Organophosphorus Hydrolase at the Air−Water Interface: Secondary Structure and Interaction with Paraoxon

The secondary structure of organophosphorus hydrolase (OPH) at the air-water interface was studied using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The shape and position of the amide I and amide II bands were used to estimate the surface conformation and orientation of OPH. The PM-IRRAS results indicated that the enzyme did not unfold for the range of surface pressure used (0-30 mN/m). At low surface pressures, the signal of amide I was very weak and the intensity was almost the same as amide II. Upon further compression, the PM-IRRAS signal and the ratio of the intensity of amide I and amide II both increase, implying an increased interfacial concentration of the enzyme. From the amide I/amide II ratio and the band position, it was deduced that the enzyme adopts a conformation which gives a higher occupied surface at low surface pressure and rotates to a more vertical orientation at high surface pressures. The compression and decompression of the OPH monolayer indicated that the fingerprint of the secondary structure at the air-water interface was reversible. PM-IRRAS was also used to investigate the pH effect of the subphase on the secondary structure of OPH. The secondary structure of OPH at the air-water interface was well defined when the pH of the subphase was near its isoelectric point (IP, pH 7.6). However, it adopted a different orientation when the subphase pH values were higher or lower than the IP with formation of random coil structure. The hydrolysis of organophosphorus compound paraoxon by OPH was also studied at the air-water interface by PM-IRRAS. The pH effect and the interaction with paraoxon both seem to orientate the enzyme more in the plane of the interface and to produce random coil structure.

Peptidolipid as binding site of acetylcholinesterase: molecular recognition of paraoxon in Langmuir films

Peptidolipid C18H35O (stearoyl)-Phe-Trp-Ser-His-Glu (peptidolipid A) was synthesized and spread at the air-water interface to study the interaction with an organophosphorus compound. Paraoxon, sodium dihydrogen phosphate, or 4-nitrophenyl phosphate disodium was added to the subphase, but only paraoxon changed the surface pressure-area (pi-A) isotherm of peptidolipid A. This indicated a specific interaction between paraoxon and peptidolipid A. To clarify which amino acid residue of peptidolipid A was responsible for the interaction, peptidolipid B, namely, C18H35O-Gly-His-Ser-Glu-Glu, was synthesized and studied as a Langmuir film. The difference between the pi-A isotherms of peptidolipid B in the absence and presence of paraoxon in the subphase was minimal; consequently, the presence of amino acids phenylalanine (Phe) and tryptophan (Trp) in peptidolipid A may explain the interaction between peptidolipid A and paraoxon. The compression-decompression cycles and kinetic studies of peptidolipid A showed that the Langmuir film was stable. The in situ optical properties of the peptidolipid A Langmuir film such as UV-vis and fluorescence spectroscopies were examined to elucidate the interaction between peptidolipid A and paraoxon. UV-vis absorption of peptidolipid A was investigated in the presence and absence of paraoxon in the subphase. The emission maximum of fluorescence of Trp in peptidolipid A was observed at 351 nm on pure water, and the band intensity decreased when the concentration of paraoxon increased in the subphase. This suggested that the Trp was involved in the molecular recognition process. Epifluorescence micrographs showed domains of peptidolipid A on the pure water subphase. In the presence of paraoxon in the subphase, the Langmuir film of peptidolipid A showed a homogeneity, which was another indication of the recognition between paraoxon and peptidolipid A.

Squaric monoamide monoester as a new class of reactive immunization hapten for catalytic antibodies

A squaric monoester monoamide motif was employed as an effective reactive immunogen for the discovery of monoclonal antibodies with reactive residue(s) in their combining sites. Two antibodies, 2D4 and 3C8, were uncovered that enhance paraoxon hydrolysis over background. Kinetic analysis of these antibodies was performed and interestingly both undergo a single turnover event due to covalent modification within the antibody combining site. Because antibodies 2D4 and 3C8 result in covalent attachment and thus inactivation of paraoxon, they could be useful probes for investigating paraoxon intoxication.

