ENZYME-CATALYZED ENOLIZATION REACTIONS - A THEORETICAL-STUDY ON THE ENERGETICS OF CONCERTED AND STEPWISE PATHWAYS

The enzyme-catalyzed enolization of acetaldehyde has been studied usin g ab initio methods. The energetics of concerted and stepwise proton t ransfer pathways are compared, with the participation of a general aci dic and/or a general basic catalyst. Two different stepwise pathways a re possible and involve the formation of enolate and oxocarbonium inte rmediates, respectively, while the concerted pathway involves a transi tion state with partial proton transfer to both the carbonyl group fro m the general acidic catalyst and from the acidic carbon atom to the g eneral basic catalyst. Potential energy surfaces are constructed at th e RHF/6-31G level of theory for two models of the general acid/base po rtions of the active site; one model involves the ammonium/ammonia pai r of molecules representing the general acid and general base, respect ively, while the other model involves the acetic acid/acetate pair. Tw o reaction coordinates, which correspond roughly to the two separate s tepwise 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 g eometries of the stable intermediates and transition state along the c oncerted reaction are reoptimized at a higher level of theory, the res ults of which corroborate the qualitative conclusions made at the lowe r level of theory. The more exact calculations involve larger basis se ts, a correlated wavefunction using second-order Moller-Plesset pertur bation theory, and a self consistent reaction field to estimate the ef fects of solvent. The calculations show that the energy of the transit ion state for the concerted pathway is significantly lower than that f or either stepwise processes, and that stable hydrogen-bonded intermed iates are the key to the stability of this pathway. The results provid e 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 (alpha-proton of a carbon acid) an d help explain the fast rates observed for reactions such as the enoli zation of acetaldehyde.