Stereochemical Constraints on the Catalytic Hydrolysis of Organophosphate Nerve Agents by Phosphotriesterase

Abstract

The chiral specificity of the bacterial phosphotriesterase for enantiomeric substrates has been examined. Paraoxon (diethyl p-nitrophenyl phosphate) is the best substrate for this enzyme but this protein is quite tolerant of significant alterations to this structure. One or both of the two ethyl groups in paraoxon can be replaced with various combinations of methyl, isopropyl, or phenyl substituents. The individual enantiomers, as well as racemic mixtures, were tested as substrates for the phosphotriesterase. The enantiomeric preference of the phosphotriesterase depends on the absolute configuration at the phosphorus center and the steric bulk of the alkyl substituents. The kinetic constants for the enzyme-catalyzed hydrolysis of the S_p-(−)-enantiomer of ethyl phenyl p-nitrophenyl phosphate were 1–2 orders of magnitude greater than those for the corresponding R_p-(+)-enantiomer. Similar differences were also obtained for other pairs of enantiomers.

Keywords::

• phosphotriesterase
• organophosphorus nerve agents
• acetylcholinesterase
• stereochemistry