Activation, inhibition, and destabilization of Thermomyces lanuginosus lipase by detergents

Lipases catalyze the hydrolysis of triglycerides and are activated at the water-lipid interface. Thus, their interaction with amphiphiles such as detergents is relevant for an understanding of their enzymatic mechanism. In this study, we have characterized the effect of nonionic, anionic, cationic, and zwitterionic detergents on the enzymatic activity and thermal stability of Thermomyces lanuginosus lipase (TlL). For all detergents, low concentrations enhance the activity of TlL toward p-nitrophenyl butyrate by more than an order of magnitude; at higher detergent concentrations, the activity declines, leveling off close to the value measured in the absence of detergent. Surprisingly, these phenomena mainly involve monomeric detergent, as activation and inhibition occur well below the cmc for the nonionic and zwitterionic detergents. For anionic and cationic detergents, activation straddles the monomer-micelle transition. The data can be fitted to a three state interaction model, comprising free TlL in the absence of detergent, an activated complex with TlL at low detergent concentrations, and an enzyme-inhibiting complex at higher concentrations. For detergents with the same headgroup, there is an excellent correspondence between carbon chain length and ability to activate and inhibit TlL. However, the headgroup and number of chains also modulate these effects, dividing the detergents overall into three broad groups with rising activation and inhibition ability, namely, anionic and cationic detergents, nonionic and single-chain zwitterionic detergents, and double-chain zwitterionic detergents. As expected, only anionic and cationic detergents lead to a significant decrease in lipase thermal stability. Since nonionic detergents activate TlL without destabilizing the protein, activation/inhibition and destabilization must be independent processes. We conclude that lipase-detergent interactions occur at many independent levels and are governed by a combination of general and structurally specific interactions. Furthermore, activation of TlL by detergents apparently does not involve the classical interfacial activation phenomenon as monomeric detergent molecules are in most cases responsible for the observed increase in activity.

Modeling impact of parathion and its metabolite paraoxon on the nematode Caenorhabditis elegans in soil

Parathion is an insecticide of a group of highly toxic organophosphorous compounds. In vivo, it is activated to the toxic metabolite paraoxon. Laboratory experiments have shown that a single relationship between the variable (concentration x time of application) and the percentage of paralyzed nematodes is relevant. Aqueous (0.01 M CaCl2) extracts from soil that had received a dose of parathion as used in practice during an incubation experiment had no effect on nematodes, because sorption and biodegradation of the pesticide decreased the pesticide concentration in the soluble phase. To predict the toxicological effects of parathion and paraoxon on nematodes under various soil conditions during a simulation period of 20 d, we used a model predicting the concentrations of parathion and paraoxon over time in the soil liquid phase. In this model, sorption and biodegradation of both parathion and paraoxon were taken into account, and the results indicated that sorption effects were dominant and determined the differential toxicological risks between soils. Variable effects were predicted for short times (typically <5 d), and critical toxicological conditions were predicted for longer duration (typically >10-15 d), in all cases.

Sensitive detection of organophosphates in river water by means of a piezoelectric biosensor

A highly sensitive piezoelectric biosensor has been developed for detection of cholinesterase inhibitors. The inhibitor benzoylecgonine-1,8-diamino-3,4-dioxaoctane (BZE-DADOO) was immobilized on a monolayer of 11-mercaptomonoundecanoic acid (MUA) self-assembled on the gold surface of the sensor. The binding of high-molecular-weight cholinesterase to the immobilized cocaine derivative was monitored with a mass sensitive piezoelectric quartz crystal (quartz crystal nanobalance; QCN). In the presence of an inhibiting substance in the sample, the binding of cholinesterase to the immobilized inhibitor was reduced. The decrease of the rate of mass change was proportional to the concentration of free inhibitor in the sample. This way the affinity sensor followed anti-cholinesterase toxicity and the enzyme activity of ChE was not addressed. A assay for detection of organophosphates (OP) was optimized. Regeneration of the sensor surface was achieved with 1 mol L(-1) formic acid, which enabled 40 measurements with one sensor. All assays were carried out in a flow-through arrangement. The total measurement time (binding+regeneration) was 25 min and the detection limit for different OP (paraoxon, diisopropylfluorophosphate, chlorpyriphos, and chlorfenvinphos) was down to 10(-10) mol L(-1) (0.02 microg L(-1)). This sensor was used for determination of organophosphate (diisopropylfluorophosphate) levels in river water samples.

