Inhibitory Potency against Human Acetylcholinesterase and Enzymatic Hydrolysis of Fluorogenic Nerve Agent Mimics by Human Paraoxonase 1 and Squid Diisopropyl Fluorophosphatase

Effective remediation and decontamination of organophosphorus compounds using enzymes: From rational design to potential applications

Organophosphorus compounds (OPs) have been widely used in agriculture for decades because of their high insecticidal efficiency, which maintains and increases crop yields worldwide. More importantly, OPs, as typical chemical warfare agents, are a serious concern and significant danger for military and civilian personnel. The widespread use of OPs, superfluous and unreasonable use, has caused great harm to the environment and food chain. Developing efficient and environmentally friendly solutions for the decontamination of OPs is a long-term challenge. Microbial enzymes show potential application as natural and green biocatalysts. Thus, utilizing OP-degrading enzymes for environmental decontamination presents significant advantages, as these enzymes can rapidly hydrolyze OPs; are environmentally friendly, nonflammable, and noncorrosive; and can be discarded safely and easily. Here, the properties, structure and catalytic mechanism of various typical OP-degrading enzymes are reviewed. The methods and effects utilized to improve the expression level, catalytic performance and stability of OP-degrading enzymes were systematically summarized. In addition, the immobilization of OP-degrading enzymes was explicated emphatically, and the latest progress of cascade reactions based on immobilized enzymes was discussed. Finally, the latest applications of OP-degrading enzymes were summarized, including biosensors, nanozyme mimics and medical detoxification. This review provides guidance for the future development of OP-degrading enzymes and promotes their application in the field of environmental bioremediation and medicine.

Enzymatic Degradation of Organophosphorus Pesticides and Nerve Agents by EC: 3.1.8.2

The organophosphorus substances, including pesticides and nerve agents (NAs), represent highly toxic compounds. Standard decontamination procedures place a heavy burden on the environment. Given their continued utilization or existence, considerable efforts are being made to develop environmentally friendly methods of decontamination and medical countermeasures against their intoxication. Enzymes can offer both environmental and medical applications. One of the most promising enzymes cleaving organophosphorus compounds is the enzyme with enzyme commission number (EC): 3.1.8.2, called diisopropyl fluorophosphatase (DFPase) or organophosphorus acid anhydrolase from Loligo Vulgaris or Alteromonas sp. JD6.5, respectively. Structure, mechanisms of action and substrate profiles are described for both enzymes. Wild-type (WT) enzymes have a catalytic activity against organophosphorus compounds, including G-type nerve agents. Their stereochemical preference aims their activity towards less toxic enantiomers of the chiral phosphorus center found in most chemical warfare agents. Site-direct mutagenesis has systematically improved the active site of the enzyme. These efforts have resulted in the improvement of catalytic activity and have led to the identification of variants that are more effective at detoxifying both G-type and V-type nerve agents. Some of these variants have become part of commercially available decontamination mixtures.

Theoretical Studies on Catalysis Mechanisms of Serum Paraoxonase 1 and Phosphotriesterase Diisopropyl Fluorophosphatase Suggest the Alteration of Substrate Preference from Paraoxonase to DFP

The calcium-dependent β-propeller proteins mammalian serum paraoxonase 1 (PON1) and phosphotriesterase diisopropyl fluorophosphatase (DFPase) catalyze the hydrolysis of organophosphorus compounds and enhance hydrolysis of various nerve agents. In the present work, the phosphotriesterase activity development between PON1 and DFPase was investigated by using the hybrid density functional theory method B3LYP. Based on the active-site difference between PON1 and DFPase, both the wild type and the mutant (a water molecule replacing Asn270 in PON1) models were designed. The results indicated that the substitution of a water molecule for Asn270 in PON1 had little effect on the enzyme activity in kinetics, while being more efficient in thermodynamics, which is essential for DFP hydrolysis. Structure comparisons of evolutionarily related enzymes show that the mutation of Asn270 leads to the catalytic Ca²⁺ ion indirectly connecting the buried structural Ca²⁺ ion via hydrogen bonds in DFPase. It can reduce the plasticity of enzymatic structure, and possibly change the substrate preference from paraoxon to DFP, which implies an evolutionary transition from mono- to dinuclear catalytic centers. Our studies shed light on the investigation of enzyme catalysis mechanism from an evolutionary perspective.

