Thiamine pyrophosphate (tpp) is a coenzyme derived from vitamin b1 that is used by the enzyme pyruvate dehydrogenase to convert pyruvate to acetyl coa. tpp has a relatively acidic proton that can produce a carbanion which can act as an electron sink and aid in the decarboxylation of Î±â€“keto acids. the first step of the pyruvate dehydrogenase mechanism is the reaction of pyruvate with tpp, followed by decarboxylation. this gives an intermediate that can donate a two-carbon unit to the lipoamide prosthetic group to form acetyl lipoamide. identify the second intermediate by dragging in the correct structure.