[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Pentose phosphate pathway [Biochemistry](https://durgical.com/en/biochemistry/) · Metabolic pathway # Pentose phosphate pathway This pathway uses glucose 6-phosphate to make NADPH, which protects cells from oxidative damage, and ribose for DNA and RNA. Updated 2026-11-01 ## Interactive map **Balance:** nadph +2 CO₂ +1 ## Key facts - Oxidative phase (steps 1–3), irreversible: each glucose 6-phosphate yields 2 NADPH and 1 CO₂, leaving ribulose 5-phosphate. It makes and uses no ATP. - Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting step; NADP⁺ activates it and NADPH holds it back. - Non-oxidative phase (steps 4–7), reversible: transketolase (with thiamine, B1) and transaldolase swap carbons and return fructose 6-phosphate and glyceraldehyde 3-phosphate to glycolysis. - It is very active in liver, adipose tissue, adrenal cortex, lactating mammary gland and red cells: wherever fatty acids and steroids are made or antioxidant defence is needed. - It takes place in the cytosol. ## In detail - NADPH keeps glutathione reduced (glutathione reductase); reduced glutathione removes hydrogen peroxide in the red cell. - If the cell needs more ribose than NADPH (a dividing cell), the non-oxidative phase runs backwards and makes ribose 5-phosphate from fructose 6-phosphate and glyceraldehyde 3-phosphate, skipping the oxidative phase. - If it needs more NADPH than ribose, the pentoses return to glycolysis as fructose 6-phosphate and glyceraldehyde 3-phosphate, which can be recycled to glucose 6-phosphate. - NADPH is also used to make fatty acids, cholesterol and steroids, by neutrophil NADPH oxidase and by cytochrome P450. - Overall when everything returns to glycolysis: 3 glucose 6-phosphate → 6 NADPH + 3 CO₂ + 2 fructose 6-phosphate + 1 glyceraldehyde 3-phosphate. **Also called** Hexose monophosphate shunt, Phosphogluconate pathway ## Regulation - The NADP⁺/NADPH ratio controls G6PD: when NADPH is used up (fat synthesis, oxidative stress) the pathway speeds up. - Insulin and a carbohydrate-rich diet increase G6PD synthesis in liver and adipose tissue. - The non-oxidative phase is regulated by substrate availability. ## The steps, one by one - **Step 1 · [Glucose-6-phosphate dehydrogenase (G6PD)](https://durgical.com/en/biochemistry/enzymes/glucose-6-phosphate-dehydrogenase/)** irreversible [Glucose 6-phosphate](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/) + nadp → 6-Phosphoglucono-δ-lactone + [NADPH](https://durgical.com/en/biochemistry/molecules/nadph/) It oxidises glucose 6-phosphate and makes the first NADPH: the rate-limiting, committed step. **If it fails:** G6PD deficiency is the world's most common enzyme defect: acute haemolysis with oxidants and neonatal jaundice. [Glucose-6-phosphate dehydrogenase (G6PD) deficiency →](https://durgical.com/en/conditions/glucose-6-phosphate-dehydrogenase-deficiency/) - **Step 2 · 6-Phosphogluconolactonase** irreversible 6-Phosphoglucono-δ-lactone + H₂O → 6-Phosphogluconate It opens the lactone ring with water, leaving 6-phosphogluconate. - **Step 3 · 6-Phosphogluconate dehydrogenase** irreversible 6-Phosphogluconate + nadp → Ribulose 5-phosphate + [NADPH](https://durgical.com/en/biochemistry/molecules/nadph/) + CO₂ It oxidises and decarboxylates: the second NADPH and one CO₂ leave, and a pentose, ribulose 5-phosphate, remains. - **Step 4 · Ribose-5-phosphate isomerase** Ribulose 5-phosphate ⇌ [Ribose 5-phosphate](https://durgical.com/en/biochemistry/molecules/ribose-5-phosphate/) It turns the ketose into an aldose: ribose 5-phosphate goes to nucleotides (or on through the non-oxidative phase). Another part of the ribulose 5-phosphate becomes xylulose 5-phosphate (epimerase). - **Step 5 · [Transketolase](https://durgical.com/en/biochemistry/enzymes/transketolase/)** [Ribose 