Similar Active Sites and Mechanisms Do Not Lead to Cross-Promiscuity in Organophosphate Hydrolysis: Implications for Biotherapeutic Engineering

Organophosphate hydrolases are proficient catalysts of the breakdown of neurotoxic organophosphates and have great potential as both biotherapeutics for treating acute organophosphate toxicity and as bioremediation agents. However, proficient organophosphatases such as serum paraoxonase 1 (PON1) and the organophosphate-hydrolyzing lactonase SsoPox are unable to hydrolyze bulkyorganophosphates with challenging leaving groups such as diisopropyl fluorophosphate (DFP) or venomous agent X, creating a major challenge for enzyme design. Curiously, despite their mutually exclusive substrate specificities, PON1 and diisopropyl fluorophosphatase (DFPase) have essentially identical active sites and tertiary structures. In the present work, we use empirical valence bond simulations to probe the catalytic mechanism of DFPase as well as temperature, pH, and mutational effects, demonstrating that DFPase and PON1 also likely utilize identical catalytic mechanisms to hydrolyze their respective substrates. However, detailed examination of both static structures and dynamical simulations demonstrates subtle but significant differences in the electrostatic properties and solvent penetration of the two active sites and, most critically, the role of residues that make no direct contact with either substrate in acting as "specificity switches" between the two enzymes. Specifically, we demonstrate that key residues that are structurally and functionally critical for the paraoxonase activity of PON1 prevent it from being able to hydrolyze DFP with its fluoride leaving group. These insights expand our understanding of the drivers of the evolution of divergent substrate specificity in enzymes with identical active sites and guide the future design of organophosphate hydrolases that hydrolyze compounds with challenging leaving groups.

Computational enzymology for degradation of chemical warfare agents: promising technologies for remediation processes

Chemical weapons are a major worldwide problem, since they are inexpensive, easy to produce on a large scale and difficult to detect and control. Among the chemical warfare agents, we can highlight the organophosphorus compounds (OP), which contain the phosphorus element and that have a large number of applications. They affect the central nervous system and can lead to death, so there are a lot of works in order to design new effective antidotes for the intoxication caused by them. The standard treatment includes the use of an anticholinergic combined to a central nervous system depressor and an oxime. Oximes are compounds that reactivate Acetylcholinesterase (AChE), a regulatory enzyme responsible for the transmission of nerve impulses, which is one of the molecular targets most vulnerable to neurotoxic agents. Increasingly, enzymatic treatment becomes a promising alternative; therefore, other enzymes have been studied for the OP degradation function, such as phosphotriesterase (PTE) from bacteria, human serum paraoxonase 1 (HssPON1) and diisopropyl fluorophosphatase (DFPase) that showed significant performances in OP detoxification. The understanding of mechanisms by which enzymes act is of extreme importance for the projection of antidotes for warfare agents, and computational chemistry comes to aid and reduce the time and costs of the process. Molecular Docking, Molecular Dynamics and QM/MM (quantum-mechanics/molecular-mechanics) are techniques used to investigate the molecular interactions between ligands and proteins.

Progress in the development of enzyme-based nerve agent bioscavengers

Acetylcholinesterase is the physiological target for acute toxicity of nerve agents. Attempts to protect acetylcholinesterase from phosphylation by nerve agents, is currently achieved by reversible inhibitors that transiently mask the enzyme active site. This approach either protects only peripheral acetylcholinesterase or may cause side effects. Thus, an alternative strategy consists in scavenging nerve agents in the bloodstream before they can reach acetylcholinesterase. Pre- or post-exposure administration of bioscavengers, enzymes that neutralize and detoxify organophosphorus molecules, is one of the major developments of new medical counter-measures. These enzymes act either as stoichiometric or catalytic bioscavengers. Human butyrylcholinesterase is the leading stoichiometric bioscavenger. Current efforts are devoted to its mass production with care to pharmacokinetic properties of the final product for extended lifetime. Development of specific reactivators of phosphylated butyrylcholinesterase, or variants with spontaneous reactivation activity is also envisioned for rapid in situ regeneration of the scavenger. Human paraoxonase 1 is the leading catalytic bioscavenger under development. Research efforts focus on improving its catalytic efficiency toward the most toxic isomers of nerve agents, by means of directed evolution-based strategies. Human prolidase appears to be another promising human enzyme. Other non-human efficient enzymes like bacterial phosphotriesterases or squid diisopropylfluorophosphatase are also considered though their intrinsic immunogenic properties remain challenging for use in humans. Encapsulation, PEGylation and other modifications are possible solutions to address this problem as well as that of their limited lifetime. Finally, gene therapy for in situ generation and delivery of bioscavengers is for the far future, but its proof of concept has been established.

