N- and O-linked)**](https://durgical.com/en/biochemistry/topics/glycoproteins-and-protein-glycosylation/) Many proteins get sugar chains attached in the ER and Golgi. The sugars help folding, sorting, stability, cell recognition and blood groups.
- [**Protein targeting and signal sequences**](https://durgical.com/en/biochemistry/topics/protein-targeting-and-signal-sequences/) Each new protein carries an 'address label' in its sequence that tells the cell where to send it: ER, nucleus, mitochondrion, peroxisome or lysosome.
- [**Molecular chaperones and heat shock proteins**](https://durgical.com/en/biochemistry/topics/molecular-chaperones-and-heat-shock-proteins/) Chaperones are helper proteins that stop new or damaged proteins from clumping and help them fold. Heat and stress switch many of them on.
- [**Ubiquitin–proteasome system**](https://durgical.com/en/biochemistry/topics/ubiquitin-proteasome-system/) Cells mark unwanted proteins with chains of a small protein, ubiquitin. The proteasome, a barrel-shaped machine, recognises the mark and chops them up.
- [**Creatine, phosphocreatine and creatinine**](https://durgical.com/en/biochemistry/topics/creatine-phosphocreatine-and-creatinine/) Creatine is made from three amino acids and stores quick energy in muscle as phosphocreatine. Its waste product, creatinine, shows how well the kidneys filter.
- [**Polyamines (putrescine, spermidine, spermine)**](https://durgical.com/en/biochemistry/topics/polyamines/) Polyamines are small, strongly positive molecules made from ornithine. Growing cells need them, so blocking their synthesis slows parasites and some tumours.
- [**Purine nucleotide cycle (muscle)**](https://durgical.com/en/biochemistry/topics/purine-nucleotide-cycle/) In working muscle, AMP is deaminated to IMP and then rebuilt. The loop keeps ATP high, feeds the Krebs cycle and releases ammonia.
- [**Fredrickson classification of hyperlipoproteinaemias**](https://durgical.com/en/biochemistry/topics/fredrickson-classification-of-hyperlipoproteinaemias/) The Fredrickson system sorts high blood fats into five types by which lipoprotein particle is raised. Each type points to typical causes and risks.
- [**Uronic acid pathway**](https://durgical.com/en/biochemistry/topics/uronic-acid-pathway/) This side route of glucose makes UDP-glucuronic acid for detoxification and for building connective tissue. It explains why humans cannot make vitamin C.
- [**Lactate dehydrogenase (LDH) as a lab marker**](https://durgical.com/en/biochemistry/topics/lactate-dehydrogenase-as-a-lab-marker/) LDH is in almost every cell, so a high blood LDH means tissue damage somewhere. It is sensitive but not specific, and haemolysis in the tube raises it falsely.
- [**Iron metabolism**](https://durgical.com/en/biochemistry/topics/iron-metabolism/) The body has no way to excrete iron: it controls how much it absorbs with hepcidin, carries it on transferrin and stores it in ferritin.
- [**Hemoglobin variants (HbA, HbA2, HbF, HbS, HbC)**](https://durgical.com/en/biochemistry/topics/hemoglobin-variants/) Hemoglobin changes chains from fetus to adult, and a single amino-acid change is enough to cause diseases such as sickle cell anemia.
- [**Fed, fasting and starvation states**](https://durgical.com/en/biochemistry/topics/fed-fasting-and-starvation-states/) The body switches fuel with the time since the last meal: it stores in the fed state and draws on reserves while fasting.
- [**Insulin signalling**](https://durgical.com/en/biochemistry/topics/insulin-signalling/) Insulin is the fed-state hormone: it makes muscle and fat take up glucose and makes the liver store instead of release.
- [**Glucagon signalling**](https://durgical.com/en/biochemistry/topics/glucagon-signalling/) Glucagon is the fasting hormone: it tells the liver to release glucose and make ketone bodies.
- [**G protein-coupled receptors and cAMP**](https://durgical.com/en/biochemistry/topics/g-protein-coupled-receptors-and-camp/) The largest receptor family: the hormone binds outside, a G protein switches on inside and a second messenger amplifies the signal.
- [**IP₃, DAG and calcium pathway**](https://durgical.com/en/biochemistry/topics/ip-dag-and-calcium-pathway/) Some hormones activate phospholipase C: IP₃ releases calcium from the reticulum and DAG activates protein kinase C.
- [**Receptor tyrosine kinases**](https://durgical.com/en/biochemistry/topics/receptor-tyrosine-kinases/) Receptors that are enzymes: when they bind the hormone they phosphorylate tyrosines and switch on growth and metabolic pathways.
- [**JAK–STAT pathway**](https://durgical.com/en/biochemistry/topics/jak-stat-pathway/) Receptors for cytokines and some hormones (GH, prolactin, leptin, erythropoietin) use attached JAK kinases and STAT factors.
- [**Nuclear receptors**](https://durgical.com/en/biochemistry/topics/nuclear-receptors/) Fat-soluble hormones (steroids, thyroid hormone, vitamin D, retinoic acid) enter the cell and activate receptors that switch genes on or off.
