Stereochemistry and secondary reactions in the irreversible inhibition of serine hydrolases by organophosphorus compounds

The stereoselectivity of phosphonylation of serine hydrolases by the ROR′P(O)X group of compounds is governed by the electronic properties of X, the size of RO groups, the active site milieu and the specific architecture of the active site of the serine hydrolase. For example, stereoselectivity in excess of 10 4 has been observed favoring the P( S ) diastereomers of 2‐(3,3‐dimethylbutyl) methylphosphonofluoridate (soman) when phosphonylating acetylcholinesterases. Based on molecular dynamics simulations, this is attributable to specific binding of the pinacolyl group to Glu199, Trp84 and Phe331, which exert compression to effect F − departure in the pentacoordinate transient (transition state or intermediate) of soman‐inhibited cholinesterases. The pinacolyl group is not so well adapted at the acyl‐binding site in the P(R) diastereomers reducing the efficiency of F − expulsion. Serine hydrolase inactivation is often followed by secondary reactions. The electronic properties of ligands attached to P are decisive in whether CO or PO bond cleavage occurs in phosphonate diesters of serine hydrolases. Phenoxide ions leave readily with PO cleavage from cholinesterases and chymotrypsin, the reaction resembling deacylation in the reaction of substrates. The architecture and electrostatic character of the active site govern the fate of a covalently attached phosphonyl fragment. Strong negative electrostatic and hydrophobic forces in the cholinesterases preferentially promote CO bond cleavage with occasional methyl migration, whereas this route of dealkylation is nearly absent in phosphonate esters of serine proteases. Dealkylation in soman‐inhibited cholinesterases is 10 orders of magnitude faster than in appropriate model reactions and occurs 10 4 times faster in the P( S ) than in the P( R ) diastereomers. It is driven by enzyme‐facilitated methyl migration, occurring most likely concerted with CO bond cleavage.