Mechanistic Insights into the Hydrolysis of Organophosphorus Compounds by Paraoxonase-1: Exploring the Limits of Substrate Tolerance in a Promiscuous Enzyme

We designed, synthesized and screened a library of analogs of the organophosphate pesticide metabolite paraoxon against a recombinant variant of human serum paraoxonase-1. Alterations of both the aryloxy leaving group and the retained alkyl chains of paraoxon analogs resulted in substantial changes to binding and hydrolysis, as measured directly by spectrophotometric methods or in competition experiments with paraoxon. Increases or decreases in the steric bulk of the retained groups generally reduced the rate of hydrolysis, while modifications of the leaving group modulated both binding and turnover. Studies on the hydrolysis of phosphoryl azide analogs as well as amino-modified paraoxon analogs, the former being developed as photo-affinity labels, found enhanced tolerance of structural modifications, when compared with O-alkyl substituted molecules. Results from computational modeling predict a predominant active site binding mode for these molecules which is consistent with several proposed catalytic mechanisms in the literature, and from which a molecular-level explanation of the experimental trends is attempted. Overall, the results of this study suggest that while paraoxonase-1 is a promiscuous enzyme, there are substantial constraints in the active site pocket, which may relate to both the leaving group and the retained portion of paraoxon analogs.

Evolved stereoselective hydrolases for broad-spectrum G-type nerve agent detoxification

A preferred strategy for preventing nerve agents intoxication is catalytic scavenging by enzymes that hydrolyze them before they reach their targets. Using directed evolution, we simultaneously enhanced the activity of a previously described serum paraoxonase 1 (PON1) variant for hydrolysis of the toxic S(P) isomers of the most threatening G-type nerve agents. The evolved variants show ≤340-fold increased rates and catalytic efficiencies of 0.2-5 × 10(7) M(-1) min(-1). Our selection for prevention of acetylcholinesterase inhibition also resulted in the complete reversion of PON1's stereospecificity, from an enantiomeric ratio (E) < 6.3 × 10(-4) in favor of the R(P) isomer of a cyclosarin analog in wild-type PON1, to E > 2,500 for the S(P) isomer in an evolved variant. Given their ability to hydrolyze G-agents, these evolved variants may serve as broad-range G-agent prophylactics.

Catalytic mechanisms for phosphotriesterases

Phosphotriesters are one class of highly toxic synthetic compounds known as organophosphates. Wide spread usage of organophosphates as insecticides as well as nerve agents has lead to numerous efforts to identify enzymes capable of detoxifying them. A wide array of enzymes has been found to have phosphotriesterase activity including phosphotriesterase (PTE), methyl parathion hydrolase (MPH), organophosphorus acid anhydrolase (OPAA), diisopropylfluorophosphatase (DFP), and paraoxonase 1 (PON1). These enzymes differ widely in protein sequence and three-dimensional structure, as well as in catalytic mechanism, but they also share several common features. All of the enzymes identified as phosphotriesterases are metal-dependent hydrolases that contain a hydrophobic active site with three discrete binding pockets to accommodate the substrate ester groups. Activation of the substrate phosphorus center is achieved by a direct interaction between the phosphoryl oxygen and a divalent metal in the active site. The mechanistic details of the hydrolytic reaction differ among the various enzymes with both direct attack of a hydroxide as well as covalent catalysis being found. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases.

