Abstract
Transketolase (TK, EC 2.2.1.1), the key enzyme of the nonβoxidative branch of pentose phosphate pathway of hydrocarbon transformation, plays an important role in a system of substrate rearrangement between pentose shunt and glycolysis, acting as a reversible link between the two metabolic pathways. In addition, it supplies precursors for biosyntheses of nucleotides, aromatic amino acids, and vitamins. In plants, the enzyme plays a central role in the Calvin cycle. TK catalyzes interconversion of sugar phosphates. Thiamine diphosphate (TDP) and bivalent cations serve as its cofactors. Being a typical TDPβdependent enzyme, TK is the least complex representative of this group of enzymes, and this accounts for its use as a model in studies of their structure and mechanism of action. TK is readily crystallized, this being the reason why the first crystal Xβray structure analysis of TDPβdependent enzymes was performed with a TK sample. Both the general structure of TK and the structures of its active centers have been studied in detail. In this article, we review experimental evidence of functional nonequivalence of the two active centers of TK, which are known to be identical by crystal Xβray structure analysis. Β© IUBMB IUBMB Life, 62(11): 797β802, 2010.