Understanding enzyme-catalyzed proton abstraction from carbon acids: Details of stepwise mechanisms for β-elimination reactions

The observed rates of enzyme-catalyzed abstraction of a proton from a carbon adjacent to a carbonyl/carboxylic acid group (α-proton of a carbon acid) require that the pK_a of the α-proton be decreased such that it is equal to (±2-3 pK_a units) or less than that of the protonated active site basic catalyst. This can be accomplished by concerted general acid-general base catalysis: Gerlt, J. A.; Kozarich, J. W.; Kenyon, G. L.; Gassman, P. G. J. Am. Chem. Soc. 1991, 113, 9667. Analysis of various enzyme-catalyzed β-elimination reactions using this principle leads to the prediction that these reactions are likely to proceed via stepwise mechanisms which involve initial concerted general acid-general base catalyzed formation of an enol intermediate followed by elimination of the β-substituent. The second step is a vinylogous E2 reaction (1,4-elimination). Thus, these enzymes are not expected to catalyze "simple" E1cb reactions as has often been proposed. The involvement of concerted general acid-general base catalysis in reducing the pK_as of carbon acids may provide insight into the observed stereochemical courses of certain enzyme-catalyzed β-elimination reactions.