# Transketolase
**Transketolase** (TK, EC 2.2.1.1) is a cytoplasmic enzyme that catalyzes the reversible transfer of a two-carbon ketol unit from a ketose phosphate donor to an aldose phosphate acceptor, and it appears to control the non-oxidative branch of the pentose phosphate pathway.[1](https://www.sciencedirect.com/science/article/pii/S1570963914001526) It requires thiamine diphosphate (ThDP), the biologically active form of vitamin B1, and a divalent metal ion as cofactors.[2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) The enzyme is ubiquitous in cellular carbon metabolism, linking the pentose phosphate pathway to glycolysis and supplying ribose units for nucleoside biosynthesis.[3](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transketolase)
| Key fact | Detail |
|---|---|
| Classification | EC 2.2.1.1, a ThDP-dependent transferase; key rate-limiting enzyme of the non-oxidative pentose phosphate pathway, located in the cytoplasm[1](https://www.sciencedirect.com/science/article/pii/S1570963914001526) |
| Cofactors | Thiamine diphosphate plus a divalent metal cation, usually Mg2+; the human enzyme depends on Ca2+ • [2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2951230/) |
| Reaction | Reversible transfer of a two-carbon ketol fragment ("active glycolaldehyde") from a ketose donor to an aldose acceptor, via cleavage of a C–C bond in the donor[1](https://www.sciencedirect.com/science/article/pii/S1570963914001526) |
| In vivo substrates | Donor d-xylulose 5-phosphate and acceptor d-ribose 5-phosphate yield d-sedoheptulose 7-phosphate and d-glyceraldehyde 3-phosphate[3](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transketolase) |
| Structure | Homodimer; the two catalytic sites are formed at the interface between the subunits, where the thiazole and pyrimidine rings of ThDP are bound[2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) |
| Discovery | Identified in 1953, independently by Horecker and Smyrniotis and by Racker and colleagues, in yeast, as an enzyme of the hexose monophosphate shunt requiring thiamine pyrophosphate[5](https://www.nejm.org/doi/full/10.1056/NEJM196209202671204) |
| Biocatalysis | Products of the reaction serve as precursors for synthetic compounds, and the enzyme has been exploited industrially; hydroxypyruvic acid as donor makes the reaction irreversible through CO2 release[2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) • [3](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transketolase) |
## Reaction and metabolic role
Transketolase catalyzes two coupled processes: cleavage of the C–C bond in a ketose donor substrate and reversible transfer of the resulting two-carbon fragment, called "active glycolaldehyde", onto an aldose acceptor.[1](https://www.sciencedirect.com/science/article/pii/S1570963914001526) In the cell the principal reaction transfers the ketol unit from d-xylulose 5-phosphate to d-ribose 5-phosphate, producing d-sedoheptulose 7-phosphate and d-glyceraldehyde 3-phosphate.[3](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transketolase)
The enzyme sits at a reversible junction between the pentose phosphate pathway and glycolysis.[6](https://iubmb.onlinelibrary.wiley.com/doi/10.1002/iub.395) It controls the supply of ribose units for nucleoside biosynthesis and, in microorganisms, the supply of erythrose 4-phosphate into the shikimate pathway for aromatic amino acid biosynthesis.[3](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transketolase) The 1998 review by Gerhard Schenk, Ronald G. Duggleby, and Peter F. Nixon of Griffith University noted that transketolase appears to control the non-oxidative branch of the pathway, although the overall flux of labeled substrates remained controversial.[2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) More recent work describes metabolic flexibility: TKT can run in reverse when the demand for nucleotides exceeds the demand for NADPH.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC12525540/)
## Structure and catalytic mechanism
Transketolase is a homodimer, and each of its two catalytic sites is formed at the interface between the two subunits, where the thiazole and pyrimidine rings of the ThDP cofactor are bound.[2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) The yeast [Saccharomyces cerevisiae](https://www.edgepedia.org/saccharomyces-cerevisiae) enzyme was among the first ThDP-dependent enzymes whose three-dimensional structure was determined: the refined structure (PDB entry 1TRK, deposited 1993) shows the homodimer at 2.0 Å resolution, with each 680-residue subunit binding one ThDP and one calcium ion, and the cofactor shielded from solvent except for the C-2 atom of the thiazolium ring.[8](https://www.rcsb.org/structure/1TRK) The human enzyme structure was later determined to 1.75 Å resolution (PDB 3MOS).[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2951230/)
ThDP adopts the canonical V-type conformation in the active site, which juxtaposes the catalytic 4′-NH2 group of the aminopyrimidine ring and the reactive C2 of the thiazolium ring.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2951230/) In the yeast structure the catalytically important C2 carbon sits 3.1 Å from the 4-NH2 group, and no enzymic base is positioned to abstract the C2 proton; the proposed mechanism instead invokes cofactor-assisted deprotonation through a hydrogen bond between the invariant residue E418 and the N1′ nitrogen of ThDP.[9](https://www.sciencedirect.com/science/article/abs/pii/S016748389800082X) In the apoenzyme, two loop regions that shield the cofactor in the holoenzyme become flexible, opening the cofactor binding site for ThDP entry.[9](https://www.sciencedirect.com/science/article/abs/pii/S016748389800082X)
