Which cofactor is required for the pyruvate dehydrogenase complex E1 enzyme?

• A. Thiamine pyrophosphate (TPP)
• B. FAD
• C. NAD+
• D. CoA

Answer: (A)

The key idea is that the E1 component of the pyruvate dehydrogenase complex uses a thiamine-derived cofactor to initiate the decarboxylation of pyruvate. Thiamine pyrophosphate (TPP) has a thiazolium ring that forms a reactive ylide, which attacks the carbonyl carbon of pyruvate to remove CO2 and generate the hydroxyethyl-TPP intermediate. This intermediate then transfers the acetyl group to the lipoamide arm of the E2 subunit, starting the transfer of the acetyl group toward acetyl-CoA. The other cofactors play roles in later steps: FAD and NAD+ are used by the E3 component to reoxidize the lipoamide and shuttle electrons, and CoA is the carrier that ultimately accepts the acetyl group to form acetyl-CoA. So TPP is the essential cofactor for the E1-catalyzed decarboxylation step.