Mechanism and stereochemical course at phosphorus of the reaction catalyzed by a bacterial phosphotriesterase

The reaction mechanism for the phosphotriesterase from Pseudomonas diminuta has been examined. When paraoxon (diethyl 4-nitrophenyl phosphate) is hydrolyzed by this enzyme in oxygen-18-labeled water, the oxygen-18 label is found exclusively in the diethyl phosphate product. The absolute configurations for the (+) and (-) enantiomers of O-ethyl phenylphosphonothioic acid have been determined by X-ray diffraction structural determination of the individual crystalline 1-phenylethylamine salts. The (+) enantiomer of the free acid corresponds to the RP configuration. The RP enantiomer of O-ethyl phenylphosphonothioic acid has been converted to the SP enantiomer of EPN O-ethyl O-(4-nitrophenyl) phenylphosphonothioate]. (SP)-EPN is hydrolyzed by the phosphotriesterase to the SP enantiomer of O-ethyl phenylphosphonothioic acid. The enzymatic reaction therefore proceeds with inversion of configuration. These results have been interpreted as an indication of a single in-line displacement by an activated water molecule directly at the phosphorus center of the phosphotriester substrate. (RP)-EPN is not hydrolyzed by the enzyme at an appreciable rate.