Detection of organophosphorus compounds using a molecularly imprinted photonic crystal

A label free molecularly imprinted photonic crystal (MIPC) was developed to detect the degradation product of nerve agents. Mono-dispersed poly-methyl methacrylate colloidal particles with the diameter of 280 nm were used to fabricate a closely packed colloidal crystal array (CCA), and a methyl phosphonic acid (MPA) imprinted hydrogel was prepared within the CCA using 2-hydroxyethyl-methacrylate and N-isopropylacrylamide as monomers, ethyleneglycol dimethacrylate and N, N'-methylenebisacrylamide as cross-linkers, a mixture of n-octanol and acetonitrile as porogen. The diffraction intensity of the MIPC decreased significantly upon the MPA adsorption with a limit of detection (LOD) of 10(-6) molL(-1). Furthermore, the diffraction intensity decreased and blue shifted with the increase of temperature, decreased and red shifted with the increase of ionic strength. At higher pH, the diffraction intensity increased without obvious diffraction shift. The MIPC provides an indirect path to detect nerve agents (Sarin, Soman, VX and R-VX) by monitoring the MPA released from the hydrolysis of nerve agents, with LODs of 3.5 × 10(-6) molL(-1), 2.5 × 10(-5) molL(-1), 7.5 × 10(-5) molL(-1) and 7.5 × 10(-5) molL(-1) for Sarin, Soman, VX and R-VX, respectively.

The evolution of function in strictosidine synthase-like proteins

The exponential growth of sequence data provides abundant information for the discovery of new enzyme reactions. Correctly annotating the functions of highly diverse proteins can be difficult, however, hindering use of this information. Global analysis of large superfamilies of related proteins is a powerful strategy for understanding the evolution of reactions by identifying catalytic commonalities and differences in reaction and substrate specificity, even when only a few members have been biochemically or structurally characterized. A comparison of >2500 sequences sharing the six-bladed β-propeller fold establishes sequence, structural, and functional links among the three subgroups of the functionally diverse N6P superfamily: the arylesterase-like and senescence marker protein-30/gluconolactonase/luciferin-regenerating enzyme-like (SGL) subgroups, representing enzymes that catalyze lactonase and related hydrolytic reactions, and the so-called strictosidine synthase-like (SSL) subgroup. Metal-coordinating residues were identified as broadly conserved in the active sites of all three subgroups except for a few proteins from the SSL subgroup, which have been experimentally determined to catalyze the quite different strictosidine synthase (SS) reaction, a metal-independent condensation reaction. Despite these differences, comparison of conserved catalytic features of the arylesterase-like and SGL enzymes with the SSs identified similar structural and mechanistic attributes between the hydrolytic reactions catalyzed by the former and the condensation reaction catalyzed by SS. The results also suggest that despite their annotations, the great majority of these >500 SSL sequences do not catalyze the SS reaction; rather, they likely catalyze hydrolytic reactions typical of the other two subgroups instead. This prediction was confirmed experimentally for one of these proteins.

Production of ES1 plasma carboxylesterase knockout mice for toxicity studies

The LD(50) for soman is 10-20-fold higher for a mouse than a human. The difference in susceptibility is attributed to the presence of carboxylesterase in mouse but not in human plasma. Our goal was to make a mouse lacking plasma carboxylesterase. We used homologous recombination to inactivate the carboxylesterase ES1 gene on mouse chromosome 8 by deleting exon 5 and by introducing a frame shift for amino acids translated from exons 6 to 13. ES1-/- mice have no detectable carboxylesterase activity in plasma but have normal carboxylesterase activity in tissues. Homozygous ES1-/- mice and wild-type littermates were tested for response to a nerve agent model compound (soman coumarin) at 3 mg/kg sc. This dose intoxicated both genotypes but was lethal only to ES1-/- mice. This demonstrated that plasma carboxylesterase protects against a relatively high toxicity organophosphorus compound. The ES1-/- mouse should be an appropriate model for testing highly toxic nerve agents and for evaluating protection strategies against the toxicity of nerve agents.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.

