O

Acetyl-CoA

NAD+

Pyruvate dehydrogenase, E1

Dihydrolipoyl transacetylase, E2

Dihydrolipoyl dehydrogenase, E3

NAD+

Pyruvate dehydrogenase, E1

Dihydrolipoyl transacetylase, E2

Dihydrolipoyl dehydrogenase, E3

FIGURE 16-6 Oxidative decarboxylation of pyruvate to acetyl-CoA

by the PDH complex. The fate of pyruvate is traced in red. In step ( pyruvate reacts with the bound thiamine pyrophosphate (TPP) of pyruvate dehydrogenase (E-,), undergoing decarboxylation to the hydroxyethyl derivative (see Fig. 14-13). Pyruvate dehydrogenase also carries out step the transfer of two electrons and the acetyl group from TPP to the oxidized form of the lipoyllysyl group of the core enzyme, dihydrolipoyl transacetylase (E2), to form the acetyl thioester of the reduced lipoyl group. Step (3) is a transesterification in which the

—SH group of CoA replaces the —SH group of E2 to yield acetyl-CoA and the fully reduced (dithiol) form of the lipoyl group. In step (( dihydrolipoyl dehydrogenase (E3) promotes transfer of two hydrogen atoms from the reduced lipoyl groups of E2 to the FAD prosthetic group of E3, restoring the oxidized form of the lipoyllysyl group of E2. In step (( the reduced FADH2 of E3 transfers a hydride ion to NAD+, forming NADH. The enzyme complex is now ready for another catalytic cycle. (Subunit colors correspond to those in Fig. 16-5b.)

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