Synthesis of 2-substituted beta-cyclodextrin derivatives with a hydrolytic activity against the organophosphorylester paraoxon

Beta-cyclodextrin was substituted by an iodosobenzoic acid derivative to create a catalytic hydrolytic activity against neurotoxic organophosphorus agents. The catalytic moiety was introduced on a secondary hydroxy group at the position 2 of a glucose unit. Several beta-cyclodextrin derivatives were obtained. In these derivatives, the methylene linker occupied all potential positions on the aromatic ring. Kinetic assays were carried out with paraoxon as organophosphate model. Three regioisomers hydrolyzed paraoxon, although the paraoxon-leaving group, para-nitrophenol, was not released from the beta-cyclodextrin torus.

The interaction between OPH and paraoxon at the air–water interface studied by AFM and epifluorescence microscopies

The paraoxon hydrolysis reaction catalyzed by organophosphorus hydrolase (OPH) monolayer at the air-water interface was studied. OPH-paraoxon interactions, occurring at the two-dimensional interface, by close-packed, highly orientated OPH monolayer, were investigated by several different surface chemistry techniques; e.g. surface pressure area isotherms, atomic force microscopy (AFM), and in situ epifluorescence microscopy. The characterization of OPH Langmuir and Langmuir-Blodgett films prepared in both the presence and absence of paraoxon, demonstrated significantly distinctive feature when compared with one another. Continuous growth of the OPH aggregates is a distinct phenomenon associated with hydrolysis, in addition to the pH changes in the local environment of the enzyme macromolecules.

Organophosphate nerve agent detection with europium complexes

We explore the detection of paraoxon, a model compound for nonvolatile organophosphate nerve agents such as VX. The detection utilizes europium complexes with 1,10 phenanthroline and thenoyltrifluoroacetone as sensitizing ligands. Both europium luminescence quenching and luminescence enhancement modalities are involved in the detection, which is simple, rapid, and sensitive. It is adaptable as well to the more volatile fluorophosphate nerve agents. It involves nothing more than visual luminescence observation under sample illumination by an ordinary hand-held ultraviolet lamp.

Zn2+-Catalyzed Methanolysis of Phosphate Triesters: A Process for Catalytic Degradation of the Organophosphorus Pesticides Paraoxon and Fenitrothion

The methanolyses of two neutral phosphorus triesters, paraoxon (1) and fenitrothion (3), were investigated as a function of added Zn(OTf)(2) or Zn(ClO(4))(2) in methanol at 25 degrees C either alone or in the presence of equimolar concentrations of the ligands phenanthroline (4), 2,9-dimethylphenanthroline (5), and 1,5,9-triazacyclododecane (6). The catalysis requires the presence of methoxide, and when studied as a function of added NaOCH(3), the rate constants (k(obs)) for methanolysis of Zn(2+) alone or in the presence of equimolar 4 or 5 maximize at different [(-)OCH(3)]/[Zn(2+)](total) ratios of 0.3, 0.5, and 1.0, respectively. Plots of k(obs) vs [Zn(2+)](total) either alone or in the presence of equimolar ligands 4 and 5 at the [(-)OCH(3)]/[Zn(2+)](total) ratios corresponding to the rate maxima are curved and show a nonlinear dependence on [Zn(2+)](total). In the cases of 4 and 5, this is explained as resulting from formation of a nonactive dimer, formulated as a bis-mu-methoxide-bridged form (L:Zn(2+)((-)OCH(3))(2)Zn(2+):L) in equilibrium with an active monomeric form (L:Zn(2+)((-)OCH(3))). In the case of the Zn(2+):6 system, no dimeric forms are present as can be judged by the strict linearity of the plots of k(obs) vs [Zn(2+)](total) in the presence of equimolar 6 and (-)OCH(3). Analysis of the potentiometric titration curves for Zn(2+) alone and in the presence of the ligands allows calculation of the speciation of the various Zn(2+) forms and shows that the binding to ligands 4 and 6 is very strong, while the binding to ligand 5 is weaker. Overall the best catalytic system is provided by equimolar Zn(2+), 5, and (-)OCH(3), which exhibits excellent turnover of the methanolysis of paraoxon when the substrate is in excess. At a concentration of 2 mM in each of these components, which sets the pH of the solution at 9.5, the acceleration of the methanolysis of paraoxon and fenitrothion relative to the methoxide reaction is 1.8 x 10(6)-fold and 13 x 10(6)-fold, respectively. A mechanism for the catalyzed reactions is proposed which involves a dual role for the metal ion as a Lewis acid and source of nucleophilic Zn(2+)-bound (-)OCH(3).

