# Non-oxidative phase of the pentose phosphate pathway
The non-oxidative phase of the pentose phosphate pathway is the reversible, cytosolic set of sugar-interconversion reactions that converts pentose phosphates into glycolytic intermediates and back again, without producing NADPH or ATP. It is the second arm of the pentose phosphate pathway (PPP), a route of glucose metabolism that runs alongside glycolysis: the oxidative arm (reactions 1–3 of the pathway's eight) produces NADPH and ribulose 5-phosphate, while the non-oxidative arm (reactions 4–8) rearranges the carbon skeletons of phosphorylated sugars[1](http://reactome.org/content/detail/R-HSA-71336). Four enzymes carry out this work: ribose-5-phosphate isomerase (RPIA), ribulose-5-phosphate-3-epimerase (RPE), transketolase (TKT), and transaldolase (TALDO1)[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/).
| Key fact | Detail |
|---|---|
| Position in the pathway | Reactions 4–8 of the eight-reaction pentose phosphate pathway[1](http://reactome.org/content/detail/R-HSA-71336) |
| Enzymes | Ribose-5-phosphate isomerase, ribulose-5-phosphate-3-epimerase, transketolase, transaldolase[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/) |
| Carbon-unit transfers | Transketolase moves two-carbon units; transaldolase moves three-carbon units[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/) |
| Net conversion | Three pentoses → two hexoses + one triose phosphate[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html) |
| Reversibility | Fully reversible; the oxidative phase is considered unidirectional[4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY) • [5](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140) |
| Cofactors | Thiamine pyrophosphate (TPP) for transketolase[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/) |
| Output | No NADPH, no ATP; carbon rearrangement only[6](https://www.ncbi.nlm.nih.gov/sites/books/NBK551687/) • [4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY) |
| Clinical marker | Erythrocyte transketolase activity is a standard assay for thiamine deficiency[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/) |
## What the non-oxidative phase is
The phase is a network of reversible reactions in which phosphorylated sugars are interconverted, generating xylulose 5-phosphate, ribulose 5-phosphate, and ribose 5-phosphate among other intermediates[7](https://www.kegg.jp/entry/map00030). Like glycolysis, it takes place in the cytosol[4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY).
Its character is defined by what it does not produce. No NADPH is created in this part of the pathway[6](https://www.ncbi.nlm.nih.gov/sites/books/NBK551687/), and no energy or reducing power is generated at all; its sole purpose is carbon rearrangement[4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY). Because every reaction is reversible, molecules can enter at different stages and be transformed into any other intermediate in the branch[4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY). This contrasts with the oxidative phase, which is considered unidirectional[5](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140).
## The reactions, step by step
Two preparative reactions set up the carbon-shuffling steps. Ribulose 5-phosphate, the ketose product of the oxidative phase, is interconverted with ribose 5-phosphate by phosphopentose isomerase and with xylulose 5-phosphate by [phosphopentose epimerase](https://www.edgepedia.org/phosphopentose-epimerase)[8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml). The epimerase matters because xylulose 5-phosphate is the appropriate epimer for the transketolase reaction[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html).
The three linking reactions catalyzed by transketolase and transaldolase then interconvert pentoses, hexoses, and trioses[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html):
1. **First transketolase step.** Ribose-5-phosphate plus xylulose-5-phosphate is rearranged to sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate[8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml). Transketolase (EC 2.2.1.1) detaches a two-carbon unit from xylulose 5-phosphate (a ketose donor) and transfers it to ribose 5-phosphate, extending it to the seven-carbon sedoheptulose 7-phosphate[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/) • [5](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140).
2. **Transaldolase step.** Sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate are rearranged to erythrose-4-phosphate and fructose-6-phosphate[8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml). Transaldolase transfers a three-carbon unit, using a [Schiff base](https://www.edgepedia.org/schiff-base) formed between the substrate and a lysine residue in the enzyme[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/).
3. **Second transketolase step.** Erythrose-4-phosphate plus xylulose-5-phosphate yields fructose-6-phosphate and glyceraldehyde-3-phosphate, again by two-carbon transfer[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html).
Both transketolase steps require thiamine pyrophosphate (TPP) as cofactor, the vitamin-derived carrier of the transferred two-carbon unit[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/). All of these interconversions are reversible[9](https://pressbooks.lib.vt.edu/cellbio/chapter/pentose-phosphate-pathway-ppp-purine-and-pyrimidine-metabolism/).
