Backbone Conformation Shifts in X-ray Structures of Human Acetylcholinesterase upon Covalent Organophosphate Inhibition

Conformations of C alpha backbones in X-ray structures of most organophosphate (OP)-inhibited human acetylcholinesterases (hAChEs) have been previously shown to be similar to that of the native hAChE. One of the exceptions is the structure of the diethylphosphoryl-hAChE conjugate, where stabilization of a large ethoxy group into the acyl pocket (AP) of hAChE-triggered notable loop distortions and consequential dissociation of the hAChE homodimer. Recently, six X-ray structures of hAChE conjugated with large OP nerve agents of the A-type, Novichoks, have been deposited to PDB. In this study we analyzed backbone conformation shifts in those structures, as well as in OP-hAChE conjugates formed by Paraoxon, Soman, Tabun, and VX. A Java-based pairwise alpha carbon comparison tool (PACCT 3) was used for analysis. Surprisingly, despite the snug fit of large substituents on phosphorus, inside Novichok-conjugated hAChEs only minor conformational changes were detected in their backbones. Small magnitudes of observed changes were due to a 1.2-2.4 & ANGS; shift of the entire conjugated OP away from the AP. It thus appears that the small AP of AChEs can accommodate, without distortion, substituents of the size of ethoxy or butyryl groups, provided that conjugated OP is "pulled " away from the AP. This observation has practical consequences in the structure-based design of nucleophilic reactivation antidotes as well as in the definition of the AChE specificity that relies on the size of its AP.