Evolution of and perspectives on therapeutic approaches to nerve agent poisoning

After more than 70 years of considerable efforts, research on medical defense against nerve agents has come to a standstill. Major progress in medical countermeasures was achieved between the 50s and 70s with the development of anticholinergic drugs and carbamate-based pretreatment, the introduction of pyridinium oximes as antidotes, and benzodiazepines in emergency treatments. These drugs ensure good protection of the peripheral nervous system and mitigate the acute effects of exposure to lethal doses of nerve agents. However, pyridostigmine and cholinesterase reactivators currently used in the armed forces do not protect/reactivate central acetylcholinesterases. Moreover, other drugs used are not sufficiently effective in protecting the central nervous system against seizures, irreversible brain damages and long-term sequelae of nerve agent poisoning.New developments of medical counter-measures focus on: (a) detoxification of organophosphorus molecules before they react with acetylcholinesterase and other physiological targets by administration of stoichiometric or catalytic scavengers; (b) protection and reactivation of central acetylcholinesterases, and (c) improvement of neuroprotection following delayed therapy.Future developments will aim at treatment of acute and long-term effects of low level exposure to nerve agents, research on alternative routes for optimizing drug delivery, and therapies. Though gene therapy for in situ generation of bioscavengers, and cell therapy based on neural progenitor engraftment for neuronal regeneration have been successfully explored, more studies are needed before practical medical applications can be made of these new approaches.

Phosphotriesterase variants with high methylphosphonatase activity and strong negative trade-off against phosphotriesters

The most lethal organophosphorus nerve agents (NA), like sarin, soman, agent-VX and Russian-VX, share a methylphosphonate moiety. Pseudomonas diminuta phosphotriesterase (PTE) catalyses the hydrolysis of methylphosphonate NA analogues with a catalytic efficiency orders of magnitude lower than that towards the pesticide paraoxon. With a view to obtaining PTE variants that more readily accept methylphosphonate NA, ~75,000 PTE variants of the substrate-binding residues Gly-60, Ile-106, Leu-303 and Ser-308 were screened with fluorogenic analogues of the NA Russian-VX and cyclosarin. Seven new PTE variants were isolated, purified and their k(cat)/K(M) determined against five phosphotriesters and five methylphosphonate analogues of sarin, cyclosarin, soman, agent-VX and Russian-VX. The novel PTE variants exhibited as much as a 10-fold increase in activity towards the methylphosphonate compounds--many reaching a k(cat)/K(M) of 10⁶ M⁻¹ s⁻¹--and as much as a 29,000-fold decrease in their phosphotriesterase activity. The mutations found in two of the variants, SS0.5 (G60V/I106L/S308G) and SS4.5 (G60V/I106A/S308G), were modelled into a high-resolution structure of PTE-wild type and docked with analogues of cyclosarin and Russian-VX using Autodock 4.2. The kinetic data and docking simulations suggest that the increase in activity towards the methylphosphonates and the loss of function against the phosphotriesters were due to an alteration of the shape and hydrophobicity of the binding pocket that hinders the productive binding of non-chiral racemic phosphotriesters, yet allows the binding of the highly asymmetric methylphosphonates.

Enzyme promiscuity: a mechanistic and evolutionary perspective

Many, if not most, enzymes can promiscuously catalyze reactions, or act on substrates, other than those for which they evolved. Here, we discuss the structural, mechanistic, and evolutionary implications of this manifestation of infidelity of molecular recognition. We define promiscuity and related phenomena and also address their generality and physiological implications. We discuss the mechanistic enzymology of promiscuity--how enzymes, which generally exert exquisite specificity, catalyze other, and sometimes barely related, reactions. Finally, we address the hypothesis that promiscuous enzymatic activities serve as evolutionary starting points and highlight the unique evolutionary features of promiscuous enzyme functions.

X-ray structure of perdeuterated diisopropyl fluorophosphatase (DFPase): perdeuteration of proteins for neutron diffraction

The signal-to-noise ratio is one of the limiting factors in neutron macromolecular crystallography. Protein perdeuteration, which replaces all H atoms with deuterium, is a method of improving the signal-to-noise ratio of neutron crystallography experiments by reducing the incoherent scattering of the hydrogen isotope. Detailed analyses of perdeuterated and hydrogenated structures are necessary in order to evaluate the utility of perdeuterated crystals for neutron diffraction studies. The room-temperature X-ray structure of perdeuterated diisopropyl fluorophosphatase (DFPase) is reported at 2.1 A resolution. Comparison with an independently refined hydrogenated room-temperature structure of DFPase revealed no major systematic differences, although the crystals of perdeuterated DFPase did not diffract neutrons. The lack of diffraction is examined with respect to data-collection and crystallographic parameters. The diffraction characteristics of successful neutron structure determinations are presented as a guideline for future neutron diffraction studies of macromolecules. X-ray diffraction to beyond 2.0 A resolution appears to be a strong predictor of successful neutron structures.

