Metabolic functions of thiamin
Thiamin has a central role in energy-yielding metabolism, and especially the metabolism of carbohydrates. Thiamin diphosphate (also known as thiamin pyro-phosphate, see Figure 8.8) is the coenzyme for three oxidative decarboxylation reactions: pyruvate dehy-drogenase in carbohydrate metabolism, α-keto-glutarate dehydrogenase in the citric acid cycle, and the branched-chain keto-acid dehydrogenase involved in the metabolism of leucine, isoleucine, and valine. These three enzymes are multienzyme complexes that catalyze oxidative decarboxylation of the substrate linked to reduction of enzyme-bound lipoamide, and eventually reduction of NAD+ to NADH.
Thiamin diphosphate is also the coenzyme for transketolase, in the pentose phosphate pathway of carbohydrate metabolism. This is the major pathway of carbohydrate metabolism in some tissues, and an important alternative to glycolysis in all tissues, being the source of half of the NADPH required for fatty acid synthesis.
Thiamin triphosphate has a role in nerve conduc-tion, as the phosphate donor for phosphorylation of a nerve membrane sodium transport protein.
Figure 8.8 Thiamin (vitamin B1) and the coenzyme thiamin diphosphate.