One source of acetyl-CoA molecules for the citric acid cycle is oxidative decarboxylation of pyruvate, catalyzed by the pyruvate dehydrogenase complex. The complex is composed of three enzymes

1. Pyruvate decarboxylase (E1)

2. Dihydrolipoamide transacetylase (E2)

3. Dihydrolipoamide dehydrogenase (E3)

and five coenzymes

1. Thiamine Pyrophosphate (TPP)

2. Lipoic Acid - lipoamide

3. FAD

4. NAD⁺

5. CoASH

The reaction mechanism is described here and is depicted in Figure 14.10.

The pyruvate dehydrogenase complex resembles the -ketoglutarate dehydrogenase complex (AKGDH). The three enzyme activities are analogous and the five coenzymes are the same. The pyruvate dehydrogenase complex also catalyzes a similar reaction - oxidative decarboxylation of an -keto acid. The pyruvate dehydrogenase complex differs from the AKGDH in being regulated allosterically and and controlled via covalent modification.