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.

This group includes two proteins from the yeast Saccharomyces cerevisiae but excludes dihydroxyactetone synthases (formaldehyde transketolases) from various yeasts and the even more distant mammalian transketolases. Among the family of thiamine diphosphate-dependent enzymes that includes transketolases, dihydroxyacetone synthases, pyruvate dehydrogenase E1-beta subunits, and deoxyxylulose-5-phosphate synthases, mammalian and bacterial transketolases seem not to be orthologous.