Computational Modeling of Human Paraoxonase 1: Preparation of Protein Models, Binding Studies, and Mechanistic Insights

The enzyme human paraoxonase 1 (huPON1) has demonstrated significant potential for use as a bioscavenger for treatment of exposure to organophosphorus (OP) nerve agents. Herein we report the development of protein models for the human isoform derived from a crystal structure of a chimeric version of the protein (pdb ID: 1V04) and a homology model derived from the related enzyme diisopropylfluorophosphatase (pdb ID: 1XHR). From these structural models, binding modes for OP substrates are predicted, and these poses are found to orient substrates in proximity to residues known to modulate specificity of the enzyme. Predictions are made with regard to the role that residues play in altering substrate binding and turnover, in particular with regard to the stereoselectivity of the enzyme, and the known differences in activity related to a natural polymorphism in the enzyme. Potential mechanisms of action of the protein for catalytic hydrolysis of OP substrates are also evaluated in light of the proposed binding modes.

Stereo-specific synthesis of analogs of nerve agents and their utilization for selection and characterization of paraoxonase (PON1) catalytic scavengers

Fluorogenic organophosphate inhibitors of acetylcholinesterase (AChE) homologous in structure to nerve agents provide useful probes for high throughput screening of mammalian paraoxonase (PON1) libraries generated by directed evolution of an engineered PON1 variant with wild-type like specificity (rePON1). Wt PON1 and rePON1 hydrolyze preferentially the less-toxic R(P) enantiomers of nerve agents and of their fluorogenic surrogates containing the fluorescent leaving group, 3-cyano-7-hydroxy-4-methylcoumarin (CHMC). To increase the sensitivity and reliability of the screening protocol so as to directly select rePON1 clones displaying stereo-preference towards the toxic S(P) enantiomer, and to determine accurately K(m) and k(cat) values for the individual isomers, two approaches were used to obtain the corresponding S(P) and R(P) isomers: (a) stereo-specific synthesis of the O-ethyl, O-n-propyl, and O-i-propyl analogs and (b) enzymic resolution of a racemic mixture of O-cyclohexyl methylphosphonylated CHMC. The configurational assignments of the S(P) and R(P) isomers, as well as their optical purity, were established by X-ray diffraction, reaction with sodium fluoride, hydrolysis by selected rePON1 variants, and inhibition of AChE. The S(P) configuration of the tested surrogates was established for the enantiomer with the more potent anti-AChE activity, with S(P)/R(P) inhibition ratios of 10-100, whereas the R(P) isomers of the O-ethyl and O-n-propyl were hydrolyzed by wt rePON1 about 600- and 70-fold faster, respectively, than the S(P) counterpart. Wt rePON1-induced R(P)/S(P) hydrolysis ratios for the O-cyclohexyl and O-i-propyl analogs are estimated to be >>1000. The various S(P) enantiomers of O-alkyl-methylphosphonyl esters of CHMC provide suitable ligands for screening rePON1 libraries, and can expedite identification of variants with enhanced catalytic proficiency towards the toxic nerve agents.

Stable adducts of nerve agents sarin, soman and cyclosarin with TRIS, TES and related buffer compounds--characterization by LC-ESI-MS/MS and NMR and implications for analytical chemistry

Buffering compounds like TRIS are frequently used in chemical, biochemical and biomedical applications to control pH in solution. One of the prerequisites of a buffer compound, in addition to sufficient buffering capacity and pH stability over time, is its non-reactivity with other constituents of the solution. This is especially important in the field of analytical chemistry where analytes are to be determined quantitatively. Investigating the enzymatic hydrolysis of G-type nerve agents sarin, soman and cyclosarin in buffered solution we have identified stable buffer adducts of TRIS, TES and other buffer compounds with the nerve agents. We identified the molecular structure of these adducts as phosphonic diesters using 1D (1)H-(31)P HSQC NMR and LC-ESI-MS/MS techniques. Reaction rates with TRIS and TES are fast enough to compete with spontaneous hydrolysis in aqueous solution and to yield substantial amounts (up to 20-40%) of buffer adduct over the course of several hours. A reaction mechanism is proposed in which the amino function of the buffer serves as an intramolecular proton acceptor rendering the buffer hydroxyl groups nucleophilic enough for attack on the phosphorus atom of the agents. Results show that similar buffer adducts are formed with a range of hydroxyl and amino function containing buffers including TES, BES, TRIS, BIS-TRIS, BIS-TRIS propane, Tricine, Bicine, HEPES and triethanol amine. It is recommended to use alternative buffers like MOPS, MES and CHES when working with G-type nerve agents especially at higher concentrations and over prolonged times.

