Computer simulations of acetylcholinesterase inhibition by nerve agent soman

Acetylcholinesterase (AChE) is a crucial enzyme in the cholinergic nervous system that hydrolyses neurotransmitter acetylcholine (ACh) and terminates synaptic signals by reducing the effective concentration of ACh in the synaptic clefts. The catalytic serine of AChE can be phosphonylated by organophosphate compounds such as snake venoms, insecticides and nerve agents (nerve toxins) and form stable covalent enzyme-inhibitor adducts. These covalent adducts can either be reactivated by nucleophilic compounds with oxime functional units to some limited extent or undergo a very fast dealkylation reaction, also termed "aging". This phosphonylation and aging process leads to irreversible AChE inhibition, results in accumulation of excess ACh at the synaptic clefts and causes neuromuscular paralysis. By employing Born-Oppenheimer ab initio QM/MM molecular dynamics simulations with umbrella sampling, a state-of-the-art approach to simulate enzyme reactions, we have characterized the phosphonylation reaction mechanism between AChE and soman as well as the aging mechanism of soman phosphonylated AChE and determined their free energy profiles. The phosphonylation reaction between AChE and soman is an associative nucleophilic substitution mechanism that is initiated when the nucleophilic Ser200 attacks soman's phosphorus atom, with His440 acting as a general base. In the elimination step, Try121 of the catalytic gorge forms hydrogen bonds with the leaving fluorine atom prior to its dissociation from the active site. Once a stable covalent adduct forms, the aging reaction begins with scission of the alkoxyl covalent bond, connecting the bound soman's alkyl group to the phosphonate moiety. Cleavage of the alkyl chain is swiftly followed with a methyl group rearrangement of the alkyl group resulting in a stable tertiary carbenium, which is hydrated to tertiary alcohol by a reactive water facilitated by Tyr121. Our characterized mechanisms and simulation results provide new detailed insights into this biologically important process. Mechanistic details are of significant interest for the development of novel strategies to reduce the toxic effects of soman by facilitating the search and design of novel compounds capable of slowing the aging of nerve agent inhibited AChEs as well as the design of effective reactivators for the aged conjugates. Most importantly, this work would facilitate the design of catalytic enzymes capable of hydrolyzing nerve agents

Aging Mechanism of Soman Inhibited Acetylcholinesterase

Acetylcholinesterase (AChE) is a crucial enzyme in the cholinergic nervous system that hydrolyzes neurotransmitter acetylcholine (ACh) and terminates synaptic signals. The catalytic serine of AChE can be phosphonylated by soman, one of the most potent nerve agents, and subsequently undergo an aging reaction. This phosphonylation and aging process leads to irreversible AChE inhibition, results in accumulation of excess ACh at the synaptic clefts, and causes neuromuscular paralysis. By employing Born–Oppenheimer <i>ab initio</i> QM/MM molecular dynamics simulations with umbrella sampling, a state-of-the-art approach to simulate enzyme reactions, we have characterized the aging mechanism of soman phosphonylated AChE and determined its free energy profile. This aging reaction starts with the scission of the O2–Cα bond, which is followed by methyl migration, and results in a tertiary carbenium intermediate. At the transition state, the scissile O2–Cα bond is already cleaved with an average O–C distance of 3.2 ± 0.3 Å and the migrating methyl group is shared between Cα and Cβ carbons with C–C distances of 1.9 ± 0.1 and 1.8 ± 0.1 Å, respectively. The negatively charged phosphonate group is stabilized by a salt bridge with the imidazole ring of the catalytic histidine. A major product of aging, 2,3-dimethyl-2-butanol can be formed swiftly by the reaction of a water molecule. Our characterized mechanism and simulation results provide new detailed insights into this important biochemical process