Comparison of Chlorpyrifos-Oxon and Paraoxon Acetylcholinesterase Inhibition Dynamics: Potential Role of a Peripheral Binding Site

The primary mechanism of action for organophosphorus (OP) insecticides, like chlorpyrifos and parathion, is to inhibit acetylcholinesterase (AChE) by their oxygenated metabolites (oxons), due to the phosphorylation of the serine hydroxyl group located in the active site of the molecule. The rate of phosphorylation is described by the bimolecular inhibitory rate constant (k(i)), which has been used for quantification of OP inhibitory capacity. It has been proposed that a peripheral binding site exists on the AChE molecule, which, when occupied, reduces the capacity of additional oxon molecules to phosphorylate the active site. The aim of this study was to evaluate the interaction of chlorpyrifos oxon (CPO) and paraoxon (PO) with rat brain AChE to assess the dynamics of AChE inhibition and the potential role of a peripheral binding site. The k(i) values for AChE inhibition determined at oxon concentrations of 1-100 nM were 0.206 +/- 0.018 and 0.0216 nM(-1)h(-1) for CPO and PO, respectively. The spontaneous reactivation rates of the inhibited AChE for CPO and PO were 0.084-0.087 (two determinations) and 0.091 +/- 0.023 h(-1), respectively. In contrast, the k(i) values estimated at a low oxon concentration (1 pM) were approximately 1,000- and 10,000-fold higher than those determined at high CPO and PO concentrations, respectively. At low concentrations, the k(i) estimates were approximately similar for both CPO and PO (150-180 [two determinations] and 300 +/- 180 nM(-1)h(-1), respectively). This implies that, at low concentrations, both oxons exhibited similar inhibitory potency in contrast to the marked difference exhibited at higher concentrations. These results support the potential importance of a secondary peripheral binding site associated with AChE kinetics, particularly at low, environmentally relevant concentrations.

Modeling of sorption and biodegradation of parathion and its metabolite paraoxon in soil

To investigate the distribution of parathion [O,O-diethyl O-(4-nitrophenyl) phosphorothioate] and its highly toxic metabolite paraoxon [O,O-diethyl O-(4-nitrophenyl)phosphate] between the soluble and sorbed pools in the soil, batch experiments were conducted to evaluate the rate of adsorption and desorption of 14C-labeled parathion and paraoxon in soil. The mineralization and degradation of these products were also investigated during a 56-d experiment under controlled laboratory conditions. Adsorption patterns indicated initial fast adsorption reactions occurring within 4 h for both parathion and paraoxon. We also observed the formation of nonextractable residues. The paraoxon was more intensively degraded than the parathion, and production of p-nitrophenol and other metabolites was observed. A kinetic model was developed to describe the sorption and biodegradation rates of parathion, taking into account the production, retention, and biodegradation of paraoxon, the main metabolite of parathion. After fitting the parameters of the model we made a simulation of the kinetics of the appearance and disappearance of paraoxon. From the simulation we predicted a quantity of metabolite in the liquid phase amounting to 1% of the quantity of parathion initially applied. This is in agreement with the experimental data.

New principle of direct real-time monitoring of the interaction of cholinesterase and its inhibitors by piezolectric biosensor