Two features of the enzyme family stand out. All characterized transketolases have similar kinetic and physical properties, but mammalian enzymes are more selective in substrate utilization than non-mammalian representatives; and the two active centers, identical by [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography), are functionally nonequivalent in experiment.[2](https://www.sciencedirect.com/science/article/pii/S1357272598000958) • [6](https://iubmb.onlinelibrary.wiley.com/doi/10.1002/iub.395) Because it is the least complex representative of the ThDP-dependent enzymes, transketolase serves as a model for studying the structure and mechanism of the whole group.[6](https://iubmb.onlinelibrary.wiley.com/doi/10.1002/iub.395)
## Transketolase-like proteins and cancer
The human genome encodes two transketolase-like proteins, TKTL1 and TKTL2. Whether TKTL1 is a catalytically active transketolase has been contested. Two independent research groups were unable to detect any transketolase activity of a TKTL1 mimic protein, and structural analysis indicates that the absence of critical residues such as His102 precludes binding of ThDP or the inhibitor oxythiamine.[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC4506423/) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC12525540/) A 2018 analysis concluded that the human TKTL1 and TKTL2 proteins are bona fide transketolases, illustrating the continuing disagreement over their function.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC6334435/)
Canonical TKT itself is implicated in tumor metabolism. The ribose 5-phosphate generated by TKT serves as a building block for DNA and RNA, and it is required for tumor cells to proliferate and mend damaged DNA, so inhibiting TKT has been suggested as a strategy for treating cancer.[12](https://www.sciencedirect.com/science/article/pii/S0753332224008199) • [13](https://onlinelibrary.wiley.com/doi/full/10.1002/cam4.71244) A 2025 review of transketolase in hepatocytes, adipocytes, regulatory T cells, and cancer cells reported roles of transketolase-mediated pentose metabolism in regulating nucleoside levels, genome instability, mitochondrial function, and [DNA methylation](https://www.edgepedia.org/dna-methylation).[14](https://pubmed.ncbi.nlm.nih.gov/41203350/)
## Clinical measurement
Because the enzyme requires thiamine pyrophosphate, erythrocyte transketolase activity was historically used to assess thiamine (vitamin B1) status; the measurement was published as a clinical application as early as 1962.[5](https://www.nejm.org/doi/full/10.1056/NEJM196209202671204)
## References
1. [Kochetov GA, Solovjeva ON (2014). Structure and functioning mechanism of transketolase. Biochimica et Biophysica Acta 1844(9):1608–1618.](https://www.sciencedirect.com/science/article/pii/S1570963914001526)
2. [Schenk G, Duggleby RG, Nixon PF (1998). Properties and functions of the thiamin diphosphate dependent enzyme transketolase. International Journal of Biochemistry & Cell Biology 30(12):1297–1318.](https://www.sciencedirect.com/science/article/pii/S1357272598000958)
3. [Transketolase – ScienceDirect Topics overview (Current Opinion in Biotechnology, 2000, Nicholas J Turner).](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transketolase)
4. [The Crystal Structure of Human Transketolase and New Insights into Its Mode of Action, PMC.](https://pmc.ncbi.nlm.nih.gov/articles/PMC2951230/)
5. [Clinical Application of Blood Transketolase Determinations, New England Journal of Medicine (1962).](https://www.nejm.org/doi/full/10.1056/NEJM196209202671204)
6. [Kochetov GA (2010). Functional nonequivalence of transketolase active centers. IUBMB Life.](https://iubmb.onlinelibrary.wiley.com/doi/10.1002/iub.395)
7. [Structural and Computational Insights into Transketolase-like 1 (TKTL-1), PMC (2025).](https://pmc.ncbi.nlm.nih.gov/articles/PMC12525540/)
8. [RCSB PDB 1TRK: Refined structure of transketolase from Saccharomyces cerevisiae at 2.0 Å resolution.](https://www.rcsb.org/structure/1TRK)
9. [Crystallography and mutagenesis of transketolase: mechanistic implications for enzymatic thiamin catalysis, ScienceDirect.](https://www.sciencedirect.com/science/article/abs/pii/S016748389800082X)
10. [TKTL1 expression in human malign and benign cell lines, PMC (2015).](https://pmc.ncbi.nlm.nih.gov/articles/PMC4506423/)
11. [The human transketolase-like proteins TKTL1 and TKTL2 are bona fide transketolases, PMC (2018).](https://pmc.ncbi.nlm.nih.gov/articles/PMC6334435/)
12. [Glucose-6-phosphate dehydrogenase and transketolase: key factors in breast cancer progression and therapy, Biomedicine & Pharmacotherapy (2024).](https://www.sciencedirect.com/science/article/pii/S0753332224008199)
13. [Metabolic Reprogramming of Cancer Cells and Therapeutics Targeting Cancer Metabolism, Cancer Medicine (2025).](https://onlinelibrary.wiley.com/doi/full/10.1002/cam4.71244)
14. [Transketolase in metabolic diseases, autoimmune diseases and cancer, PubMed (2025).](https://pubmed.ncbi.nlm.nih.gov/41203350/)
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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 › Transketolase*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license