- [**Hormone classes and mechanisms**](https://durgical.com/en/biochemistry/topics/hormone-classes-and-mechanisms/) Its chemistry decides whether a hormone travels free or protein-bound and whether it acts on membrane receptors or inside the cell.
- [**Adrenal enzymes and congenital adrenal hyperplasia**](https://durgical.com/en/biochemistry/topics/adrenal-enzymes-and-congenital-adrenal-hyperplasia/) Which adrenal cortex enzyme makes each hormone, and what happens when one is missing: what comes before piles up and spills into another branch.
- [**Catecholamines: synthesis and breakdown**](https://durgical.com/en/biochemistry/topics/catecholamines-synthesis-and-breakdown/) Tyrosine becomes dopamine, noradrenaline and adrenaline in four steps; the adrenal medulla releases mostly adrenaline.
- [**Exercise metabolism**](https://durgical.com/en/biochemistry/topics/exercise-metabolism/) Muscle switches fuel with intensity and duration: phosphocreatine, glycogen with lactate, and then oxidised glucose and fat.
- [**Leptin, appetite and obesity**](https://durgical.com/en/biochemistry/topics/leptin-appetite-and-obesity/) Leptin comes from fat tissue and tells the hypothalamus how much fat there is; in obesity it is plentiful but the brain responds poorly.
- [**DNA structure and chromatin**](https://durgical.com/en/biochemistry/topics/dna-structure-and-chromatin/) DNA is a double helix of two antiparallel strands held by base pairs; it wraps around histones to fit inside the nucleus.
- [**Central dogma of molecular biology**](https://durgical.com/en/biochemistry/topics/central-dogma-of-molecular-biology/) Information flows from DNA to RNA and from RNA to protein; replication copies DNA and reverse transcription goes back from RNA to DNA.
- [**DNA replication: the replication fork**](https://durgical.com/en/biochemistry/topics/dna-replication-the-replication-fork/) To copy DNA the double helix opens into a fork: one strand is copied continuously and the other in pieces (Okazaki fragments).
- [**DNA repair**](https://durgical.com/en/biochemistry/topics/dna-repair/) Cells fix thousands of DNA lesions every day with several repair systems; when one fails, cancer and inherited diseases appear.
- [**Transcription**](https://durgical.com/en/biochemistry/topics/transcription/) Transcription copies a DNA gene into an RNA molecule; in eukaryotes three RNA polymerases do it with the help of transcription factors.
- [**RNA processing (cap, poly-A tail and splicing)**](https://durgical.com/en/biochemistry/topics/rna-processing/) Newly made RNA matures in the nucleus: it gets a cap and a poly-A tail and loses its introns by splicing before it leaves for the cytoplasm.
- [**Translation and the genetic code**](https://durgical.com/en/biochemistry/topics/translation-and-the-genetic-code/) In translation the ribosome reads the mRNA three bases at a time (codons) and joins the amino acids brought by tRNAs into a protein.
- [**Transfer RNA (tRNA)**](https://durgical.com/en/biochemistry/topics/transfer-rna/) tRNA is the adaptor of translation: it carries an amino acid at one end and, at the other, an anticodon that reads the mRNA codon.
- [**The lac operon and gene regulation in bacteria**](https://durgical.com/en/biochemistry/topics/the-lac-operon-and-gene-regulation-in-bacteria/) The lac operon is the classic model of how bacteria switch genes on only when needed: they use them to eat lactose when glucose runs out.
- [**Epigenetics: DNA and histone methylation**](https://durgical.com/en/biochemistry/topics/epigenetics-dna-and-histone-methylation/) Epigenetics means chemical marks on DNA and histones that switch genes on or off without changing the sequence, and that pass on when cells divide.
- [**Types of mutations**](https://durgical.com/en/biochemistry/topics/types-of-mutations/) A mutation is a change in the DNA sequence; depending on where and how it happens it may do nothing, change one amino acid or ruin the whole protein.
- [**Trinucleotide repeat expansion disorders**](https://durgical.com/en/biochemistry/topics/trinucleotide-repeat-expansion-disorders/) Some diseases come from a three-base unit repeated too many times; the repeat tends to grow from one generation to the next.
- [**Recombinant DNA and cloning**](https://durgical.com/en/biochemistry/topics/recombinant-dna-and-cloning/) Recombinant DNA technology cuts and joins DNA from different sources so that a host cell makes a protein or copies a gene.
- [**Gene therapy**](https://durgical.com/en/biochemistry/topics/gene-therapy/) Gene therapy treats a disease by adding, silencing or correcting a gene inside the patient's cells.
- [**Polymerase chain reaction (PCR)**](https://durgical.com/en/biochemistry/topics/polymerase-chain-reaction/) PCR copies one chosen piece of DNA millions of times in a few hours; it underpins molecular diagnosis of infections and genetic diseases.
- [**ELISA (enzyme-linked immunosorbent assay)**](https://durgical.com/en/biochemistry/topics/elisa/) ELISA detects and measures antibodies or antigens in a sample using an enzyme that makes colour when the antibody binds its target.
- [**Southern, Northern and Western blots**](https://durgical.com/en/biochemistry/topics/southern-northern-and-western-blots/) Blots separate molecules by size in a gel, move them to a membrane and detect them with a probe: DNA (Southern), RNA (Northern) or protein (Western).