This paper describes a new method for the sensitive detection of cholinesterase inhibitors based on real-time monitoring using a piezoelectric biosensor. The cholinesterase inhibitor paraoxon was immobilized on the sensing surface via a chelate complex as the recognition element. At first, the conjugate of N-mercaptoundecanoic acid (MUA) with Nalpha,Nalpha-bis (carboxymethyl)-L-lysine (NTA-Lys) was chemisorbed to form a self-assembled monolayer on the surface of the gold electrode of the piezosensor. In the next step, paraoxon-spacer-hexahistidine conjugate was linked to the MUA-Lys-NTA layer via the chelate complex with Ni2+. The paraoxon-modified surface thus obtained was applied for the binding of human butyrylcholinesterase (BChE). Regeneration of the sensing surface was achieved by splitting the chelate complex with EDTA and depositing a fresh layer of Ni2+ followed by addition of the paraoxon-spacer-hexahistidine. In the presence of free inhibitors like diisopropylfluorophosphate (DFP), binding of BChE to the surface-bound paraoxon was decreased. In this way, a competitive affinity assay for organophosphorus compounds was developed. The limit of detection for DFP as a model compound was 10 nmol/l (ca. 2 microg/l). This new concept seems suitable for constructing biosensors for the group-specific detection of cholinesterase-inhibiting substances like insecticides in the field.

Sequential structural changes of Escherichia coli thioesterase/protease I in the serial formation of Michaelis and tetrahedral complexes with diethyl p-nitrophenyl phosphate

Escherichia coli thioesterase/protease I (TEP-I) belongs to a new subclass of lipolytic enzymes of the serine hydrolase superfamily. Here we report the first direct NMR observation of the formation of the Michaelis complex (MC) between TEP-I and diethyl p-nitrophenyl phosphate (DENP), an active site directed inhibitor of serine protease, and its subsequent conversion to the tetrahedral complex (TC). NMR, ESI-MS, and kinetic data showed that DENP binds to TEP-I in a two-step process, a fast formation of MC followed by a slow conversion to TC. NMR chemical shift perturbation further revealed that perturbations were confined mainly to four conserved segments comprising the active site. Comparable magnitudes of chemical shift perturbations were detected in both steps. The largest chemical shift perturbation occurred around the catalytic Ser(10). In MC, the conformation of the mobile Ser(10) was stabilized, and its amide resonance became observable. From the large chemical shift perturbation upon conversion from MC to TC, we propose that the amide protons of Ser(10) and Gly(44) serve as the oxyanion hole proton donors that stabilize the tetrahedral adduct. The pattern of residues perturbed in both steps suggests a sequential, stepwise structural change upon binding of DENP. The present study also demonstrates the important catalytic roles of conserved residues in the SGNH family of proteins.

Enhancement of the catalytic activity of an artificial phosphotriesterase using a molecular imprinting technique

An artificial phosphotriesterase (PTE) was constructed by co-polymerization of 4(5)-vinylimidazole-Zn(2+)-methacrylic acid cluster with a divinylbenzene polymer. Compared with the spontaneous hydrolysis, the resulting polymer catalyst caused 105-fold rate acceleration towards the hydrolysis of diethyl p-nitrophenyl phosphate (Paraoxon). The catalytic activity of the polymer catalyst could be enhanced for 30% using molecular imprinting technique and the molecularly-imprinted catalyst (MIC) showed a turnover rate of 7.4 x 10(-2) s(-1) towards the hydrolysis of Paraoxon. The MIC also hydrolyzed thiophosphates and phosphorothiolate triester pesticides. Construction of an amperometric sensor employing the MIC as catalyst achieved a detection limit of 0.1 mM Paraoxon.

Billion-fold acceleration of the methanolysis of paraoxon promoted by La(OTf)3 in methanol

The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)(3)] (OTf = (-)OS(O)(2)CF(3)) as a function of at 25 degrees C. Plots of the pseudo-first-order rate constants (k(obs)) for methanolysis as a function of [La(OTf)(3)](total) were obtained under buffered conditions from 5.15 to 10.97, and the slopes of the linear parts of these were used to determine the second-order rate constants (k(2)(obs)) for the La(3+)-catalyzed methanolysis of 2. Detailed analysis of the potentiometric titration data of La(OTf)(3) in methanol through fits to a multicomponent equilibrium mixture of dimers of general stoichiometry La(3+)(2)((-)OCH3)n, where n assumes values of 1-5, gives the equilibrium distribution of each as a function of. These data, when fit to a second expression describing k(2)(obs) in terms of a linear combination of individual rate constants k(2)(2:1), k(2)(2:2).k(2)(2:)n for the dimers, allow one to describe the overall catalytic profile in terms of the individual contributions. The most catalytically important species are the three dimers La(3+)(2)((-)OCH3)1, La(3+)(2)((-)OCH3)2, and La(3+)(2)((-)OCH3)3. The catalysis of the methanolysis of 2 is spectacular: a 2 x 10(-3) M solution of [La(3+)](total), at neutral, affords a 10(9)-fold acceleration relative to the base reaction (t(1/2) approximately 20 s at 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La(3+)(2)((-)OCH3)2 species is proposed.