## How the carbon bookkeeping works
The C2-versus-C3 distinction between the two transferases is what makes the arithmetic balance. Transketolase moves two-carbon fragments between ketose donors and aldose acceptors; transaldolase moves three-carbon fragments. Together they redistribute the carbon atoms of three pentoses (5 + 5 + 5 = 15 carbons) into two hexoses and one triose phosphate (6 + 6 + 3 = 15 carbons)[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html).
The reversible network supports two distinct operating modes. In the ribose-production mode, the non-oxidative reactions run so as to generate net amounts of ribose 5-phosphate with no production of NADPH, drawing on the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate as carbon sources[1](http://reactome.org/content/detail/R-HSA-71336) • [8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml). In the recycling mode, the reactions run the other way, converting pentose phosphates into fructose-6-phosphate and glyceraldehyde-3-phosphate for glycolysis, so that the oxidative phase can yield NADPH without net ribose production[8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml).
[Ribose 5-phosphate](https://www.edgepedia.org/ribose-5-phosphate) also feeds nucleotide synthesis directly: phosphoribosyl pyrophosphate (PRPP), the activated ribose donor for nucleotide synthesis, is formed from ribose-5-phosphate[7](https://www.kegg.jp/entry/map00030).
## Linkage to glycolysis and direction of flux
The pentose phosphate pathway and glycolysis are distinct pathways, but they share three intermediates: glucose 6-phosphate, glyceraldehyde 3-phosphate, and fructose 6-phosphate, so the two are interconnected[1](http://reactome.org/content/detail/R-HSA-71336). Ribose 5-phosphate produced in the pathway can be returned to glycolysis through these shared metabolites[6](https://www.ncbi.nlm.nih.gov/sites/books/NBK551687/).
Which way the network runs depends on the metabolic state of the cell and the nature of the biosynthetic reactions underway[1](http://reactome.org/content/detail/R-HSA-71336). In highly proliferative cells with high nucleotide demand, the non-oxidative phase can function independently of the oxidative phase, producing ribose 5-phosphate from fructose 6-phosphate and glyceraldehyde 3-phosphate[9](https://pressbooks.lib.vt.edu/cellbio/chapter/pentose-phosphate-pathway-ppp-purine-and-pyrimidine-metabolism/). In adipose tissue, liver, and mammary glands, which require large amounts of NADPH for fatty acid synthesis, ribose-5-phosphate is instead converted into glyceraldehyde-3-phosphate and fructose-6-phosphate by transketolase and transaldolase, returning the carbon to glycolysis[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html).
The non-oxidative branch itself is virtually ubiquitous, metabolizing fructose 6-phosphate, glyceraldehyde 3-phosphate, and sedoheptulose sugars, and yielding ribose 5-phosphate for nucleic acid synthesis as well as sugar-phosphate precursors for amino acid synthesis[5](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140). This ubiquity matters for tissues in which the oxidative phase is weak: oxidative-phase activity is low in muscle but high in liver, testis, [adrenal cortex](https://www.edgepedia.org/adrenal-cortex), and mammary gland, and in red blood cells, which use NADPH to counteract their oxidizing environment[8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml).
## By the numbers
- The full pathway comprises eight reactions, of which the non-oxidative branch accounts for five (reactions 4–8)[1](http://reactome.org/content/detail/R-HSA-71336).
- The three linking reactions convert three pentoses into two hexoses and one triose[3](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html).
- [Transketolase](https://www.edgepedia.org/transketolase) transfers two-carbon units; transaldolase transfers three-carbon units[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/).
- Classical assay methods allow quantitative determination of all four non-oxidative enzymes in tissue extracts, by coupling the reactions to the measurement of glyceraldehyde 3-phosphate through a triose phosphate isomerase/α-glycerophosphate dehydrogenase/NADH system; assay conditions can be arranged so that each enzyme in turn becomes rate-limiting, and the resulting activities have been compared across tissues with those of the two oxidative dehydrogenases[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC1198749/).
The exact net stoichiometry of the full recycling cycle (six glucose 6-phosphate oxidized to six CO₂ with recovery of five glucose 6-phosphate equivalents) is widely taught but is not stated explicitly in the sources used here, so it is not asserted further.
## How it compares with the oxidative phase
The two arms of the pathway differ on every major axis:
- **Reversibility.** The non-oxidative branch is fully reversible[4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY); the oxidative PPP is considered unidirectional[5](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140).