- [**FISH (fluorescence in situ hybridization)**](https://durgical.com/en/biochemistry/topics/fish/) FISH uses fluorescent DNA probes that stick to one exact place on a chromosome, to see under the microscope whether a piece is missing, extra or moved.
- [**DNA microarrays**](https://durgical.com/en/biochemistry/topics/dna-microarrays/) A microarray is a chip with thousands of DNA probes that measures the expression of many genes at once or looks for gains and losses of genetic material.
- [**CRISPR-Cas9 and genome editing**](https://durgical.com/en/biochemistry/topics/crispr-cas9-and-genome-editing/) CRISPR-Cas9 is RNA-guided 'molecular scissors' that cut DNA at an exact spot to switch off, correct or insert a gene.
- [**DNA sequencing**](https://durgical.com/en/biochemistry/topics/dna-sequencing/) Sequencing means reading the exact order of DNA bases; today you can read one gene, all exons (exome) or a person's whole genome.
- [**Nucleotide structure**](https://durgical.com/en/biochemistry/topics/nucleotide-structure/) A nucleotide is a nitrogenous base joined to a sugar and one or more phosphates; they build DNA and RNA and serve as the cell's energy currency.
- [**Drugs acting on nucleotide metabolism**](https://durgical.com/en/biochemistry/topics/drugs-acting-on-nucleotide-metabolism/) Many drugs against cancer, gout and transplant rejection block one specific enzyme of nucleotide synthesis or breakdown.
- [**Chemiosmosis and the proton gradient**](https://durgical.com/en/biochemistry/topics/chemiosmosis-and-the-proton-gradient/) Chemiosmosis explains how the respiratory chain and ATP synthase are linked: through a proton gradient across the inner membrane.
- [**Uncouplers of oxidative phosphorylation**](https://durgical.com/en/biochemistry/topics/uncouplers-of-oxidative-phosphorylation/) Uncouplers let protons cross the inner membrane without ATP synthase: more fuel is burned, temperature rises and ATP falls.
- [**Mitochondrial NADH shuttles**](https://durgical.com/en/biochemistry/topics/mitochondrial-nadh-shuttles/) Glycolytic NADH cannot cross the inner membrane; two shuttles carry its electrons into the mitochondrion, with different ATP yields.
- [**Reactive oxygen species and antioxidants**](https://durgical.com/en/biochemistry/topics/reactive-oxygen-species-and-antioxidants/) Reactive oxygen species are forms of oxygen with extra electrons that damage lipids, proteins and DNA; antioxidant enzymes keep them in check.
- [**Amino acids: structure, charge and pKa**](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/) The 20 amino acids share one backbone and differ in their side chain; their charge changes with pH, which decides how a protein folds and migrates.
- [**Protein structure and folding**](https://durgical.com/en/biochemistry/topics/protein-structure-and-folding/) A protein is a chain of amino acids that folds into a precise shape; if it misfolds, it stops working or piles up and damages tissues.
- [**Enzyme kinetics (Michaelis–Menten and Lineweaver–Burk)**](https://durgical.com/en/biochemistry/topics/enzyme-kinetics/) An enzyme's rate rises with substrate until it reaches a maximum; two numbers, Km and Vmax, sum up how it works and how drugs change it.
- [**Types of enzyme inhibition**](https://durgical.com/en/biochemistry/topics/types-of-enzyme-inhibition/) Many drugs and poisons work by slowing an enzyme; depending on where they bind, they change Km, Vmax or both, which shows on the Lineweaver–Burk plot.
- [**Allosteric and covalent regulation of enzymes**](https://durgical.com/en/biochemistry/topics/allosteric-and-covalent-regulation-of-enzymes/) The cell switches its enzymes on and off within seconds using molecules that bind outside the active site or chemical changes such as phosphorylation.
- [**Isoenzymes**](https://durgical.com/en/biochemistry/topics/isoenzymes/) Isoenzymes carry out the same reaction but are different proteins; each tissue has its own set, so in blood they reveal which organ is damaged.
- [**Oxygen binding: haemoglobin versus myoglobin**](https://durgical.com/en/biochemistry/topics/oxygen-binding-haemoglobin-versus-myoglobin/) Myoglobin stores oxygen and releases it late; haemoglobin picks it up in the lung and delivers it to tissues thanks to its S-shaped curve.
- [**Collagen synthesis**](https://durgical.com/en/biochemistry/topics/collagen-synthesis/) Collagen is built in steps inside and outside the cell; each step needs an enzyme or a vitamin, and each failure causes a different disease.
- [**Plasma proteins and electrophoresis**](https://durgical.com/en/biochemistry/topics/plasma-proteins-and-electrophoresis/) Electrophoresis splits serum proteins into five bands; their shape helps recognise inflammation, cirrhosis, nephrotic syndrome or myeloma.
- [**Cytochrome P450 system**](https://durgical.com/en/biochemistry/topics/cytochrome-p450-system/) A family of haem enzymes that oxidise drugs, toxins, hormones and vitamins; their induction and inhibition explain many drug interactions.
- [**Xenobiotic biotransformation: phases I, II and III**](https://durgical.com/en/biochemistry/topics/xenobiotic-biotransformation-phases-i-ii-and-iii/) The body turns fat-soluble drugs and toxins into water-soluble molecules it can pass in urine or bile, in three phases.
