Transketolase

Transketolase is involved in the pentose phosphate pathway, which is the major pathway of carbohydrate metabolism in some tissues and a significant alternative to glycolysis in all tissues. The main importance of the pentose phosphate pathway is in the production of NADPH for use in biosynthetic reactions (and especially lipogenesis) and the de novo synthesis of ribose for nucleotide synthesis.

As shown in Figure 6.4, transketolase catalyzes the transfer of a two-carbon unit from a donor ketose onto an acceptor aldose sugar. The donor ketose forms a transient intermediate with thiamin diphosphate, which then undergoes cleavage to release an aldose two carbons smaller than the ketose substrate, leaving enzyme-bound dihydroxyethyl thiamin diphosphate. This reacts with an acceptor aldose to form a ketose two carbons larger.

Although the entry of glucose 6-phosphate into the pentose phosphate pathway is controlled by the need for NADPH and pentose sugars, transketolase has a high control strength (0.74) in the nonoxidative part of the pathway, and a proportion of the enzyme is normally present as the (inactive) apo-enzyme. High intakes of thiamin, leading to more-or-less complete saturation of transketolase with its cofactor, might therefore disturb regulation of pentose metabolism (Berthon et al., 1992).

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