The Architecture of Human Acetylcholinesterase Active Center Probed by Interactions with Selected Organophosphate Inhibitors

The role of the functional architecture of human acetylcholinesterase (HuAChE) active center in facilitating reactions with organophosphorus inhibitors was examined by a combination of site-directed mutagenesis and kinetic studies of phosphorylation with organophosphates differing in size of their alkoxy substituents and in the nature of the leaving group. Replacements of residues Phe-295 and Phe-297, constituting the HuAChE acyl pocket, increase up to 80-fold the reactivity of the enzymes toward diisopropyl phosphorofluoridate, diethyl phosphorofluoridate, and p-nitrophenyl diethyl phosphate (paraoxon), indicating the role of this subsite in accommodating the phosphate alkoxy substituent. On the other hand, a decrease of up to 160-fold in reactivity was observed for enzymes carrying replacements of residues Tyr-133, Glu-202, and Glu-450, which are constituents of the hydrogen bond network in the HuAChE active center, which maintains its unique functional architecture. Replacement of residues Trp-86, Tyr-337, and Phe-338 in the alkoxy pocket affected reactivity toward diisopropyl phosphorofluoridate and paraoxon, but to a lesser extent that toward diethyl phosphorofluoridate, indicating that both the alkoxy substituent and the p-nitrophenoxy leaving group interact with this subsite. In all cases the effects on reactivity toward organophosphates, demonstrated in up to 10,000-fold differences in the values of bimolecular rate constants, were mainly a result of altered affinity of the HuAChE mutants, while the apparent first order rate constants of phosphorylation varied within a narrow range. This finding indicates that the main role of the functional architecture of HuAChE active center in phosphorylation is to facilitate the formation of enzyme-inhibitor Michaelis complexes and that this affinity, rather than the nucleophilic activity of the enzyme catalytic machinery, is a major determinant of HuAChE reactivity toward organophosphates.