Biochemistry · Metabolic pathway

Gluconeogenesis !

Gluconeogenesis makes new glucose from lactate, alanine or glycerol during a long fast; it happens mainly in the liver.

Updated 2026-11-01

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Balance: ATP -4GTP -2NADH -2

Key facts

It is not glycolysis run backwards: it shares 7 reversible steps but goes around the 3 irreversible ones with 4 enzymes of its own (pyruvate carboxylase, PEPCK, fructose-1,6-bisphosphatase and glucose-6-phosphatase).
Cost per glucose: 4 ATP, 2 GTP and 2 NADH (6 high-energy bonds), against the 2 ATP that glycolysis yields.
Site: liver (90 %) and renal cortex; in prolonged fasting the kidney supplies up to 40 %. The first step is mitochondrial and the last takes place in the endoplasmic reticulum.
Substrates: lactate (Cori cycle), alanine and other glucogenic amino acids (glucose–alanine cycle), glycerol from triglycerides (enters as DHAP) and propionate. Even-chain fatty acids and acetyl-CoA give no net glucose.
It takes over after about 12–18 h of fasting, once liver glycogen runs out.

Also called: Glucose synthesis from non-carbohydrate precursors

Regulation

Glucagon and cortisol switch it on; insulin switches it off (represses PEPCK and glucose-6-phosphatase).
Acetyl-CoA from beta-oxidation activates pyruvate carboxylase: fatty acids pay the energy bill and push pyruvate toward glucose.
Fructose-2,6-bisphosphate inhibits fructose-1,6-bisphosphatase and activates PFK-1; glucagon lowers it, so the cell does not run both pathways at once (futile cycle).
AMP inhibits fructose-1,6-bisphosphatase: with little energy, no glucose is made.

The steps, one by one

Step 1 · Pyruvate carboxylase irreversible Pyruvate + CO₂ + ATP → oxaloacetate + ADP + phosphate (Pi)In the mitochondrion it adds CO₂ to pyruvate to form oxaloacetate: the first detour around the pyruvate kinase step.If it fails: Pyruvate carboxylase deficiency causes lactic acidosis, hypoglycaemia and neurological damage from birth. Lactic acidosis →
Step 2 · Phosphoenolpyruvate carboxykinase (PEPCK) irreversible oxaloacetate + GTP → phosphoenolpyruvate + gdp + CO₂It removes the CO₂ and adds a phosphate from GTP: oxaloacetate becomes phosphoenolpyruvate.If it fails: Cytosolic PEPCK deficiency is very rare: fasting hypoglycaemia and liver damage.
Step 3 · Enolase (in reverse) phosphoenolpyruvate + H₂O ⇌ 2-phosphoglycerateIt adds water to phosphoenolpyruvate to form 2-phosphoglycerate; from here on the glycolytic enzymes are used.
Step 4 · Phosphoglycerate mutase (in reverse) 2-phosphoglycerate ⇌ 3-phosphoglycerateIt moves the phosphate from carbon 2 to carbon 3.
Step 5 · Phosphoglycerate kinase (in reverse) 3-phosphoglycerate + ATP ⇌ 1,3-bisphosphoglycerate + ADPIt spends one ATP to form 1,3-bisphosphoglycerate.
Step 6 · Glyceraldehyde-3-phosphate dehydrogenase (in reverse) 1,3-bisphosphoglycerate + NADH ⇌ glyceraldehyde 3-phosphate + NAD⁺ + phosphate (Pi)It uses NADH to reduce 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate. Part of it becomes DHAP (triose phosphate isomerase), where glycerol also enters.
Step 7 · Aldolase (in reverse) glyceraldehyde 3-phosphate + dihydroxyacetone phosphate ⇌ Fructose 1,6-bisphosphateIt joins the two triose phosphates into fructose 1,6-bisphosphate.
Step 8 · Fructose-1,6-bisphosphatase (FBPase-1) irreversible Fructose 1,6-bisphosphate + H₂O → fructose 6-phosphate + phosphate (Pi)It removes the phosphate on carbon 1: it bypasses the PFK-1 step and is the pathway's main control point.If it fails: FBPase-1 deficiency causes hypoglycaemia with lactic acidosis during fasting or fever, and worsens with fructose.
Step 9 · Phosphoglucose isomerase (in reverse) fructose 6-phosphate ⇌ Glucose 6-phosphateIt turns fructose 6-phosphate into glucose 6-phosphate.
Step 10 · Glucose-6-phosphatase irreversible Glucose 6-phosphate + H₂O → Glucose + phosphate (Pi)In the endoplasmic reticulum of liver and kidney it frees glucose, which goes out into the blood; muscle lacks this enzyme.If it fails: Its deficiency is glycogen storage disease type I (von Gierke): severe hypoglycaemia with high lactate, uric acid and triglycerides. Von Gierke disease (glycogen storage disease type I) →

Clinical notes

Alcohol uses up liver NAD⁺: excess NADH diverts pyruvate to lactate and oxaloacetate to malate, and stalls gluconeogenesis. Hence hypoglycaemia in the fasting drinker.
Metformin lowers hepatic glucose output; in kidney or liver failure lactate can build up.
Glucose-6-phosphatase deficiency (von Gierke) blocks the last step shared by gluconeogenesis and glycogenolysis: severe fasting hypoglycaemia.
In type 2 diabetes hepatic gluconeogenesis stays high despite insulin and accounts for much of the fasting glucose.

Test yourself

Think of the answer, then tap the question to see it.

Which four enzymes are unique to gluconeogenesis?

Pyruvate carboxylase, PEPCK, fructose-1,6-bisphosphatase and glucose-6-phosphatase: they go around the three irreversible steps of glycolysis.

Why do even-chain fatty acids give no net glucose?

They yield acetyl-CoA and PDH is irreversible; the two carbons of acetyl-CoA leave as CO₂ in the Krebs cycle.

Why can muscle not release glucose into the blood?

It lacks glucose-6-phosphatase; its glucose 6-phosphate stays inside the fibre.

Which molecule activates pyruvate carboxylase?

Acetyl-CoA: it signals plenty of fatty acids and diverts pyruvate toward glucose.

Why does alcohol during fasting cause hypoglycaemia?

Its oxidation floods the liver with NADH; pyruvate goes to lactate and oxaloacetate to malate, leaving gluconeogenesis without substrate.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Gluconeogenesis (Reactome, 2026); PC: Pyruvate carboxylase, mitochondrial (human) (UniProt, 2026); G6PC1: Glucose-6-phosphatase catalytic subunit 1 (human) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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