- **Output.** The non-oxidative phase produces no NADPH[6](https://www.ncbi.nlm.nih.gov/sites/books/NBK551687/) and no energy or reducing power of any kind[4](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY); the oxidative phase is the pathway's NADPH source.
- **Regulation.** The non-oxidative phase is not regulated as such; the key regulatory enzyme of the pathway is glucose 6-phosphate dehydrogenase in the oxidative portion, inhibited by high NADPH levels via negative feedback[9](https://pressbooks.lib.vt.edu/cellbio/chapter/pentose-phosphate-pathway-ppp-purine-and-pyrimidine-metabolism/).
- **Tissue distribution.** The non-oxidative branch is virtually ubiquitous[5](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140), whereas oxidative-phase activity varies strongly by tissue, being low in muscle and high in liver, testis, adrenal cortex, mammary gland, and red blood cells[8](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml).
## When it fails: deficiencies and thiamine dependence
**Transaldolase deficiency** is an autosomal recessive disorder caused by inherited mutations in the TALDO1 gene. Presentation is variable between patients, but most manifest symptoms in the neonatal period: hepatosplenomegaly, bleeding diathesis, abnormal liver function, cholestatic jaundice, and elevated liver enzymes, with later [hepatic fibrosis or cirrhosis](https://www.edgepedia.org/hepatic-fibrosis-or-cirrhosis)[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/). Biochemically, the block leads to accumulation of seven-carbon carbohydrates including sedoheptulose, sedoheptulose 7-phosphate, and mannoheptulose, as well as open-chain polyols[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/).
**Thiamine status** acts on the phase through transketolase, the one non-oxidative enzyme requiring a vitamin-derived cofactor, TPP[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/). This dependence is exploited diagnostically: transketolase is expressed at high levels in red blood cells, which are easy to isolate and analyze, and the only vitamin-derived cofactor it requires is TPP, so assay of transketolase activity in red blood cell lysates is highly diagnostic in cases of suspected thiamine deficiency[2](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/). Reduced transketolase activity in a blood sample accordingly indicates thiamine deficiency[9](https://pressbooks.lib.vt.edu/cellbio/chapter/pentose-phosphate-pathway-ppp-purine-and-pyrimidine-metabolism/).
The sources reviewed here do not cover the clinical presentation of transketolase (TKT) variants, so that question is left open.
## References
1. [Reactome: Pentose phosphate pathway](http://reactome.org/content/detail/R-HSA-71336)
2. [Pentose Phosphate Pathway – The Medical Biochemistry Page](https://themedicalbiochemistrypage.org/pentose-phosphate-pathway/)
3. [Tymoczko Biochemistry 3e, Chapter 26](https://digfir-published.macmillanusa.com/tymoczko3e/tymoczko3e_ch26_2.html)
4. [MetaTobaccoCyc pentose phosphate pathway (Cornell SolCyc)](https://solcyc.sgn.cornell.edu/METATOB/NEW-IMAGE?object=PENTOSE-P-PWY&type=PATHWAY)
5. [Stincone et al., The return of metabolism: biochemistry and physiology of the pentose phosphate pathway, Biological Reviews](https://onlinelibrary.wiley.com/doi/10.1111/brv.12140)
6. [Biochemistry, Hexose Monophosphate Pathway (StatPearls/NCBI Bookshelf)](https://www.ncbi.nlm.nih.gov/sites/books/NBK551687/)
7. [KEGG PATHWAY: map00030 (Pentose phosphate pathway)](https://www.kegg.jp/entry/map00030)
8. [Pentose Phosphate Pathway (NYU School of Medicine teaching material)](http://education.med.nyu.edu/mbm/carbohydrates/pentosePathway.shtml)
9. [Pentose Phosphate Pathway (PPP), Purine and Pyrimidine Metabolism – Virginia Tech Pressbooks](https://pressbooks.lib.vt.edu/cellbio/chapter/pentose-phosphate-pathway-ppp-purine-and-pyrimidine-metabolism/)
10. [Novello & McLean, The pentose phosphate pathway of glucose metabolism. Measurement of the non-oxidative reactions of the cycle, Biochem J](https://pmc.ncbi.nlm.nih.gov/articles/PMC1198749/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Carbohydrate and energy metabolism › Pentose phosphate pathway › Non-oxidative phase of the pentose phosphate pathway*
*Initially written Sep 17, 2026 · Edited: Oct 11, 2026; Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license