- [**Glucuronidation**](https://durgical.com/en/biochemistry/topics/glucuronidation/) The most important phase II conjugation: UGT enzymes attach glucuronic acid to drugs, bilirubin and hormones so they can be excreted.
- [**Acetaminophen metabolism and NAPQI**](https://durgical.com/en/biochemistry/topics/acetaminophen-metabolism-and-napqi/) At therapeutic doses acetaminophen is conjugated and excreted; in overdose those routes saturate and NAPQI destroys the liver.
- [**Oncogenes and tumour suppressor genes**](https://durgical.com/en/biochemistry/topics/oncogenes-and-tumour-suppressor-genes/) Cancer arises from mutations that speed up division (oncogenes) or remove its brakes (tumour suppressors); this is the basic outline.
- [**Warburg effect (aerobic glycolysis in cancer)**](https://durgical.com/en/biochemistry/topics/warburg-effect/) Many tumour cells burn glucose and make lactate even with oxygen present; this gives them building blocks to grow fast.
- [**Cobalamin (vitamin B12)**](https://durgical.com/en/biochemistry/topics/cobalamin/) Vitamin B12 contains cobalt and only two enzymes use it, but its lack causes megaloblastic anaemia and spinal cord damage.
- [**Iron**](https://durgical.com/en/biochemistry/topics/iron/) Iron carries oxygen in haemoglobin and moves electrons in mitochondria; the body has no way to excrete it, so it controls absorption.
- [**Zinc**](https://durgical.com/en/biochemistry/topics/zinc/) Zinc is part of hundreds of enzymes and of the proteins that read DNA; it is key for growth, wound healing and fighting infection.
- [**Copper**](https://durgical.com/en/biochemistry/topics/copper/) Copper powers enzymes that use oxygen, build collagen and melanin and move iron; two genetic diseases show its lack and its excess.
- [**Iodine**](https://durgical.com/en/biochemistry/topics/iodine/) Iodine is the raw material of thyroid hormones; its lack causes goitre, hypothyroidism and, in pregnancy, brain damage in the baby.
- [**Calcium and phosphate**](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/) Calcium and phosphate form bone mineral and control nerves, muscles and energy; three hormones keep their blood levels steady.
- [**Magnesium**](https://durgical.com/en/biochemistry/topics/magnesium/) Magnesium partners ATP in almost every energy-using reaction and steadies nerves and muscles, including the heart.
- [**Selenium**](https://durgical.com/en/biochemistry/topics/selenium/) Selenium is part of antioxidant enzymes and of the enzymes that activate thyroid hormone; both lack and excess are harmful.
- [**Fluoride**](https://durgical.com/en/biochemistry/topics/fluoride/) Fluoride hardens tooth enamel and protects it from decay; too much in childhood stains the teeth and harms bone.

## Metabolic diseases

- [Ornithine transcarbamylase (OTC) deficiency](https://durgical.com/en/conditions/ornithine-transcarbamylase-deficiency/)
- [Hyperammonaemia](https://durgical.com/en/conditions/hyperammonaemia/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Portal hypertension](https://durgical.com/en/conditions/portal-hypertension/)
- [Phenylketonuria (PKU)](https://durgical.com/en/conditions/phenylketonuria/)
- [Tyrosinaemia type I (hepatorenal)](https://durgical.com/en/conditions/tyrosinaemia-type-i/)
- [Alkaptonuria](https://durgical.com/en/conditions/alkaptonuria/)
- [Oculocutaneous albinism](https://durgical.com/en/conditions/oculocutaneous-albinism/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Helicobacter pylori gastritis](https://durgical.com/en/conditions/helicobacter-pylori-gastritis/)
- [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/)
- [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/)
- [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/)
- [Pyruvate dehydrogenase deficiency](https://durgical.com/en/conditions/pyruvate-dehydrogenase-deficiency/)
- [Phaeochromocytoma](https://durgical.com/en/conditions/phaeochromocytoma/)
- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Carcinoid syndrome](https://durgical.com/en/conditions/carcinoid-syndrome/)
- [Hartnup disease](https://durgical.com/en/conditions/hartnup-disease/)
- [Homocystinuria](https://durgical.com/en/conditions/homocystinuria/)
- [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/)
- [Methylmalonic acidaemia](https://durgical.com/en/conditions/methylmalonic-acidaemia/)
- [Propionic acidaemia](https://durgical.com/en/conditions/propionic-acidaemia/)
- [Viral hepatitis](https://durgical.com/en/conditions/viral-hepatitis/)
- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)
- [Maple syrup urine disease](https://durgical.com/en/conditions/maple-syrup-urine-disease/)
- [Parkinson disease](https://durgical.com/en/conditions/parkinson-disease/)
