Figure 18.7 Reactions of the pyruvate dehydrogenase complex. At the top (left), the enzyme (represented by a yellow, a green, and two red spheres) is unmodified and ready for a catalytic cycle. (1) Pyruvate is decarboxylated to form hydroxyethyl-TPP by E₁. (2) The lipoamide arm of E₂ moves into the active site of E₁. (3) E₁ catalyzes the transfer of the two-carbon group to the lipoamide group to form the acetyl–lipoamide complex. (4) E₂ catalyzes the transfer of the acetyl moiety to CoA to form the product acetyl CoA. The dihydrolipoamide arm then swings to the active site of E₃. E₃ catalyzes (5) the oxidation of the dihydrolipoamide acid and (6) the transfer of the protons and electrons to NAD⁺ to complete the reaction cycle.