Biological Functions

Thiamin functions as the coenzyme TDP in the metabolism of carbohydrates and branched-chain amino acids (a-keto-isocaproic, a-keto-^-methyl valeric, and a-keto-isovaleric acids). In association with Mg2+ ions, TDP is important (1) in various dehydrogenase complexes for the oxidation of a-keto acids (pyruvate, a-ketoglutarate, and the branched-chain a-keto acids) and (2) in the formation of a-ketols among the hexose and pentose phosphates catalyzed by transketolase (EC 2.2.1.1). Thus, a deficiency of thiamin has severe consequences for energy generation and amino acid interconnections, and these have important links with lipid metabolism, cell replication, and neural activity.

Two principal dehydrogenase complexes that require the participation of TDP are pyruvate dehydrogenase, which generates acetyl-CoA, and the oxidative decarboxylation of a-ketoglutarate to succinyl-CoA (Figure 3). Pyruvate dehydrogenase is situated at the junction of the glycolysis pathway,

Figure 3 The four principal sites of action of thiamin diphosphate coenzyme in carbohydrate metabolism.

Cytoplasm

Glucose

, Glucose-6-Glycolysisi phosphate

Pyruvate dehydrogenase complex

Ribose-5-phosphate

/ Transketolase

Pyruvate

Fructose-6-phosphate

Mitochondria

Acetyl CoA

Acetyl CoA

Isovaleryl CoA \

Oxaloacetate

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