Reversed enantioselectivity of diisopropyl fluorophosphatase against organophosphorus nerve agents by rational design

Diisopropyl fluorophosphatase (DFPase) from Loligo vulgaris is an efficient and robust biocatalyst for the hydrolysis of a range of highly toxic organophosphorus compounds including the nerve agents sarin, soman, and cyclosarin. In contrast to the substrate diisopropyl fluorophosphate (DFP) the nerve agents possess an asymmetric phosphorus atom, which leads to pairs of enantiomers that display markedly different toxicities. Wild-type DFPase prefers the less toxic stereoisomers of the substrates which leads to slower detoxification despite rapid hydrolysis. Enzyme engineering efforts based on rational design yielded two quadruple enzyme mutants with reversed enantioselectivity and overall enhanced activity against tested nerve agents. The reversed stereochemical preference is explained through modeling studies and the crystal structures of the two mutants. Using the engineered mutants in combination with wild-type DFPase leads to significantly enhanced activity and detoxification, which is especially important for personal decontamination. Our findings may also be of relevance for the structurally related enzyme human paraoxonase (PON), which is of considerable interest as a potential catalytic in vivo scavenger in case of organophosphorus poisoning.

Molecular interactions of cholinesterases inhibitors using in silico methods: current status and future prospects

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a low amount of acetylcholine (ACh) in hippocampus and cortex. Acetylcholinesterase (AChE) is one of the most important enzymes in many living organisms including human being and other vertebrates, insects like mosquitoes, among others. Several reports have been published where it has been clearly shown that the genesis of amyloid protein plaques associated with AD is connected to modifications of both AChE and butyrylcholinesterase (BChE), since the plaque is significantly decreased in AD patients using cholinesterase inhibitors (ChEIs). This review gives some examples of these inhibitors discovered during past couple of years that have shown very prominent interactions at the active site triad of the proteins as well as different other parts of the active site like, peripheral anionic site (PAS), oxyanionic hole, anionic subsite or acyl binding pocket (ABP). Most of the inhibition and their interactions have been visualized by X-ray crystallography, but some of the other inhibitors have been studied either by molecular docking or molecular dynamic (MD) simulations or by both the in silico methods. Some of these prominent studies have been crucially observed and reported here.

Rapid determination of hydrogen positions and protonation states of diisopropyl fluorophosphatase by joint neutron and X-ray diffraction refinement

Hydrogen atoms constitute about half of all atoms in proteins and play a critical role in enzyme mechanisms and macromolecular and solvent structure. Hydrogen atom positions can readily be determined by neutron diffraction, and as such, neutron diffraction is an invaluable tool for elucidating molecular mechanisms. Joint refinement of neutron and X-ray diffraction data can lead to improved models compared with the use of neutron data alone and has now been incorporated into modern, maximum-likelihood based crystallographic refinement programs like CNS. Joint refinement has been applied to neutron and X-ray diffraction data collected on crystals of diisopropyl fluorophosphatase (DFPase), a calcium-dependent phosphotriesterase capable of detoxifying organophosphorus nerve agents. Neutron omit maps reveal a number of important features pertaining to the mechanism of DFPase. Solvent molecule W33, coordinating the catalytic calcium, is a water molecule in a strained coordination environment, and not a hydroxide. The smallest Ca-O-H angle is 53 degrees, well beyond the smallest angles previously observed. Residue Asp-229, is deprotonated, supporting a mechanism involving nucleophilic attack by Asp-229, and excluding water activation by the catalytic calcium. The extended network of hydrogen bonding interactions in the central water filled tunnel of DFPase is revealed, showing that internal solvent molecules form an important, integrated part of the overall structure.