- [Spina bifida (myelomeningocele)](https://durgical.com/en/conditions/spina-bifida/)
- [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/)
- [Pyruvate kinase deficiency](https://durgical.com/en/conditions/pyruvate-kinase-deficiency/)
- [Tarui disease (muscle phosphofructokinase deficiency)](https://durgical.com/en/conditions/tarui-disease/)
- [Arsenic poisoning](https://durgical.com/en/conditions/arsenic-poisoning/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Wernicke encephalopathy (thiamine deficiency)](https://durgical.com/en/conditions/wernicke-encephalopathy/)
- [Fumarase deficiency](https://durgical.com/en/conditions/fumarase-deficiency/)
- [Hereditary paraganglioma and pheochromocytoma (SDH)](https://durgical.com/en/conditions/hereditary-paraganglioma-and-pheochromocytoma/)
- [Fluoroacetate poisoning](https://durgical.com/en/conditions/fluoroacetate-poisoning/)
- [Glioblastoma](https://durgical.com/en/conditions/glioblastoma/)
- [Von Gierke disease (glycogen storage disease type I)](https://durgical.com/en/conditions/von-gierke-disease/)
- [Hereditary fructose intolerance](https://durgical.com/en/conditions/hereditary-fructose-intolerance/)
- [Pompe disease (glycogen storage disease type II)](https://durgical.com/en/conditions/pompe-disease/)
- [Cori disease (glycogen storage disease type III)](https://durgical.com/en/conditions/cori-disease/)
- [Andersen disease (glycogen storage disease type IV)](https://durgical.com/en/conditions/andersen-disease/)
- [McArdle disease (glycogen storage disease type V)](https://durgical.com/en/conditions/mcardle-disease/)
- [Hers disease (glycogen storage disease type VI)](https://durgical.com/en/conditions/hers-disease/)
- [Glucose-6-phosphate dehydrogenase (G6PD) deficiency](https://durgical.com/en/conditions/glucose-6-phosphate-dehydrogenase-deficiency/)
- [Essential fructosuria](https://durgical.com/en/conditions/essential-fructosuria/)
- [Diabetic retinopathy](https://durgical.com/en/conditions/diabetic-retinopathy/)
- [Cataract](https://durgical.com/en/conditions/cataract/)
- [Classic galactosaemia](https://durgical.com/en/conditions/classic-galactosaemia/)
- [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/)
- [Cushing syndrome](https://durgical.com/en/conditions/cushing-syndrome/)
- [Gout](https://durgical.com/en/conditions/gout/)
- [Malaria](https://durgical.com/en/conditions/malaria/)
- [Obesity](https://durgical.com/en/conditions/obesity/)
- [Atherosclerosis and dyslipidaemia](https://durgical.com/en/conditions/atherosclerosis-and-dyslipidaemia/)
- [X-linked adrenoleukodystrophy](https://durgical.com/en/conditions/x-linked-adrenoleukodystrophy/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Gaucher disease](https://durgical.com/en/conditions/gaucher-disease/)
- [Tay–Sachs disease](https://durgical.com/en/conditions/tay-sachs-disease/)
- [Niemann–Pick disease](https://durgical.com/en/conditions/niemann-pick-disease/)
- [Fabry disease](https://durgical.com/en/conditions/fabry-disease/)
- [Krabbe disease (globoid cell leukodystrophy)](https://durgical.com/en/conditions/krabbe-disease/)
- [Metachromatic leukodystrophy](https://durgical.com/en/conditions/metachromatic-leukodystrophy/)
- [Cystinuria](https://durgical.com/en/conditions/cystinuria/)
- [Primary carnitine deficiency](https://durgical.com/en/conditions/primary-carnitine-deficiency/)
- [Zellweger syndrome (Zellweger spectrum)](https://durgical.com/en/conditions/zellweger-syndrome/)
- [MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes)](https://durgical.com/en/conditions/melas/)
- [I-cell disease (mucolipidosis II)](https://durgical.com/en/conditions/i-cell-disease/)
- [Hurler syndrome (mucopolysaccharidosis type I)](https://durgical.com/en/conditions/hurler-syndrome/)
- [Hunter syndrome (mucopolysaccharidosis type II)](https://durgical.com/en/conditions/hunter-syndrome/)
- [Sandhoff disease](https://durgical.com/en/conditions/sandhoff-disease/)
- [Sanfilippo syndrome (mucopolysaccharidosis type III)](https://durgical.com/en/conditions/sanfilippo-syndrome/)
- [Morquio syndrome (mucopolysaccharidosis type IV)](https://durgical.com/en/conditions/morquio-syndrome/)
- [Knee Osteoarthritis](https://durgical.com/en/conditions/knee-osteoarthritis/)
- [GM2 activator deficiency (GM2 gangliosidosis, AB variant)](https://durgical.com/en/conditions/gm2-activator-deficiency/)
- [Smith–Lemli–Opitz syndrome](https://durgical.com/en/conditions/smith-lemli-opitz-syndrome/)
- [Familial hypercholesterolemia](https://durgical.com/en/conditions/familial-hypercholesterolemia/)
- [Familial chylomicronemia syndrome (lipoprotein lipase deficiency)](https://durgical.com/en/conditions/familial-chylomicronemia-syndrome/)
