Pyruvate produced by glycolysis must be oxidized to acetate prior to entry into the citric acid cycle. The enzyme catalyzing this reaction is the pyruvate dehydrogenase complex. The overall reaction:

Pyruvate + NAD⁺ + CoA-SH <-> Acetyl-CoA + NADH + CO2

is very energetically favorable ( = -33.5 kJ/mol) and is essentially irreversible in vivo. The reaction is somewhat involved. Catalysis of the reaction by the pyruvate dehydrogenase complex involves three enzymatic activities (below) and five coenzymes (CoA-SH, NADH, thiamine pyrophosphate, FAD, lipoic acid). The pyruvate dehydrogenase complex is a highly organized multienzyme assembly.

Enzymatic activities contained in the pyruvate dehydrogenase complex include:

Pyruvate decarboxylase (E1)

Dihydrolipoamide transacetylase (E2)

Dihydrolipoamide dehydrogenase (E3)