Do multiple cytochrome P450 isoforms contribute to parathion metabolism in man?

Phosphorothioate compounds are widely used in agriculture and public health for the control of unwanted pests. The phosphorothioate parathion was metabolised to the toxic metabolite paraoxon (0.038-0.683 nmol/min per mg protein) and p-nitrophenol (0.023-2.10 nmol/min per mg protein) by human liver microsomes ( n=27) in an NADPH-dependent reaction. There was a significant correlation ( P<0.02) between nifedipine oxidation and paraoxon formation from parathion (200 micro M) by human liver microsomes and with cytochrome P450 (CYP) 3A4/5 expression ( P<0.05), although not with midazolam 1'-hydroxylation or testosterone 6beta-hydroxylation. Paclitaxel 6'-hydroxylation and CYP2C8 expression correlated with paraoxon formation ( P<0.01), indicating CYP2C8 involvement. Of nine recombinant P450 isoforms, CYPs 3A4, 3A5, 1A2 and 2D6 activated parathion to paraoxon at the highest rates (6.5, 8.5, 5.7 and 6.2 pmol/pmol P450 per h) with K(m) values of 12.6, 2.7, 1.5 and 9.2 micro M, respectively. Similar K(m) values were seen with the human liver microsomes. These data indicate that CYP3A4/5 and CYP2C8, which constitute up to 40% of human liver P450s, are the most significant participants in the metabolism of parathion. However, several other isoforms could play an important role when CYP3A and CYP2C8 are poorly expressed due to environmental factors or the presence of a genetic polymorphism.

Structure-activity relationship on human serum paraoxonase (PON1) using substrate analogues and inhibitors

Substrate analogues based on the parent compounds paraoxon and phenyl acetate were tested on human serum paraoxonase (PON1) to explore the active site of the enzyme. Replacement of the nitro group of paraoxon with an amine or hydrogen, as well as electronic changes to the parent compound, converted these analogues into inhibitors. Introduction of either electron-withdrawing or donating groups onto phenyl acetate resulted in reduction in their rate of hydrolysis by PON1.

Optical biosensor for simultaneous detection of captan and organophosphorus compounds

The optical biosensor consisting of GST and acetylcholinesterase (AChE)-immobilized gel film was developed to detect captan and organophosphorus compounds simultaneously in contaminated water. The sensing scheme was based on the measurement of decrease of products formation (s-(2,4-dinitrobenzene) glutathione and alpha-naphthol by GST and AChE, respectively) due to the inhibition by captan and organophosphorus compounds. The absorbance of s-(2,4-dinitrobenzene) glutathione and alpha-naphthol was detected at 400 and 500 nm, respectively, by a proposed optical biosensor system. It was observed that AChE was inhibited by both captan and organophosphorus compounds, and GST was inhibited only by captan. The simultaneous detection and quantification of captan and organophosphorus compounds could be successfully achieved by the proposed sensor system. The proposed biosensor could successfully detect the captan and organophosphorus compounds concentration from 0 to 2 ppm.

Ghrelin can bind to a species of high density lipoprotein associated with paraoxonase

Ghrelin is a 28-residue peptide hormone that is principally released from the stomach during fasting and prior to eating. Two forms are present in human plasma: the unmodified peptide and a less abundant acylated version, in which octanoic acid is attached to the third residue, a serine, via an ester linkage. The acylated form of ghrelin acts as a ligand for the growth hormone secretagogue receptor and can stimulate the release of growth hormone from the pituitary gland. It also initiates behavioral and metabolic adaptations to fasting. Here we show that an immobilized form of ghrelin specifically binds a species of high density lipoprotein associated with the plasma esterase, paraoxonase, and clusterin. Both free ghrelin and paraoxon, a substrate for paraoxonase, can inhibit this interaction. An endogenous species of ghrelin is found to co-purify with high density lipoprotein during density gradient centrifugation and subsequent gel filtration. This interaction links the orexigenic peptide hormone ghrelin to lipid transport and metabolism. Furthermore, the interaction of the esterified hormone ghrelin with a species of HDL containing an esterase suggests a possible mechanism for the conversion of ghrelin to des-acyl ghrelin.

