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Decarboxylation of a-keto acids

Activation e0 e0 e0-P-0-P-0

Thiamin diphosphate (ThDP)

Figure 28.24 • Catalytic cycle of thiamine diphosphate.

Activation e0 e0 e0-P-0-P-0

(aldehyde)

(aldehyde)

All ThDP-dependent enzymes need a proton donor to initiate the first half-reaction and a proton acceptor to complete the second half-reaction. In most cases, the aminopyrimi-dine N4' in concert with an invariant glutamate is the proton donor-acceptor in the first half-reaction, although there are no universally conserved residues for the second half-reaction, and differ according to the enzyme class.143

In the decarboxylation of pyruvate, the coenzyme interacts with pyruvic acid to form the so-called active aldehyde. This active aldehyde intermediates then interacts with thioc-tic acid to form acetyl-thioctate, which is responsible for acetylating CoA-SH to form acetyl-CoA. In deficiency states, the oxidation of a-keto acids is decreased, resulting in increased pyruvate levels in the blood.

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