- [Acute pancreatitis](https://durgical.com/en/conditions/acute-pancreatitis/)
- [Tangier disease](https://durgical.com/en/conditions/tangier-disease/)
- [Metabolic syndrome](https://durgical.com/en/conditions/metabolic-syndrome/)
- [Gallstones (cholelithiasis)](https://durgical.com/en/conditions/gallstones/)
- [Abetalipoproteinemia (Bassen–Kornzweig syndrome)](https://durgical.com/en/conditions/abetalipoproteinemia/)
- [Familial dysbetalipoproteinemia (type III hyperlipoproteinemia)](https://durgical.com/en/conditions/familial-dysbetalipoproteinemia/)
- [Alzheimer disease](https://durgical.com/en/conditions/alzheimer-disease/)
- [Crohn disease](https://durgical.com/en/conditions/crohn-disease/)
- [Adrenal insufficiency](https://durgical.com/en/conditions/adrenal-insufficiency/)
- [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/)
- [High anion gap metabolic acidosis](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/)
- [Normal anion gap (hyperchloraemic) metabolic acidosis](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/)
- [Metabolic alkalosis](https://durgical.com/en/conditions/metabolic-alkalosis/)
- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)
- [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Hypernatraemia](https://durgical.com/en/conditions/hypernatraemia/)
- [Choledocholithiasis](https://durgical.com/en/conditions/choledocholithiasis/)
- [Gilbert syndrome](https://durgical.com/en/conditions/gilbert-syndrome/)
- [Hepatocellular carcinoma](https://durgical.com/en/conditions/hepatocellular-carcinoma/)
- [Wilson disease](https://durgical.com/en/conditions/wilson-disease/)
- [Hereditary hemochromatosis](https://durgical.com/en/conditions/hereditary-hemochromatosis/)
- [Rickets and osteomalacia](https://durgical.com/en/conditions/rickets-and-osteomalacia/)
- [Primary hyperparathyroidism](https://durgical.com/en/conditions/primary-hyperparathyroidism/)
- [Pancreatic cancer](https://durgical.com/en/conditions/pancreatic-cancer/)
- [Acute cholangitis](https://durgical.com/en/conditions/acute-cholangitis/)
- [Acute compartment syndrome](https://durgical.com/en/conditions/acute-compartment-syndrome/)
- [Carnitine palmitoyltransferase II (CPT-II) deficiency](https://durgical.com/en/conditions/carnitine-palmitoyltransferase-ii-deficiency/)
- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Chronic pancreatitis](https://durgical.com/en/conditions/chronic-pancreatitis/)
- [Mumps](https://durgical.com/en/conditions/mumps/)
- [Small bowel obstruction](https://durgical.com/en/conditions/small-bowel-obstruction/)
- [Ectopic pregnancy](https://durgical.com/en/conditions/ectopic-pregnancy/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)
- [Autosomal dominant polycystic kidney disease](https://durgical.com/en/conditions/autosomal-dominant-polycystic-kidney-disease/)
- [Upper gastrointestinal bleeding](https://durgical.com/en/conditions/upper-gastrointestinal-bleeding/)
- [Hypocalcaemia](https://durgical.com/en/conditions/hypocalcaemia/)
- [Refeeding syndrome](https://durgical.com/en/conditions/refeeding-syndrome/)
- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)
- [Rheumatoid arthritis](https://durgical.com/en/conditions/rheumatoid-arthritis/)
- [Acute appendicitis](https://durgical.com/en/conditions/acute-appendicitis/)
- [Colorectal cancer](https://durgical.com/en/conditions/colorectal-cancer/)
- [Prostate cancer](https://durgical.com/en/conditions/prostate-cancer/)
- [Ovarian cancer](https://durgical.com/en/conditions/ovarian-cancer/)
- [Testicular cancer](https://durgical.com/en/conditions/testicular-cancer/)
- [Lung cancer](https://durgical.com/en/conditions/lung-cancer/)
- [Ulcerative colitis](https://durgical.com/en/conditions/ulcerative-colitis/)
- [Benign prostatic hyperplasia](https://durgical.com/en/conditions/benign-prostatic-hyperplasia/)
- [Prostatitis](https://durgical.com/en/conditions/prostatitis/)
- [Urinary tract infection (acute cystitis)](https://durgical.com/en/conditions/urinary-tract-infection/)
- [Endometriosis](https://durgical.com/en/conditions/endometriosis/)
- [Pelvic inflammatory disease](https://durgical.com/en/conditions/pelvic-inflammatory-disease/)
- [Uterine fibroids (leiomyomas)](https://durgical.com/en/conditions/uterine-fibroids/)
- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency](https://durgical.com/en/conditions/medium-chain-acyl-coa-dehydrogenase-deficiency/)
- [Kidney stones (urolithiasis)](https://durgical.com/en/conditions/kidney-stones/)
- [Acute pyelonephritis](https://durgical.com/en/conditions/acute-pyelonephritis/)
- [Jamaican vomiting sickness (ackee poisoning)](https://durgical.com/en/conditions/jamaican-vomiting-sickness/)
