Organophosphorus compounds include many synthetic, neurotoxic substances that are commonly used as insecticides. The toxicity of these compounds is due to their ability to inhibit the enzyme acetylcholine esterase. Some of the most toxic organophosphates have been adapted for use as chemical warfare agents; the most well known are GA, GB, GD, GF, VX and VR. All of these compounds contain a chiral phosphorus center with the S P -enantiomers being significantly more toxic than the R P -enantiomers. Phosphotriesterase (PTE) is an enzyme capable of detoxifying these agents, but the stereochemical preference of the wild-type enzyme is for the R P -enantiomers. A series of enantiomerically pure chiral nerve agent analogues has been developed containing the relevant phosphoryl centers found in GB, GD, GF, VX and VR. Wild-type and mutant forms of PTE have been tested for their ability to hydrolyze this series of compounds. Mutant forms of PTE with significantly enhanced, as well as relaxed or reversed stereoselectivity, have been identified. A number of variants showed dramatically improved kinetic constants for the catalytic hydrolysis of the more toxic S P -enantiomers. Improvements of up to three orders of magnitude relative to the wild type enzyme were observed. Some of these mutants were tested against racemic mixtures of GB and GD. The kinetic constants obtained with the chiral nerve agent analogues accurately predict the improved activity and stereoselectivity against the authentic nerve agents used in this study.Organophosphorus compounds have been utilized for more than 50 years as insecticides for the protection of agricultural crops (1) and similar compounds have been developed as chemical warfare agents (2). The structures of these latter compounds are presented in Scheme 1 and include tabun (GA), sarin (GB), soman (GD), cyclosarin (GF), VX and VR. GA has a cyanide leaving group, the three remaining G-agents (GB, GD, and GF) have a fluoride leaving group, and the two versions of VX have a thiolate leaving group. The toxicity of these organophosphonates is due to the inactivation of acetylcholinesterase (AChE), an enzyme that catalyzes the hydrolysis of acetylcholine at neural synapses, through the phosphonylation of an active site serine residue (3). GA, GB, GF, VX, and VR contain a chiral phosphorus center and thus each of these nerve agents has two stereoisomers, while soman has four stereoisomers because of an additional chiral center within the pinacolyl substituent. The enantiomers are differentially toxic; the S Pstereoisomer of sarin reacts with AChE approximately ~10 4 times faster than the R Pstereoisomer and the two S P -stereoisomers of soman react ~10 5 times faster than the two † This work was supported by the NIH (GM 68550).

The bacterial phosphotriesterase (PTE) from Pseudomonas diminuta catalyzes the hydrolysis of organophosphate esters at rates close to the diffusion limit. X-ray diffraction studies have shown that a binuclear metal center is positioned in the active site of PTE and that this complex is responsible for the activation of the nucleophilic water from solvent. In this paper the three dimensional structure of PTE was determined in the presence of the hydrolysis product, diethyl phosphate (DEP), and a product analogue, cacodylate. In the structure of the PTE-diethyl phosphate complex the DEP product is found symmetrically bridging the two divalent cations. The DEP displaces the hydroxide from solvent that normally bridges the two divalent cations in structures determined in the presence or absence of substrate analogues. One of the phosphoryl oxygen atoms in the PTE-DEP complex is 2.0 Å away from the α-metal ion while the other oxygen is 2.2 Å away from the β-metal ion. The two metal ions are separated by a distance of 4.0 Å. A similar structure is observed in the presence of cacodylate. Analogous complexes have previously been observed for the product complexes of isoaspartyl dipeptidase, D-aminoacylase, and dihydroorotase from the amidohydrolase superfamily of enzymes. The experimentally determined structure of the PTE-diethyl phosphate product complex is inconsistent with a recent proposal based upon QM/MM simulations which postulated the formation of an asymmetrical product complex bound exclusively to the β-metal ion with a metalmetal separation of 5.3 Å. This structure is also inconsistent with a chemical mechanism for substrate hydrolysis that utilizes the bridging hydroxide as a base to abstract a proton from a water molecule loosely associated with the α-metal ion. Density functional theory (DFT) calculations support a reaction mechanism that utilizes the bridging hydroxide as the direct nucleophile in the hydrolysis of organophosphate esters by PTE. † This work was supported in part by the NIH (GM71790 and GM68550) and the Robert A. Welch Foundation (A-840). JK was supported by GM76988. Supporting information available Electron density for cacodylate bound to the active site of G60A ( Figure S1) and for diethyl phosphate bound to wild type PTE ( Figures S2A and S2B). This material is available free of charge via the Internet at

Wild-type phosphotriesterase (PTE) preferentially hydrolyzes the R p -enantiomers of the nerve agents sarin (GB) and cyclosarin (GF) and their chromophoric analogues. The active site of PTE can be subdivided into three binding pockets that have been denoted as the small, large and leaving group pockets based on high resolution crystal structures. The sizes and shapes of these pockets dictate the substrate specificity and the stereoselectivity for catalysis. Mutants of PTE have been prepared that exhibit substantial changes in substrate specificity and the ability to differentiate between chiral substrates. For example, the G60A is stereoselective for the hydrolysis of the Rp-enantiomer of the chromophoric analogues of sarin and cyclosarin whereas the H254G/ H257W/L303T (GWT) mutant reverses the stereoselectivity for the enantiomers of these two compounds. Molecular dynamics simulations and high resolution X-ray structures identified the correlations between structural changes in the active site and the experimentally determined kinetic parameters for substrate hydrolysis. New high resolution structures were determined for the H257Y/L303T (YT), I106G/F132G/H257Y (GGY) and H254Q/H257F (QF) mutants of PTE. Molecular dynamics calculations were conducted using the S p -and R p -enantiomers of the analogues for sarin and cyclosarin for the wild-type PTE and the G60A, YT, GGY, QF, and GWT mutants. The experimental stereoselectivity correlated nicely with the difference in the computed angle of attack for the nucleophilic hydroxide relative to the phenolic leaving group of the substrate.Phosphotriesterase (PTE1), isolated originally from soil microbes, catalyzes the hydrolysis of a wide range of organophosphate esters, including agricultural insecticides and chemical warfare agents (1,2). The X-ray crystal structure of [Zn 2+ /Zn 2+ ]-PTE reveals a homodimeric (β/α) 8 -barrel structural fold with a binuclear metal center in the active site (3). The two zinc ions are bridged by a hydroxide and a carbamate functional group, formed by the reaction of CO 2 with the ε-amino group from an active site lysine residue (4). X-ray crystal structures from the Holden laboratory, determined in the presence of inhibitors, have identified three † This work was supported in part by the National Institutes of Health (GM 68550). The X-ray coordinates and structure factors for the QF, GGY, and YT mutants of PTE have been deposited in the Protein Data Bank (PDB accession codes: 2OQL, 2O4M, and 2OB3). The MD simulations for the G60A, H257Y/L303T, H254G/H257W/L303T, I106G/F132G/H257Y mutants are presented in the supporting information in Tables S1-S5. This information is available free of charge via the Internet at http://pubs.acs.org. 1 Abbreviations: PTE, phosphotriesterase; GWT, the H254G/H257W/L303T mutant of PTE; YT, the H257Y/L303T mutant of PTE; GGY, the I106G/F132G/H257Y mutant of PTE; and QF, the H254Q/H257F mutant of PTE. respectively (3). A three dimensional representation of the PTE active site is presented in Figure 1...