Significant and differential acceleration of dephosphorylation of the insecticides, paraoxon and parathion, caused by alkali metal ethoxidesElectronic Supplementary Information (ESI) available: kinetic data. See http://www.rsc.org/suppdata/cc/b3/b310055c/

In the reaction of paraoxon with alkali metal ethoxides, ion-paired EtO-M+ species are more reactive than the dissociated EtO- with the reactivity order EtO-Li+ EtO-Na+ > EtO-K+ > EtO-, while in the reaction of parathion, the reactivity follows the order EtO-K+ > EtO- > EtO-Na+ > EtO-Li+.

Nanocrystalline metal oxides as destructive adsorbents for organophosphorus compounds at ambient temperatures

Nanocrystals of magnesium oxide react with organophosphorus compounds at room temperature by dissociative chemisorption, which we term "destructive adsorption". This process involves cleavage of P-O and P-F bonds (but not P-C bonds) and immobilization of the resultant molecular fragments. These ultrafine powders have unusual crystalline shapes and possess high surface concentrations of reactive edge/corner and defect sites, and thereby display higher surface reactivity, normalized for surface area, than typical polycrystalline material. This high surface reactivity coupled with high surface area allows their use for effective decontamination of chemical warfare agents and related toxic substances. Herein data is presented for paraoxon, diisopropylfluorophosphate (DFP), and (CH3CH2O)2P(O)CH2-SC6H5 (DEPTMP). Solid-state NMR and IR spectroscopy indicate that all OR and F groups dissociate; this leaves bound -PO4, -F, and -OR groups for paraoxon, DFP, and DEPTMP, respectively. For paraoxon, it was shown that one monolayer reacts. For DEPTMP, the OR groups dissociate, but not the P-CH2SC6H5 group. The nanocrystalline MgO reacts much faster and in higher capacity than typical activated carbon samples, which physisorb but do not destructively adsorb these phosphorous compounds.

Short, strong hydrogen bonds at the active site of human acetylcholinesterase: proton NMR studies

Cholinesterases use a Glu-His-Ser catalytic triad to enhance the nucleophilicity of the catalytic serine. We have previously shown by proton NMR that horse serum butyryl cholinesterase, like serine proteases, forms a short, strong hydrogen bond (SSHB) between the Glu-His pair upon binding mechanism-based inhibitors, which form tetrahedral adducts, analogous to the tetrahedral intermediates in catalysis [Viragh, C., et al. (2000) Biochemistry 39, 16200-16205]. We now extend these studies to human acetylcholinesterase, a 136 kDa homodimer. The free enzyme at pH 7.5 shows a proton resonance at 14.4 ppm assigned to an imidazole NH of the active-site histidine, but no deshielded proton resonances between 15 and 21 ppm. Addition of a 3-fold excess of the mechanism-based inhibitor m-(N,N,N-trimethylammonio)trifluoroacetophenone (TMTFA) induced the complete loss of the 14.4 ppm signal and the appearance of a broad, deshielded resonance of equal intensity with a chemical shift delta of 17.8 ppm and a D/H fractionation factor phi of 0.76 +/- 0.10, consistent with a SSHB between Glu and His of the catalytic triad. From an empirical correlation of delta with hydrogen bond lengths in small crystalline compounds, the length of this SSHB is 2.62 +/- 0.02 A, in agreement with the length of 2.63 +/- 0.03 A, independently obtained from phi. Upon addition of a 3-fold excess of the mechanism-based inhibitor 4-nitrophenyl diethyl phosphate (paraoxon) to the free enzyme at pH 7.5, and subsequent deethylation, two deshielded resonances of unequal intensity appeared at 16.6 and 15.5 ppm, consistent with SSHBs with lengths of 2.63 +/- 0.02 and 2.65 +/- 0.02 A, respectively, suggesting conformational heterogeneity of the active-site histidine as a hydrogen bond donor to either Glu-327 of the catalytic triad or to Glu-199, also in the active site. Conformational heterogeneity was confirmed with the methylphosphonate ester anion adduct of the active-site serine, which showed two deshielded resonances of equal intensity at 16.5 and 15.8 ppm with phi values of 0.47 +/- 0.10 and 0.49 +/- 0.10 corresponding to average hydrogen bond lengths of 2.59 +/- 0.04 and 2.61 +/- 0.04 A, respectively. Similarly, lowering the pH of the free enzyme to 5.1 to protonate the active-site histidine (pK(a) = 6.0 +/- 0.4) resulted in the appearance of two deshielded resonances, at 17.7 and 16.4 ppm, consistent with SSHBs with lengths of 2.62 +/- 0.02 and 2.63 +/- 0.02 A, respectively. The NMR-derived distances agree with those found in the X-ray structures of the homologous acetylcholinesterase from Torpedo californica complexed with TMTFA (2.66 +/- 0.28 A) and sarin (2.53 +/- 0.26 A) and at low pH (2.52 +/- 0.25 A). However, the order of magnitude greater precision of the NMR-derived distances establishes the presence of SSHBs at the active site of acetylcholinesterase, and detect conformational heterogeneity of the active-site histidine. We suggest that the high catalytic power of cholinesterases results in part from the formation of a SSHB between Glu and His of the catalytic triad.