- [Refsum disease (adult)](https://durgical.com/en/conditions/refsum-disease/)
- [Asthma](https://durgical.com/en/conditions/asthma/)
- [Peptic ulcer disease](https://durgical.com/en/conditions/peptic-ulcer-disease/)
- [Patent ductus arteriosus](https://durgical.com/en/conditions/patent-ductus-arteriosus/)
- [Dysmenorrhoea](https://durgical.com/en/conditions/dysmenorrhoea/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Kwashiorkor](https://durgical.com/en/conditions/kwashiorkor/)
- [Marasmus](https://durgical.com/en/conditions/marasmus/)
- [Diverticular disease and diverticulitis](https://durgical.com/en/conditions/diverticular-disease-and-diverticulitis/)
- [Irritable bowel syndrome](https://durgical.com/en/conditions/irritable-bowel-syndrome/)
- [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)
- [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/)
- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)
- [Thyroid nodule](https://durgical.com/en/conditions/thyroid-nodule/)
- [Thyroid cancer (papillary)](https://durgical.com/en/conditions/thyroid-cancer/)
- [Congenital disorder of glycosylation type Ia (PMM2-CDG)](https://durgical.com/en/conditions/congenital-disorder-of-glycosylation-type-ia/)
- [α1-antitrypsin deficiency](https://durgical.com/en/conditions/1-antitrypsin-deficiency/)
- [Primary hyperoxaluria](https://durgical.com/en/conditions/primary-hyperoxaluria/)
- [Amyloidosis](https://durgical.com/en/conditions/amyloidosis/)
- [Prion diseases (Creutzfeldt–Jakob)](https://durgical.com/en/conditions/prion-diseases/)
- [Multiple myeloma](https://durgical.com/en/conditions/multiple-myeloma/)
- [Cervical cancer](https://durgical.com/en/conditions/cervical-cancer/)
- [Human papillomavirus (HPV) infection](https://durgical.com/en/conditions/human-papillomavirus-infection/)
- [Guanidinoacetate methyltransferase (GAMT) deficiency](https://durgical.com/en/conditions/guanidinoacetate-methyltransferase-deficiency/)
- [Duchenne muscular dystrophy](https://durgical.com/en/conditions/duchenne-muscular-dystrophy/)
- [Stable angina](https://durgical.com/en/conditions/stable-angina/)
- [Erectile dysfunction](https://durgical.com/en/conditions/erectile-dysfunction/)
- [Preeclampsia](https://durgical.com/en/conditions/preeclampsia/)
- [Myoadenylate deaminase deficiency (AMP deaminase deficiency)](https://durgical.com/en/conditions/myoadenylate-deaminase-deficiency/)
- [Familial combined hyperlipidaemia](https://durgical.com/en/conditions/familial-combined-hyperlipidaemia/)
- [Familial hypertriglyceridaemia](https://durgical.com/en/conditions/familial-hypertriglyceridaemia/)
- [Scurvy (vitamin C deficiency)](https://durgical.com/en/conditions/scurvy/)
- [Crigler-Najjar syndrome](https://durgical.com/en/conditions/crigler-najjar-syndrome/)
- [Rhabdomyolysis](https://durgical.com/en/conditions/rhabdomyolysis/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Infectious mononucleosis](https://durgical.com/en/conditions/infectious-mononucleosis/)
- [Pneumocystis pneumonia (PCP)](https://durgical.com/en/conditions/pneumocystis-pneumonia/)
- [Tumour lysis syndrome](https://durgical.com/en/conditions/tumour-lysis-syndrome/)
- [Acute intermittent porphyria](https://durgical.com/en/conditions/acute-intermittent-porphyria/)
- [Porphyria cutanea tarda](https://durgical.com/en/conditions/porphyria-cutanea-tarda/)
- [Erythropoietic protoporphyria](https://durgical.com/en/conditions/erythropoietic-protoporphyria/)
- [Lead poisoning](https://durgical.com/en/conditions/lead-poisoning/)
- [Dubin-Johnson syndrome](https://durgical.com/en/conditions/dubin-johnson-syndrome/)
- [Rotor syndrome](https://durgical.com/en/conditions/rotor-syndrome/)
- [Neonatal jaundice](https://durgical.com/en/conditions/neonatal-jaundice/)
- [Goitre](https://durgical.com/en/conditions/goitre/)
- [Pituitary adenoma](https://durgical.com/en/conditions/pituitary-adenoma/)
- [Breast cancer](https://durgical.com/en/conditions/breast-cancer/)
- [Adenosine deaminase deficiency (ADA-SCID)](https://durgical.com/en/conditions/adenosine-deaminase-deficiency/)
- [Congenital adrenal hyperplasia due to 21-hydroxylase deficiency](https://durgical.com/en/conditions/congenital-adrenal-hyperplasia-due-to-21-hydroxylase-deficiency/)
- [Congenital adrenal hyperplasia due to 11β-hydroxylase deficiency](https://durgical.com/en/conditions/congenital-adrenal-hyperplasia-due-to-11-hydroxylase-deficiency/)
- [Congenital adrenal hyperplasia due to 17α-hydroxylase deficiency](https://durgical.com/en/conditions/congenital-adrenal-hyperplasia-due-to-17-hydroxylase-deficiency/)
- [Anaphylaxis](https://durgical.com/en/conditions/anaphylaxis/)
- [Obstructive sleep apnoea](https://durgical.com/en/conditions/obstructive-sleep-apnoea/)
