Thiamine pyrophosphate (TPP) functions as a coenzyme vital to tissue respiration. It is required for the oxidative decarboxylation of pyruvate to form acetyl-coenzyme A, providing entry of oxidizable substrate into the Krebs cycle for the generation of energy. It is also a coenzyme for transketolase, which functions in the pentose phosphate pathway, an alternate pathway for glucose oxidation. Thiamine is most strongly linked with carbohydrate metabolism, but TPP is also required for the oxidative decarboxylation of other alpha-keto acids such as alpha-ketoglutarate and branched-chain alpha-keto acids (i.e., 2-ketocarboxylates) derived from certain amino acids (e.g., valine, isoleucine, leucine).
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