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Reduction of intrinsic kinetic and thermodynamic barriers in enzyme-catalyzed proton transfers from carbon acid substrates

Abstract:

Many enzymes catalyze the heterolytic abstraction of the α-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 catalyzed enolization at enzyme active sites occur late on the reaction coordinate [Gerlt, J.A., Gassman, P.G. (1993) J. Am. Chem. Soc. 115, 11552-11568]. We postulate that as an enzyme evolves, it may decrease ∆G for a proton transfer step associated with substrate enolization by following the path of steepest descent on the two-dimensional surface corresponding to ∆G, as defined by Marcus formalism. We show that for an enzyme that has decreased ∆G following the path of steepest descent, the values of the intrinsic kinetic (∆Gint,E) and thermodynamic (∆G°E) barriers for proton transfer reactions on the enzyme may be predicted from the known values of ∆Gint,N and ∆G°N for the corresponding nonenzymatic reaction and the free energy of activation on the enzyme (∆GE). In addition, the enzymatic transition state will occur later on the reaction coordinate than the corresponding nonenzymatic transition state (i.e., xE > xN ) if the condition (6 − sqrt(2)) / 8 < xN < (6 + sqrt(2)) / 8  is satisfied. For enzyme-catalyzed abstraction of the α-proton from carbon acid substrates with high pKa values (e.g., pKa ~ 29), the free energy of activation for the nonenzymatic reaction (∆GN) is dominated by ∆G°N. Reduction of ∆G, via the path of steepest descent, will reduce ∆G° to a greater extent (i.e., differential binding) than ∆Gint if ∆G°N > 2 ∆G‡int,N.

Authors: Stephen L. Bearne, Raymond J. Spiteri

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