Which vitamin-derived cofactor is essential for the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase complexes?

• A. Thiamine pyrophosphate (TPP)
• B. FAD
• C. Coenzyme A (CoA)
• D. Biotin

Answer: (A)

TPP, the active form of vitamin B1, is required for the decarboxylation step that kicks off both the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase reactions. In the E1 component of these complexes, TPP forms a reactive carbanion on its thiazolium ring that enables the removal of CO2 from pyruvate or α-ketoglutarate, generating a reactive aldehyde- or acyl-TPP intermediate. This intermediate is then transferred along the E2 (lipoamide) arm and ultimately to CoA to form acetyl-CoA or succinyl-CoA, with electrons shuttled through the NAD+/FAD/E3 portion of the complexes. The other cofactors present in these systems—FAD (a riboflavin-derived redox partner in the E3 step), CoA (the acetyl group carrier), and biotin (used in carboxylation reactions, not in these decarboxylations)—play different roles and are not the initiating vitamin-derived cofactor for these dehydrogenase complexes.