Proximity Effect in Catalysed Systems: a Dramatic Effect on Ester Hydrolysis

Abstract

THE catalytic effectiveness of enzymes arises in part from the bringing together (the “approximation”) of the substrate and catalytic groups of the active site in the formation of the enzyme–substrate complex¹. Much of the thinking behind suggestions concerning the importance of approximation has derived from intra-molecular reactions where the proximity between reactive groupings is caused by covalent bonds. While this analogy is quite appropriate for the breakdown of covalent reaction intermediates—for example, acyl-enzymes²—it is not a very close one for the kinetic steps which immediately follow the formation of the enzyme–substrate complex, nor can the first order rate constants from intra-molecular reactions be compared directly with the second order rate constants from corresponding bimoleeular processes except by the incompletely satisfying concept of “effective molarity”³. The rates of inter-molecular catalytic reactions are, in contrast, much more relevant to any assessment of the kinetic importance of approximation, allow rational comparisons to be made between specific and non-specific catalysts, and provide opportunities for studying the kinetic consequences of approximation by the weak intermolecular forces on which the binding of substrate to enzyme depends⁴. We report here a very large rate enhancement in a non-polymeric system involving the imidazole-catalysed hydrolysis of esters, when the substrate and catalyst are brought together by hydro-phobic forces.