Acetylcholinesterase-Catalyzed Hydrolisis of Anilides: Acylation Reaction Dynamics and Intrinsic Chemical Transition-State Structures

Article abstract of DOI:10.1021/ja00235a038

The acylation stages of acetylcholinesterase-catalyzed hydrolyses of the anilides o-nitrochloroacetanilide (ONCA), o-nitroformanilide (ONFA), and o-nitroacetanilide (ONA) are rate limited by virtual transition states that are weighted averages of contributions from solvent isotope sensitive and solvent isotope insensitive microscopic transition states. Increasing concentrations of Me2SO(. hydrolysis increases from 1.31 +/- 0.02 in the absence of Me2SO to 1.94 +/- 0.04 in the presence of 10percent Me2SO (v/v).The solvent isotope effect for ONA hydrolysis changes from 1.55 +/- 0.06 to 1.92 +/- 0.02 in the presence of 5percent Me2SO (v/v). (d) Proton inventories for V/K of ONCA hydrolysis are linear when the reaction medium contains either 1.)percent or 10percent Me2SO (v/v).The ONA proton inventory is linear in the presence of 5percent Me2SO (v/v).These results indicate that inhibition by Me2SO is accompanied by a shift from a virtual transition state to a prominently rate limiting chemical transition state that is stabilized by a single proton bridge.Therefore, AChE behave as a simple general acid-base catalyst during the acylation stage of the mechanism.The solvent isotope effects for V/K and V of ONCA hydrolysis, along with the intrinsic solvent isotope effect determined for the corresponding Me2SO inhibited reaction, allow one to calculate the relative free energies of serial ES complexes that precede the respective acylenzymes.By using these data and relative acylation transition state free energies that are calculated from protor inventories in the absence of Me2SO (described in the preceding paper in this issue), one can formulate free energy diagrams for the acylation stage of AChE-catalyzed hydrolysis of anilides.The picture which emerges from this analysis is that acylation reaction dynamics involve succeassive ES complexes of similar free energies which respectively precede transition states of similar free energies.