Structural Study of the Complex Stereoselectivity of Human Butyrylcholinesterase for the Neurotoxic V-agents*

Nerve agents are chiral organophosphate compounds (OPs) that exert their acute toxicity by phosphorylating the catalytic serine of acetylcholinesterase (AChE). The inhibited cholinesterases can be reactivated using oximes, but a spontaneous time-dependent process called aging alters the adduct, leading to resistance toward oxime reactivation. Human butyrylcholinesterase (BChE) functions as a bioscavenger, protecting the cholinergic system against OPs. The stereoselectivity of BChE is an important parameter for its efficiency at scavenging the most toxic OPs enantiomer for AChE. Crystals of BChE inhibited in solution or in cristallo with racemic V-agents (VX, Russian VX, and Chinese VX) systematically show the formation of the P_S adduct. In this configuration, no catalysis of aging seems possible as confirmed by the three-dimensional structures of the three conjugates incubated over a period exceeding a week. Crystals of BChE soaked in optically pure VX_R-(+) and VX_S-(−) solutions lead to the formation of the P_S and P_R adduct, respectively. These structural data support an in-line phosphonylation mechanism. Additionally, they show that BChE reacts with VX_R-(+) in the presence of racemic mixture of V-agents, at odds with earlier kinetic results showing a moderate higher inhibition rate for VX_S-(−). These combined results suggest that the simultaneous presence of both enantiomers alters the enzyme stereoselectivity. In summary, the three-dimensional data show that BChE reacts preferentially with P_R enantiomer of V-agents and does not age, in complete contrast to AChE, which is selectively inhibited by the P_S enantiomer and ages.