InterPro: IPR005474 Transketolase, N-terminal

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

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