InterPro domain: IPR005475

Transketolase 2.2.1.1 (TK) catalyzes the reversible transfer of atwo-carbon ketol unit from xylulose 5-phosphate to an aldose receptor, such asribose 5-phosphate, to form sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate. This enzyme, together with transaldolase, provides a link betweenthe glycolytic and pentose-phosphate pathways.TK requires thiamine pyrophosphate as a cofactor. In most sources where TK hasbeen purified, it is a homodimer of approximately 70 Kd subunits. TK sequencesfrom a variety of eukaryotic and prokaryotic sources [ 1 , 2 ] show that theenzyme has been evolutionarily conserved.In the peroxisomes of methylotrophic yeast Pichia angusta (Yeast) (Hansenula polymorpha), there is ahighly related enzyme, dihydroxy-acetone synthase (DHAS) 2.2.1.3 (alsoknown as formaldehyde transketolase), which exhibits a very unusualspecificity by including formaldehyde amongst its substrates.

1-deoxyxylulose-5-phosphate synthase (DXP synthase) [ 3 ] is an enzyme so farfound in bacteria (gene dxs) and plants (gene CLA1) which catalyzes thethiamine pyrophosphoate-dependent acyloin condensation reaction between carbonatoms 2 and 3 of pyruvate and glyceraldehyde 3-phosphate to yield 1-deoxy-D-xylulose-5-phosphate (dxp), a precursor in the biosynthetic pathway toisoprenoids, thiamine (vitamin B1), and pyridoxol (vitamin B6). DXP synthaseis evolutionary related to TK. The N-terminal section, contains a histidine residue which appears to function inproton transfer during catalysis [ 4 ]. In the centralsection there are conserved acidic residues that are part of the active cleftand may participate in substrate-binding [ 5 ].This family includes transketolase enzymes 2.2.1.1 and also partially matches to 2-oxoisovalerate dehydrogenasebeta subunit P37941 1.2.4.4 . Both these enzymesutilise thiamine pyrophosphate as a cofactor, suggestingthere may be common aspects in their mechanism of catalysis.

1. Nucleotide and predicted amino acid sequence of a cDNA clone encoding part of human transketolase. Biochem. Biophys. Res. Commun. 183, 1159-66
2. DNA sequence of the yeast transketolase gene. Biochemistry 31, 1892-6
3. Identification of a thiamin-dependent synthase in Escherichia coli required for the formation of the 1-deoxy-D-xylulose 5-phosphate precursor to isoprenoids, thiamin, and pyridoxol. Proc. Natl. Acad. Sci. U.S.A. 94, 12857-62
4. Three-dimensional structure of transketolase, a thiamine diphosphate dependent enzyme, at 2.5 A resolution. EMBO J. 11, 2373-9