Investigations on the pyruvate decarboxylase catalysed oxidative decarboxylation of 2-oxoacids by 2.6-dichlorophenolindophenol.

Pyruvate decarboxylase, a thiamine pyrophosphate and Mg2+ dependent enzyme, catalyzes normally the simple decarboxylation of its substrate. However, in the presence of suitable hydrogen acceptors, such as dichlorophenolindophenol, it catalyzes an artificial reaction, the oxidative decarboxylation of 2-oxoacids to the corresponding carboxylic acids. As a result of kinetic studies a mechanism is presented in this paper, which describes the synchronous progress of both the oxidative ping-pong-type decarboxylation reaction and the physiological non-oxidative decarboxylation reaction. Moreover, experiments using phenylglyoxylic acids that carry suitable substituents in the 4-position have shown that the electronic influence of the substituents (causing a decrease in the rate constants with increasing electron pressure) is in quantitative agreement for both types of reactions. A common rate limiting transition state preceding the alpha-carbanion intermediate of the enzymic reaction (2-(1-hydroxybenzyl)-thiamine pyrophosphate carbanion) must therefore be assumed for both reactions. Acetaldehyde which acts as noncompetitive inhibitor in the normal enzyme mechanism does not influence the oxidative decarboxylation reaction. 4'-hydroxy-4'-deamino-thiamine pyrophosphate is inactive as coenzyme for both types of enzyme reactions. This confirms again the essential role of the 4'-amino group in the cofactor function.