Human acetylcholinesterase (AChE) is a significant target for therapeutic drugs. Here we present high resolution crystal structures of human AChE, alone and in complexes with drug ligands; donepezil, an Alzheimer's disease drug, binds differently to human AChE than it does to Torpedo AChE. These crystals of human AChE provide a more accurate platform for further drug development than previously available.
Over
50 years ago, the toxicity of irreversible organophosphate
inhibitors targeting human acetylcholinesterase (hAChE) was observed
to be stereospecific. The therapeutic reversal of hAChE inhibition
by reactivators has also been shown to depend on the stereochemistry
of the inhibitor. To gain clarity on the mechanism of stereospecific
inhibition, the X-ray crystallographic structures of hAChE inhibited
by a racemic mixture of VX (P
R/S
) and
its enantiomers were obtained. Beyond identifying hAChE structural
features that lend themselves to stereospecific inhibition, structures
of the reactivator HI-6 bound to hAChE inhibited by VX enantiomers
of varying toxicity, or in its uninhibited state, were obtained. Comparison
of hAChE in these pre-reactivation and post-reactivation states along
with enzymatic data reveals the potential influence of unproductive
reactivator poses on the efficacy of these types of therapeutics.
The recognition of structural features related to hAChE’s stereospecificity
toward VX shed light on the molecular influences of toxicity and their
effect on reactivators. In addition to providing a better understanding
of the innate issues with current reactivators, an avenue for improvement
of reactivators is envisioned.
The recent use of organophosphate nerve agents in Syria, Malaysia, Russia, and the United Kingdom has reinforced the potential threat of their intentional release. These agents act through their ability to inhibit human acetylcholinesterase (hAChE; E.C. 3.1.1.7), an enzyme vital for survival. The toxicity of hAChE inhibition via G-series nerve agents has been demonstrated to vary widely depending on the G-agent used. To gain insight into this issue, the structures of hAChE inhibited by tabun, sarin, cyclosarin, soman, and GP were obtained along with the inhibition kinetics for these agents. Through this information, the role of hAChE active site plasticity in agent selectivity is revealed. With reports indicating that the efficacy of reactivators can vary based on the nerve agent inhibiting hAChE, human recombinatorially expressed hAChE was utilized to define these variations for HI-6 among various G-agents. To identify the structural underpinnings of this phenomenon, the structures of tabun, sarin, and somaninhibited hAChE in complex with HI-6 were determined. This revealed how the presence of G-agent adducts impacts reactivator access and placement within the active site. These insights will contribute toward a path of next-generation reactivators and an improved understanding of the innate issues with the current reactivators.
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