Enol mechanism, Claisen enzymes

Enol Mechanism for Claisen Enzymes. An alternative mechanism for Claisen enzymes involves initial conversion of the nucleophilic substrate into the corresponding enol. In this case, addition of the carbon of the enol to the carbonyl of the electrophilic reactant can be assisted by acid-base catalysis. Application of the mechanism to the malate synthase reaction is shown in the Scheme... 

There are, however, clear stereomechanistic differences between these two classes of enzyme-catalyzed reactions. The Claisen-type condensations uniformly involve inversion of configuration at the a-carbon of the esteratic substrate, involving C-C bond formation at either the re or the si face of the ketonic or aldehydic substrate (Table VII) (196-211). Moreover, neither Schiff bases nor metal ions have been directly implicated in the catalytic mechanisms of these enzymes. Unlike the aldolases, these enzymes do not catalyze rapid enolization of the nucleophilic substrate in the absence of the second substrate. Inversion of configuration suggests that at least two catalytic groups, perhaps operating in concert, facilitate C-C bond formation. Physicochemical measurements on citrate synthase are consistent with this interpretation of inversion of configuration. 

Such a condensation is mediated by the enzyme 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS). The mechanism of this catalysis is outlined in Figure 1.21. An initial frani-thioesterase step transfers the acetyl group of the first acetyl-CoA to an enzymatic cysteine. In the Claisen condensation phase of the reaction, the a-carbon of a second acetyl-CoA is deprotonated, forming an enolate. The enolate carbon attacks the electrophilic thioester carbon, forming a tetrahedral intermediate which quickly collapses to expel the cysteine thiol [22]. 

In the Claisen condensation catalyzed by the enzyme thiolase, acetyl-CoA is converted to its enolate anion, which then attacks the carbonyl group of a second molecule of acetyl-CoA to form a tetrahedral carbonyl addition intermediate. Collapse of this intermediate by the loss of CoAnSH gives acetoacetyl-CoA. The mechanism for this condensation reaction is exactly the same as that of the Claisen condensation (Section 15.3A) ... 

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