- [Gynaecomastia](https://durgical.com/en/conditions/gynaecomastia/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Bloom syndrome](https://durgical.com/en/conditions/bloom-syndrome/)
- [Xeroderma pigmentosum](https://durgical.com/en/conditions/xeroderma-pigmentosum/)
- [Lynch syndrome (hereditary non-polyposis colorectal cancer)](https://durgical.com/en/conditions/lynch-syndrome/)
- [Hereditary breast and ovarian cancer syndrome (BRCA1/BRCA2)](https://durgical.com/en/conditions/hereditary-breast-and-ovarian-cancer-syndrome/)
- [Ataxia-telangiectasia](https://durgical.com/en/conditions/ataxia-telangiectasia/)
- [Fanconi anaemia](https://durgical.com/en/conditions/fanconi-anaemia/)
- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)
- [Systemic lupus erythematosus](https://durgical.com/en/conditions/systemic-lupus-erythematosus/)
- [Fragile X syndrome](https://durgical.com/en/conditions/fragile-x-syndrome/)
- [Huntington disease](https://durgical.com/en/conditions/huntington-disease/)
- [Dengue](https://durgical.com/en/conditions/dengue/)
- [Interstitial lung disease](https://durgical.com/en/conditions/interstitial-lung-disease/)
- [Lesch-Nyhan syndrome](https://durgical.com/en/conditions/lesch-nyhan-syndrome/)
- [Hereditary xanthinuria](https://durgical.com/en/conditions/hereditary-xanthinuria/)
- [Hereditary orotic aciduria](https://durgical.com/en/conditions/hereditary-orotic-aciduria/)
- [Pleural effusion](https://durgical.com/en/conditions/pleural-effusion/)
- [Multiple sclerosis](https://durgical.com/en/conditions/multiple-sclerosis/)
- [Cyanide poisoning](https://durgical.com/en/conditions/cyanide-poisoning/)
- [Leber hereditary optic neuropathy (LHON)](https://durgical.com/en/conditions/leber-hereditary-optic-neuropathy/)
- [Leigh syndrome](https://durgical.com/en/conditions/leigh-syndrome/)
- [Menkes disease](https://durgical.com/en/conditions/menkes-disease/)
- [Pernicious anaemia](https://durgical.com/en/conditions/pernicious-anaemia/)
- [Pellagra (niacin deficiency)](https://durgical.com/en/conditions/pellagra/)
- [GM1 gangliosidosis](https://durgical.com/en/conditions/gm1-gangliosidosis/)
- [Farber disease (Farber lipogranulomatosis)](https://durgical.com/en/conditions/farber-disease/)
- [Migraine](https://durgical.com/en/conditions/migraine/)
- [Osteogenesis imperfecta](https://durgical.com/en/conditions/osteogenesis-imperfecta/)
- [Ehlers–Danlos syndrome](https://durgical.com/en/conditions/ehlers-danlos-syndrome/)
- [Osteoporosis](https://durgical.com/en/conditions/osteoporosis/)
- [Abdominal aortic aneurysm](https://durgical.com/en/conditions/abdominal-aortic-aneurysm/)
- [Aortic dissection](https://durgical.com/en/conditions/aortic-dissection/)
- [Carpal tunnel syndrome](https://durgical.com/en/conditions/carpal-tunnel-syndrome/)
- [Hypertensive heart disease](https://durgical.com/en/conditions/hypertensive-heart-disease/)
- [Atrial fibrillation](https://durgical.com/en/conditions/atrial-fibrillation/)
- [Intracerebral haemorrhage](https://durgical.com/en/conditions/intracerebral-haemorrhage/)
- [Molar pregnancy (hydatidiform mole)](https://durgical.com/en/conditions/molar-pregnancy/)
- [Gastric cancer](https://durgical.com/en/conditions/gastric-cancer/)
- [Gastro-oesophageal reflux disease (GERD)](https://durgical.com/en/conditions/gastro-oesophageal-reflux-disease/)
- [Alcoholic liver disease (biochemistry)](https://durgical.com/en/conditions/alcoholic-liver-disease/)
- [Disulfiram–ethanol reaction](https://durgical.com/en/conditions/disulfiram-ethanol-reaction/)
- [Methanol poisoning](https://durgical.com/en/conditions/methanol-poisoning/)
- [Ethylene glycol poisoning](https://durgical.com/en/conditions/ethylene-glycol-poisoning/)
- [Oesophageal cancer](https://durgical.com/en/conditions/oesophageal-cancer/)
- [Acetaminophen toxicity](https://durgical.com/en/conditions/acetaminophen-toxicity/)
- [Vitamin A deficiency (night blindness and xerophthalmia)](https://durgical.com/en/conditions/vitamin-a-deficiency/)
- [Hypervitaminosis A (vitamin A toxicity)](https://durgical.com/en/conditions/hypervitaminosis-a/)
- [Vitamin K deficiency bleeding of the newborn](https://durgical.com/en/conditions/vitamin-k-deficiency-bleeding-of-the-newborn/)
- [Beriberi (thiamine deficiency)](https://durgical.com/en/conditions/beriberi/)
- [Zinc deficiency (acrodermatitis enteropathica)](https://durgical.com/en/conditions/zinc-deficiency/)
- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Dental caries](https://durgical.com/en/conditions/dental-caries/)

Sources: RCSB PDB (CC0), PubChem (public domain), Reactome, UniProt, MedlinePlus.

Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/