Enolization enzyme-catalysed mechanism

Ah initio methods have been used to compare enzyme-catalysed enolization mechanisms.130 Acid- and base-catalysed stepwise mechanisms have been compared with the concerted reaction the latter is favoured by several hydrogen-bonding interactions. 

The mechanism in the frame shows the enol of acetyl CoA attacking the reactive ketone. In nature the enolization is catalysed by a basic carboxylate group (Asp) and an acidic histidine, both part of the enzyme, so that even this easy reaction goes faster. 

This enzyme catalyses the decarboxylation of the ) -ketoacid oxaloacetate, with the same stoichiometry as acetoacetate decarboxylase. The former however, requires a Mn ion for activity and is insensitive to the action of sodium borohydride. This duality of mechanism is not unlike the one observed for enzymatic aldol condensation, where enzymes of Class 1 react by forming Schiff-base intermediates, whereas enzymes of Class II show metal ion requirements [47]. Oxaloacetate decarboxylase from cod also catalyses the reduction by borohydride of the enzymatic reaction product pyruvate. This is evidenced by the accumulation of D-lactate in presence of enzyme, reducing agent, and manganous ions. It has been proposed that both reduction and decarboxylation occur by way of an enzyme-metal ion-substrate complex in which the metal ion acts as an electron sink, thereby stabilizing the enolate ion formed in the decarboxylation reaction [48] ... 

Several possible mechanisms for the formation of triple bonds have been considered. One involves the dehydration of enols corresponding to polj keto acids. Examples of the reverse of this reaction are known. The conversion of acetylene dicarboxylic acid to oxaloacetic acid has been shown to be brought about by extracts of Enterohacteriaceae, and the conversion of acetylene monocarboxyhc acid to malonic semialdehyde is catalysed by an enzyme from a strain of Psetidomonas fluorescens. ... 

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