Aging mechanism of butyrylcholinesterase inhibited by an N-methyl analogue of tabun: Implications of the trigonal–bipyramidal transition state rearrangement for the phosphylation or reactivation of cholinesterases

Cholinesterases are the main target of organophosphorus nerve agents (OPs). Their inhibition results in cholinergic syndrome and death. The enzymes are inhibited by phosphylation of the catalytic serine enzyme, but can be reactivated by oximes to some extent. However, phosphylated cholinesterases undergo a side reaction that progressively prevents their reactivatability. This unimolecular reaction, termed “aging”, has been investigated for decades. It was shown that most OP–ChE conjugates aged by O-dealkylation of an alkoxy substituent of the phosphorus atom, a mechanism involving the stabilization of a transient carbocation. In this paper we present structural data supporting a substitution-based mechanism for aging of the huBChE conjugate of an N-mono-methyl analogue of tabun. This mechanism involves an adjacent nucleophilic attack followed by Berry pseudorotation. A similar adjacent attack and subsequent rearrangement of the transition state have been recently proposed for tabun phosphylation of AChE. We suggest that a similar mechanism is also possible for oxime reactivation of phosphylated cholinesterases. This opens new perspectives in terms of reactivator design.