Thiamin coenzymes in metabolism and nerve conduction
THE CONTEXT
Thiamin has a central role in energy-yielding metabolism, particularly carbohydrate metabolism. Its diphosphate and triphosphate forms have distinct functions.
Thiamin diphosphate-dependent enzymes
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Thiamin diphosphate is the coenzyme for three multienzyme complexes that catalyze oxidative decarboxylation.
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These complexes are pyruvate dehydrogenase in carbohydrate metabolism, α-ketoglutarate dehydrogenase in the citric acid cycle, and branched-chain keto acid dehydrogenase in leucine, isoleucine, and valine metabolism.
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Thiamin diphosphate is also the coenzyme for transketolase in the pentose phosphate pathway.
Role in oxidative decarboxylation
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A reactive carbon on the thiazole moiety of thiamin diphosphate forms a carbanion that adds to a substrate carbonyl group, such as that of pyruvate.
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The resulting addition compound is decarboxylated, eliminating carbon dioxide.
Thiamin triphosphate in nerves
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Thiamin triphosphate participates in nerve conduction by phosphorylating and activating a chloride channel in the nerve membrane.