<p>Transketolase <db_xref db="EC" dbkey="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 [<cite idref="PUB00000209"/>, <cite idref="PUB00000362"/>] show that theenzyme has been evolutionarily conserved.In the peroxisomes of methylotrophic yeast <taxon tax_id="4905">Pichia angusta</taxon> (Yeast) (Hansenula polymorpha), there is ahighly related enzyme, dihydroxy-acetone synthase (DHAS) <db_xref db="EC" dbkey="2.2.1.3"/> (alsoknown as formaldehyde transketolase), which exhibits a very unusualspecificity by including formaldehyde amongst its substrates.</p> <p>1-deoxyxylulose-5-phosphate synthase (DXP synthase) [<cite idref="PUB00004906"/>] 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 [<cite idref="PUB00001222"/>]. In the centralsection there are conserved acidic residues that are part of the active cleftand may participate in substrate-binding [<cite idref="PUB00001222"/>].This family includes transketolase enzymes <db_xref db="EC" dbkey="2.2.1.1"/>and also partially matches to 2-oxoisovalerate dehydrogenasebeta subunit <db_xref db="SWISSPROT" dbkey="P37941"/> <db_xref db="EC" dbkey="1.2.4.4"/>. Both these enzymesutilise thiamine pyrophosphate as a cofactor, suggestingthere may be common aspects in their mechanism of catalysis.</p>