Coenzyme-A

The entry point into the TCA cycle involves pyruvate, the end point of glycolysis. The pyruvate is transported across the inner membrane of a mitochondrion where the fuel molecule ATP is made in the process of cellular respiration . The first step with the pyruvate is to decarboxylate it to form a two-carbon acetyl group (-CH₃COO^-). This acetyl group is transferred to a coenyzme-A complex to produce acetyl CoA, the next step toward the TCA cycle.

The coenzyme-A is a complex organic compound derived from the vitamin pantothenic acid. It uses yet another catalyst, the giant multienzyme complex pyruvate dehydrogenase, to accomplish the decarboxylation of of pyruvate and the transfer to the coenzyme-A. From this point the acetyl CoA is produced and initiates the TCA cycle.