Reactivation of Tabun-inhibited Acetylcholinesterase Investigated by Two Oximes and Mutagenesis

Abstract

The reactivation of tabun-inhibited acetylcholinesterase (AChE) site-directed mutants assisted by two bispyridinium oximes, K048 (N-[4-(4-hydroxyiminomethylpyridinio)butyl]-4-carbamoyl-pyridinium dibromide) and K033 ((N, N´-butano)bis(2-hydroxyiminomethylpyridinium bromide) was studied to analyse the constraints on oxime-assisted reactivation. AChE was modified within the acyl pocket (F295L, F297I) and choline binding site (Y337A) of the active site gorge. Results show that introduced mutations affected both the affinity of phosphorylated enzyme for oximes and the rate of nucleophilic displacement of phosphoryl moiety from the catalytic serine. Mutations significantly lowered the overall reactivation efficacy of K048, but slightly enhanced the potency of K033 to reactivate tabun-inhibited AChE. It seems that the replacement of aromatic residues with the aliphatic ones at the acyl pocket and choline binding site mostly interfered with the stabilization of the oxime’s pyridinium ring(s) within the active site gorge needed to obtain the proper orientation of the oxime group toward the phosphorylated active site serine.