transketolas

Transketolas is not a standard term in modern biochemistry and is rarely used in primary references. It is sometimes used informally to refer to transketolases, a group of enzymes that catalyze two-carbon transfer reactions in the pentose phosphate pathway. In most literature, the core enzymes are called transketolase (TKT) and transketolase-like proteins (such as TKTL1 and TKTL2).

Transketolases catalyze the transfer of a two-carbon unit from a ketose donor to an aldose acceptor. They require thiamine pyrophosphate (TPP) as a cofactor and a divalent metal ion, typically Mg2+. In the canonical reactions, xylulose-5-phosphate serves as the donor and ribose-5-phosphate or sedoheptulose-7-phosphate as acceptors, yielding glyceraldehyde-3-phosphate (GAP) and sedoheptulose-7-phosphate (S7P) in one step, and GAP plus fructose-6-phosphate (F6P) in a second step. These reactions are part of the non-oxidative branch of the pentose phosphate pathway (PPP).

Functionally, transketolases help rearrange carbon skeletons to balance the PPP, generating ribose-5-phosphate for nucleotide synthesis and regenerating glycolytic intermediates such as GAP and F6P for glycolysis. The PPP also produces NADPH, which is important for reductive biosynthesis and antioxidant defense; transketolase activity links NADPH production to the supply of sugars for nucleotide synthesis and metabolism.

Distribution and variants: in humans, the primary enzyme is transketolase (TKT); related proteins such as TKTL1 and TKTL2 are transketolase-like. The exact roles and enzymatic activities of TKTL1 in humans are subjects of ongoing research and debate, with some studies emphasizing transketolase-like activity rather than canonical transketolase function.

Clinical relevance: thiamine (vitamin B1) status affects transketolase activity, and erythrocyte transketolase activity measurements are used to assess thiamine deficiency. Altered transketolase expression has been observed in certain diseases, including cancers, but its precise diagnostic and therapeutic implications remain under study. See also pentose phosphate pathway.