Enzyme-catalyzed enolization reactions: a theoretical study on the energetics of concerted and stepwise pathways

Abstract

The enzyme-catalyzed enolization of acetaldehyde has been studied using ab initio methods. The energetics of concerted and stepwise proton transfer pathways are compared, with the participation of a general acidic and/or a general basic catalyst. Two different stepwise pathways are possible and involve the formation of enolate and oxocarbonium intermediates, respectively, while the concerted pathway involves a transition state with partial proton transfer to both the carbonyl group from the general acidic catalyst and from the acidic carbon atom to the general basic catalyst. Potential energy surfaces are constructed at the RHF/6-31G level of theory for two models of the general acid/base portions of the active site; one model involves the ammonium/ammonia pair of molecules representing the general acid and general base, respectively, while the other model involves the acetic acid/acetate pair. Two reaction coordinates, which correspond roughly to the two separate stepwise mechanisms of the proton transfer reaction, are defined and 8 points along each reaction coordinate are mapped out to yield a total of 64 points on the potential energy surface. From this surface, the geometries of the stable intermediates and transition state along the concerted reaction are reoptimized at a higher level of theory, the results of which corroborate the qualitative conclusions made at the lower level of theory. The more exact calculations involve larger basis sets, a correlated wavefunction using second-order Møller-Plesser perturbation theory, and a self consistent reaction field to estimate the effects of solvent. The calculations show that the energy of the transition state for the concerted pathway is significantly lower than that for either stepwise processes, and that stable hydrogen-bonded intermediates are the key to the stability of this pathway. The results provide insight into the mechanisms of enzyme-catalyzed reactions which are initiated by abstraction of a proton from a carbon atom adjacent to a carbonyl or a carboxylic acid group (α-proton of a carbon acid) and help explain the fast rates observed for reactions such as the enolization of acetyldehyde.