Enhancement, relaxation, and reversal of the stereoselectivity for phosphotriesterase by rational evolution of active site residues

The factors that govern the substrate reactivity and stereoselectivity of phosphotriesterase (PTE) toward organophosphotriesters containing various combinations of methyl, ethyl, isopropyl, and phenyl substituents at the phosphorus center were determined by systematic alterations in the dimensions of the active site. The wild type PTE prefers the S(P)-enantiomers over the corresponding R(P)-enantiomers by factors ranging from 10 to 90. Enlargement of the small subsite of PTE with the substitution of glycine and alanine residues for Ile-106, Phe-132, and/or Ser-308 resulted in significant improvements in k(cat)/K(a) for the R(P)-enantiomers of up to 2700-fold but had little effect on k(cat)/K(a) for the corresponding S(P)-enantiomers. The kinetic preferences for the S(P)-enantiomers were thus relaxed without sacrificing the inherent catalytic activity of the wild type enzyme. A reduction in the size of the large subsite with the mutant H257Y resulted in a reduction in k(cat)/K(a) for the S(P)-enantiomers, while the values of k(cat)/K(a) for the R(P)-enantiomers were essentially unchanged. The initial stereoselectivity observed with the wild type enzyme toward the chiral substrate library was significantly reduced with the H257Y mutant. Simultaneous alternations in the sizes of the large and small subsites resulted in the complete reversal of the chiral specificity. With this series of mutants, the R(P)-enantiomers were preferred as substrates over the corresponding S(P)-enantiomers by up to 500-fold. These results have demonstrated that the stereochemical determinants for substrate hydrolysis by PTE can be systematically altered through a rational reconstruction of the dimensions of the active site.

Degradation of organophosphorous nerve agents by enzyme-polymer nanocomposites: efficient biocatalytic materials for personal protection and large-scale detoxification

The biocatalytic destruction of organophosphates has become an important focus area, as efficient "clean" technologies are sought for chemical weapons decommissioning, counteracting nerve agent attacks, and protecting against organophosphate pesticide poisoning. A novel method is advanced for immobilizing the broad-spectrum enzyme organophosphorous hydrolase (OPH) from Pseudomonas diminuta, based on the formation of nanocomposite protein-silicone polymers. The resulting materials are highly active, stable, and versatile biocatalysts for the liquid and gas phase detoxification of organophosphates, and can be fabricated as monoliths, sheets, thick films, granulates, or macroporous foams. This approach offers an efficient avenue to robust, high-performance biocatalytic OPH-containing polymers that outperform immobilized OPH catalysts reported to date. The method provides for the first time a route to biocatalytic materials that may be suitable for "active" protective wear, as well as bulk catalysts for the destruction of large volumes of organophosphates. The preparation of OPH-silicone biocomposites, their performances in the liquid and gas phase detoxification of paraoxon, dichlorvos, and diisopropyl fluorophosphate, and their features are discussed.