Ildiko M. Kovach

Structure and Dynamics of Serine Hydrolase-Organophosphate Adducts

The structural profile for the interactions between serine proteases and organophosphorus (OP) compounds can be deduced from recent NMR and X-ray crystallographic data. Using the rationale proposed for serine proteases, dynamic data on the inhibition of acetylcholinesterase by OP compounds is also consistent with structural constraints and an impairment of the proton switch mechanism during phosphorylation.

Serine hydrolase-phosphyl ester interactions: Molecular modeling

Abstract Energies of interactions for trypsin adducts phosphorylated at the active-site Ser-195, seryl 2-propylphosphate anion, with and without a proton on His-57 at the active-site, and seryl diisopropylphosphate with the protonated His-57 were calculated with the molecular mechanics program YETI. The total energy of stabilization is similar for the adducts with protonated His-57, 20–60 kcal mol–1 lower than for the unprotonated adduct, and dependent on the choice of the dielectric parameter. In all three adducts, two H-bonding interactions were calculated in the oxyanion hole to give –3.5 to −4.5 kcal mol−1 mostly dependent on the dielectric parameter. Additional stabilizing interactions between the protonated His-57 and the phosphate anion are −2 kcal mol−1 due to H-bonding and at least −5 kcal mol−1 due to electrostatic forces. For this adduct, the energies of interactions at the active-site total to

COMPETITIVE IRREVERSIBLE INHIBITION OF ENZYMES IN THE PRESENCE OF A SUBSTRATE: SCOPE AND LIMITATIONS

Rapid irreversible inhibition of enzymes constitutes a difficult problem and demands sophisticated techniques to meet contemporary expectations of accuracy and precision. Modern computerized, analytical techniques now allow inhibition to be measured in the presence of a chromogenic substrate, the decomposition product of which can be followed by a conventional method and in a continuous mode. This article has been written to fulfill a need for guidelines to aid the designer of experiments for the irreversible inhibition of enzymes. Thus the scope and limitations of the continuous competitive method for the irreversible inhibition of enzymes is examined here. Examples of acetylcholinesterase inhibition by two diagonally different phosphonate inhibitors are used for illustrating accuracy and precision of the competitive irreversible inhibition technique at different levels of enzyme saturation with inhibitor and substrate.

Transition states of some acyl transfer reactions in acetonitrile 1

Abstract β-Deuterium isotope effect (β-DIE) studies of acyl transfer from aryl acetates to acetate ion in acetonitrile indicate the degree of tetrahedral character at the transition state (TS) to be small, and the same as in protic solvents.

Comparative Study of Nucleophilic am) Enzymic Reactions of 2-Propyl Methylphosphonate Derivatives

Abstract Solvent isotope effects have been measured for the reactions of 4-nitrophenyl 2-propyl methylphosphonate (IMN) with acetylcholinesterase (AChE), chymotrypsin, imidazole base, hydroxide ion, phosphate dianion, and water, and for the reactions of 2-propyl methyl phosphonofluoridate (sarin) with AChE. Kinetics and structural features of the dealkylation of AChE and chymotrypsin adducts were also studied.

Enzymatic determination of the concentration of intact soman and sarin in aqueous solutions

Abstract Intact S-soman at 0.5–13 μM and S- and R-sarin at 50–100 μM in aqueous solutions were determined by a spectrofluorometric method, which detects chymotrypsin before and after its reaction with either inhibitor and the difference in active-site concentration measures concentration of the inhibitor. The chymotryptic method of analysis of organophosphates is sensitive, inexpensive, and specific for intact inhibitors. It is free of interference from organophosphorus decomposition products.