Effect of Reactivity on Virtual Transition-State Structure for the Acylation Stage of Acetylcholinesterase-Catalyzed Hydrolysis of Aryl Esters and Anilides

Article abstract of DOI:10.1021/ja00235a037

The acylation stage of acetylcholinesterase-catalyzed hydrolysis of p-methoxyphenyl formate and of three anilides (o-nitrochloroacetanilide, o-nitroacetanilide, and o-nitroformanilide) has been studied by measuring substrate secondary and solvent isotope effects and by determining pL (L = H, D)-rate profiles and Eyring plots.The results of each of these probes support a model for acylation rate determination that involves a virtual transition state that contains contributions from the transition states of sequential physical and chemical steps.Eyring plots for all substrates are nonlinear and are interpreted in terms of temperature-dependent changes in fractional rate determination of sequential microscopic steps.For all substrates acylation reactivity increases sigmoidally with pH and depends on pK_a^H2O = 5.6-5.8, which is well below the intrinsic pK_a = 6.3 of the active site histidine.Solvent isotope effects for the anilide substrates are in the range 1.3-1.6.Proton inventory experiments indicate that intrinsic solvent isotope effects of ca 2 that arise from general acid-base stabilization of the chemical transition state partially masked by a solvent isotope-insensitive transition state that contributes 58-67percent to acylation rate determination.For the most reactive substrate, p-methoxyphenyl formate, the solvent isotope effect is 1.09, which indicates that the solvent isotope-insensitive transition state is almost entirely rate determining.Substrate secondary deuterium kinetic isotope effects are consistent with decreasing nucleophilic interaction at the carbonyl carbon of the scissile bond of the substrate in the virtual acylation transition state with increasing k_cat/K_m.Hence, both solvent and substrate isotope effects indicate a general trend toward less acylation rate determination by chemical transition states as reactivity increases.The virtual transition-state model delineated herein lends quantitative support to Rosenberry's notion that the acylation stage of acetylcholinesterase-catalyzed hydrolysis of neutral substrates is prominently rate limited by an induced fit conformation change that precedes chemical catalysis.