5-phosphate](https://durgical.com/en/biochemistry/molecules/ribose-5-phosphate/) + Xylulose 5-phosphate ⇌ Sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate It moves two carbons from xylulose 5-phosphate to ribose 5-phosphate: sedoheptulose 7-phosphate (7 C) and glyceraldehyde 3-phosphate (3 C) come out. **If it fails:** In thiamine deficiency transketolase loses activity: the basis of the red cell transketolase test. [Wernicke encephalopathy (thiamine deficiency) →](https://durgical.com/en/conditions/wernicke-encephalopathy/) - **Step 6 · Transaldolase** Sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate ⇌ Erythrose 4-phosphate + fructose 6-phosphate It moves three carbons from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate: erythrose 4-phosphate and fructose 6-phosphate come out. **If it fails:** Transaldolase deficiency is very rare: liver damage from birth. - **Step 7 · [Transketolase (second time)](https://durgical.com/en/biochemistry/enzymes/transketolase/)** Erythrose 4-phosphate + Xylulose 5-phosphate ⇌ fructose 6-phosphate + glyceraldehyde 3-phosphate It moves two carbons from another xylulose 5-phosphate to erythrose 4-phosphate: fructose 6-phosphate and glyceraldehyde 3-phosphate come out and return to glycolysis. ## Clinical notes - G6PD deficiency leaves the red cell without NADPH: oxidants such as fava beans, primaquine, dapsone or an infection cause haemolysis with Heinz bodies. - Red cell transketolase activity measures thiamine (B1) status; it falls in alcoholism and Wernicke encephalopathy. - Without NADPH, neutrophil NADPH oxidase cannot produce the respiratory burst (chronic granulomatous disease when that oxidase fails). ## Related conditions - [Glucose-6-phosphate dehydrogenase (G6PD) deficiency](https://durgical.com/en/conditions/glucose-6-phosphate-dehydrogenase-deficiency/) - [Wernicke encephalopathy (thiamine deficiency)](https://durgical.com/en/conditions/wernicke-encephalopathy/) ## Related - [Glucose 6-phosphate](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/) - [Glucose-6-phosphate dehydrogenase (G6PD)](https://durgical.com/en/biochemistry/enzymes/glucose-6-phosphate-dehydrogenase/) - [Transketolase](https://durgical.com/en/biochemistry/enzymes/transketolase/) - [NADPH](https://durgical.com/en/biochemistry/molecules/nadph/) - [Ribose 5-phosphate](https://durgical.com/en/biochemistry/molecules/ribose-5-phosphate/) - [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/) - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) ## Test yourself Think of the answer, then tap the question to see it. ## What are the two main products of the pentose phosphate pathway? NADPH (antioxidant defence and fat and steroid synthesis) and ribose 5-phosphate (nucleotides). ## What is the rate-limiting enzyme and what regulates it? G6PD; NADP⁺ activates it and NADPH inhibits it. ## Which vitamin does transketolase need? Thiamine (B1), as thiamine pyrophosphate; this is why red cell transketolase measures B1 deficiency. ## Why does the red cell depend so much on this pathway? It is its only source of NADPH, which keeps glutathione reduced to remove peroxides. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/pathways/pentose-phosphate-pathway/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Pentose phosphate pathway](https://reactome.org/PathwayBrowser/#/R-HSA-71336) (Reactome, 2026); [G6PD: Glucose-6-phosphate 1-dehydrogenase (human)](https://www.uniprot.org/uniprotkb/P11413/entry) (UniProt, 2026); [TKT: Transketolase (human)](https://www.uniprot.org/uniprotkb/P29401/entry) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/pathways/pentose-phosphate-pathway/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/pathways/pentose-phosphate-pathway/