First reaction requires thiamine pyrophosphate 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.

Unlike b-keto acids, a-keto acids cannot stabilize the 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.