Reduction of intrinsic kinetic and thermodynamic barriers for enzyme-catalysed proton transfers from carbon acid substrates

Abstract

Many enzymes catalyse the heterolytic abstraction of the alpha-proton from a carbon acid substrate. Gerlt and Gassman have applied Marcus formalism to such proton transfer reactions to argue that transition states for concerted general acid-general base catalysed enolization at enzyme active sites occur late on the reaction coordinate (J. Am. Chem. Soc. 115 (1993) 11552). We postulate that as an enzyme evolves, it may decrease deltaG++ for a proton transfer step associated with substrate enolization by following the path of steepest descent on the two-dimensional surface corresponding to deltaG++, as defined by Marcus formalism. We show that for an enzyme that has decreased deltaG++ following the path of steepest descent, the values of the intrinsic kinetic (deltaG++(int,E)) and thermodynamic (deltaG(E)0) barriers for proton transfer reactions on the enzyme may be predicted from the known values of deltaG++(int,N) and deltaG(N)0 for the corresponding non-enzymic reaction and the free energy of activation on the enzyme (deltaG++(E)). In addition, the enzymic transition state will occur later on the reaction coordinate than the corresponding non-enzymic transition state (i.e. x++(E)>x++(N)) if the condition (6 - square root 2)/8<x++(N)<(6 + square root 2)/8 is satisfied. For enzyme-catalysed abstraction of the alpha-proton from carbon acid substrates with high pK(a) values (e.g. pK(a) approximately 29), the free energy of activation for the non-enzymic reaction (deltaG++(N)) is dominated by deltaG(N)0. Reduction of deltaG++, via the path of steepest descent will reduce deltaG0 to a greater extent (i.e. differential binding) than deltaG++(int) if deltaG(N)0>2deltaG++(int,N).