The first reaction requires thiamine pyrophosphate (TPP) as a coenzyme. •The vitamin is structurally complex, but its conversion to the coenzyme form, thiamine pyrophosphate, or TPP, involves simply an ATP-dependent pyrophosphorylation.•Thiamine pyrophosphate is the coenzyme for al decarboxylations of a-keto acids. (氧化脫羧反應)•Unlike b-keto acids, a-keto acids cannot stabilizethe carbanion transition state that develops during decarboxylation, and they thus require the aid of a cofactor (TPP).•The thiazole ring of TPP is the functional part of the coenzyme, allowing it to bind and transfer activated aldehydes.