Subunits of the Pyruvate Dehydrogenase Complex.
Pyruvic acid is oxidatively decarboxylated to form Acetyl CoA, by the pyruvate dehydrogenase enzyme complex. This complex is made up of three subunits (trimer), E1, E2, and E3. E1 is pyruvate decarboxylase. This subunit has a coenzyme called thiamine pyrophosphate. Pyruvate binds to H-TPP and is decarboxylated. The carbon is realeased as CO2, leaving a hydroethyl derrivitive. E2 is dihydrolipoyl transacetylase. This subunit has a cofactor called lipoic acid (in the disulphide form), as well as Co enzyme A. Transfer of the hydroethyl derrivitive from H-TPP to lipoic acid results in oxidation of the hydroethyl derrivitive. Co enzyme A is then added to the derrivitive, causing its release from lipoic acid as Acetyl CoA and leaving the disulphide form of lipoic acid in the reduced form. E3 is dihydrolipoate dehydrogenase. It has two cofactors, FAD and NAD. FAD oxidises the reduced lipoic acid and is itself reduced. FADH2 is in turn oxidised back to FAD by NAD which is reduced to NADH+H+. E3 therefore is not directly involved in the oxidative decarboxylation of pyruvic acid, but it functions in preparing E2 for its role in the process.
