Thiamine pyrophosphate is an essential cofactor for enzymes that cat-
alyze the oxidative decarboxylation of a-keto acids to form an acylated
coenzyme A (acyl CoA). These include pyruvate dehydrogenase (pyruvate
^ acetyl-CoA), a-ketoglutarate dehydrogenase (a-KG ^ succinyl-CoA)
and branched-chain a-keto acid dehydrogenase. These three enzymes oper-
ate by a similar catalytic mechanism (Figure 15-2). Each of these enzymes is
a multi-subunit complex with three enzymatic activities that require the par-
ticipation of several cofactors. The first activity (E1) is the dehydrogenase
complex, which has TPP as the enzyme-bound cofactor. The carbanion form
of TPP attacks the carbonyl carbon of the a-keto acid, releasing CO2 and leav-
ing a hydroxyalkyl-TPP intermediate. The hydroxyalkyl group reacts with the
oxidized form of lipoamide, the prosthetic group of dihydrolipoyl transacety-
lase (E2), which is the second component of the complex. The resulting acyl-
lipoamide reacts with coenzyme A (HSCoA) to form the product, acyl CoA,
and leaving lipoamide in the fully reduced form. To regenerate the fully oxi-
dized form of lipoamide for further rounds of the reaction, it interacts with the
third component of the complex, dihydrolipoyl dehydrogenase (E3), which
has covalently bound FAD. The FAD accepts the reducing equivalents from
the reduced lipoamide to form FADH2 and oxidized lipoamide. The reducing
equivalents are then transferred to n Ad + to form NADH, which is regener-
ated via the electron transport system with the production of ATP.
Figure 15-2. The catalytic mechanism shared by the enzymes pyruvate dehy-
drogenase, a-ketoglutarate dehydrogenase, and branched-chain a-ketoacid
dehydrogenase. E1 is the dehydrogenase complex; E2 is the dihydrolipoyl
transacetylase subunit, and E3 is the dihydrolipoyl dehydrogenase component.
E1 and E2 are specific to each enzyme, and E3 is common to all three enzymes.
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