Nerve agents such as soman (GD), sarin (GB) and VX belong to a family of compounds known as organophosphorus compounds (OPs). The OPs are known to inhibit the activity of acetylcholinesterase (AChE), a protein responsible for the hydrolysis of the neurotransmitter acetylcholine. The catalytic center of AChE is a triad comprised of serine-200, histidine-440, and glutamate-327.1
Although acetylcholine is hydrolyzed by AChE at nearly diffusion-controlled rates,2
nerve agents bind to the catalytic serine, thereby inhibiting cholinesterase activity. The inhibited enzyme could be reactivated by a sufficiently strong nucleophile, such as oximes, which are currently employed as therapeutics for OP exposure.3
However, the efficacy of AChE reactivation by various oximes is dependent on the nerve agent with which the cholinesterase was inhibited as well as on the ChE source.3
OPs bound to AChE can also undergo dealkylation,4
a process referred to as aging, which leads to irreversible enzyme inhibition. Due to the rapid aging rates of some nerve agents (with half lives ranging from minutes to days)3
as well as the varying selectivity of oximes towards reactivation of inhibited AChE, it is desirable to develop a protein capable of hydrolyzing a broad spectrum of nerve agents to act as a prophylactic for those in high-risk positions of nerve agent exposure.
Previous studies have shown that the substitution of selenocysteine (U) for cysteine (C) in enzymatic systems has yielded increases in activity varying from 2-fold to several thousand-fold.5
We have therefore determined the variable binding of OPs to serine, cysteine, and selenocysteine analogues (replacing the nucleophile oxygen with sulfur or selenium, respectively) to glean the effects of varying nucleophilic strength on hydrolysis.
Unfortunately, computational modeling of the reaction between OPs and AChE is extremely complex due to the requirement for high accuracy and the size of the protein. Numerous theoretical studies have been conducted on phosphate ester hydrolysis with model nucleophiles; however, most have not addressed the actual mechanism of hydrolysis, but focused instead on the existence of dianionic phosphorane intermediates.6
Similarly, while many studies have examined the general reaction pathway for phosphate ester hydrolysis,6,7
the number of studies that provide data for OP binding to AChE are far more sparse.7,8
It is important to benchmark the predictive abilities of model systems for evaluating the thermochemistry for the reaction between a family of nerve agents and AChE due to the computational expense of treating the full protein structure. In addition, the reaction energetics of OPs with model nucleophiles will serve as a foundation for later calculations, revealing the perturbations created by the protein environment near the active site of cholinesterases. To that end, this study will evaluate the reaction thermochemistry for the binding of 5 common nerve agents with model nucleophiles, including novel cysteine (C) and selenocysteine (U) mutatations of the catalytic serine (S200).