Stereoselective Detoxification of Chiral Sarin and Soman Analogues by Phosphotriesterase

Abstract

The catalytic activity of the bacterial phosphotriesterase (PTE) toward a series of chiral analogues of the chemical warfare agents sarin and soman was measured. Chemical procedures were developed for the chiral syntheses of the S P- and R P-enantiomers of O-isopropyl p-nitrophenyl methylphosphonate (sarin analogue) in high enantiomeric excess. The R P-enantiomer of the sarin analogue ( k cat=2600 s −1) was the preferred substrate for the wild-type PTE relative to the corresponding S P-enantiomer ( k cat=290 s −1). The observed stereoselectivity was reversed using the PTE mutant, I106A/F132A/H254Y where the k cat values for the R P- and S P-enantiomers were 410 and 4200 s −1, respectively. A chemo-enzymatic procedure was developed for the chiral synthesis of the four stereoisomers of O-pinacolyl p-nitrophenyl methylphosphonate (soman analogue) with high diastereomeric excess. The R P R C-stereoisomer of the soman analogue was the preferred substrate for PTE. The k cat values for the soman analogues were measured as follows: R P R C, 48 s −1; R P S C, 4.8 s −1; S P R C, 0.3 s −1, and S P S C, 0.04 s −1. With the I106A/F132A/H254Y mutant of PTE the stereoselectivity toward the chiral phosphorus center was reversed. With the triple mutant the k cat values for the soman analogues were found to be as follows: R P R C, 0.3 s −1; R P S C, 0.3 s −1; S P R C, 11 s −1, and S P S C, 2.1 s −1. Prior investigations have demonstrated that the S P-enantiomers of sarin and soman are significantly more toxic than the R P-enantiomers. This investigation has demonstrated that mutants of the wild-type PTE can be readily constructed with enhanced catalytic activities toward the most toxic stereoisomers of sarin and soman.