Operational control of stereoselectivity during the enzymatic hydrolysis of racemic organophosphorus compounds

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Degradation of pesticides diazinon and diazoxon by phosphotriesterase: insight into divergent mechanisms from QM/MM and MD simulations

Enzymatic hydrolysis by phosphotriesterase (PTE) is one of the most effective ways of degrading organophosphorus pesticides, but the catalytic efficiency depends on the structural features of substrates. Here the enzymatic degradation of diazinon (DIN) and diazoxon (DON), characterized by PS and PO, respectively, have been investigated by QM/MM calculations and MM MD simulations. Our calculations demonstrate that the hydrolysis of DON (with PO) is inevitably initiated by the nucleophilic attack of the bridging-OH^- on the phosphorus center, while for DIN (with PS), we proposed a new degradation mechanism, initiated by the nucleophilic attack of the Zn_α-bound water molecule, for its low-energy pathway. For both DIN and DON, the hydrolytic reaction is predicted to be the rate-limiting step, with energy barriers of 18.5 and 17.7 kcal mol^-1, respectively. The transportation of substrates to the active site, the release of the leaving group and the degraded product are generally verified to be favorable by MD simulations via umbrella sampling, both thermodynamically and dynamically. The side-chain residues Phe132, Leu271 and Tyr309 play the gate-switching role to manipulate substrate delivery and product release. In comparison with the DON-enzyme system, the degraded product of DIN is more easily released from the active site. These new findings will contribute to the comprehensive understanding of the enzymatic degradation of toxic organophosphorus compounds by PTE.

Enzyme-Catalyzed Kinetic Resolution of Chiral Precursors to Antiviral Prodrugs

Nucleoside analogues are among the most common medications given for the treatment of viral infections and cancers. The therapeutic effectiveness of nucleoside analogues can be dramatically improved by phosphorylation. The ProTide approach was developed using a phosphorylated nucleoside that is masked by esterification with an amino acid and phenol forming a chiral phosphorus center. The biological activity of the ProTides depends, in part, on the stereochemistry at phosphorus, and thus, it is imperative that efficient methods be developed for the chemical synthesis and isolation of diastereomerically pure ProTides. Chiral ProTides are often synthesized by direct displacement of a labile phenol (p-nitrophenol or pentafluorophenol) from a chiral phosphoramidate precursor with the appropriate nucleoside analogue. The ability to produce these chiral products is dictated by the synthesis of the chiral phosphoramidate precursors. The enzyme phosphotriesterase (PTE) from Pseudomonas diminuta is well-known for its high stereoselectivity and broad substrate profile. Screening PTE variants from enzyme evolution libraries enabled the identification of variants of PTE that can stereoselectively hydrolyze the chiral phosphoramidate precursors. The variant G60A-PTE exhibits a 165-fold preference for hydrolysis of the R_P isomer, while the variant In1W-PTE has a 1400-fold preference for hydrolysis of the S_P isomer. Using these mutants of PTE, the S_P and R_P isomers were isolated on a preparative scale with no detectable contamination of the opposite isomer. Combining the simplicity of the enzymatic resolution of the precursor with the latest synthetic strategy will facilitate the production of diastereometrically pure nucleotide phosphoramidate prodrugs.

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.

Conformational variability of organophosphorus hydrolase upon soman and paraoxon binding

The bacterial enzyme organophosphorus hydrolase (OPH) exhibits both catalytic and substrate promiscuity. It hydrolyzes bonds in a variety of phosphotriester (P-O), phosphonothioate (P-S), phosphofluoridate (P-F), and phosphonocyanate (F-CN) compounds. However, its catalytic efficiency varies markedly for different substrates, limiting the broad-range application of OPH as catalyst in the bioremediation of pesticides and chemical war agents. In the present study, pK(a) calculations and multiple explicit-solvent molecular dynamics (MD) simulations were performed to characterize and contrast the structural dynamics of OPH bound to two substrates hydrolyzed with very distinct catalytic efficiencies: the nerve agent soman (O-pinacolylmethylphosphonofluoridate) and the pesticide paraoxon (diethyl p-nitrophenyl phosphate). pK(a) calculations for the substrate-bound and unbound enzyme showed a significant pK(a) shift from standard values (ΔpK(a) = ±3 units) for residues His254 and Arg275. MD simulations of protonated His254 revealed a dynamic hydrogen bond network connecting the catalytic residue Asp301 via His254 to Asp232, Asp233, Arg275, and Asp235, and is consistent with a previously postulated proton relay mechanism to ferry protons away from the active site with substrates that do not require activation of the leaving group. Hydrogen bonds between Asp301 and His254 were persistent in the OPH-paraoxon complex but not in the OPH-soman one, suggesting a potential role for such interaction in the more efficient hydrolysis of paraoxon over soman by OPH. These results are in line with previous mutational studies of residue His254, which led to an increase of the catalytic efficiency of OPH over soman yet decreased its efficiency for paraoxon. In addition, comparative analysis of the molecular trajectories for OPH bound to soman and paraoxon suggests that binding of the latter facilitates the conformational transition of OPH from the open to the closed substate promoting a tighter binding of paraoxon.

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.

The role of nonbonded interactions in the conformational dynamics of organophosphorous hydrolase adsorbed onto functionalized mesoporous silica surfaces

The enzyme organophosphorous hydrolase (OPH) catalyzes the hydrolysis of a wide variety of organophosphorous compounds with high catalytic efficiency and broad substrate specificity. The immobilization of OPH in functionalized mesoporous silica (FMS) surfaces increases significantly its catalytic specific activity, as compared to the enzyme in solution, with important applications for the detection and decontamination of insecticides and chemical warfare agents. Experimental measurements of immobilization efficiency as a function of the charge and coverage percentage of different functional groups have been interpreted as electrostatic forces being the predominant interactions underlying the adsorption of OPH onto FMS surfaces. Explicit solvent molecular dynamics simulations have been performed for OPH in bulk solution and adsorbed onto two distinct interaction potential models of the FMS functional groups to investigate the relative contributions of nonbonded interactions to the conformational dynamics and adsorption of the protein. Our results support the conclusion that electrostatic interactions are responsible for the binding of OPH to the FMS surface. However, these results also show that van der Waals forces are detrimental for interfacial adhesion. In addition, it is found that OPH adsorption onto the FMS models favors a protein conformation whose active site is fully accessible to the substrate, in contrast to the unconfined protein.

Differentiation of chiral phosphorus enantiomers by P and H NMR spectroscopy using amino acid derivatives as chemical solvating agents

The ability of commercially available amino acid derivatives, especially Fmoc-Trp(Boc)-OH, to differentiate enantiomers of chiral phosphonates, phosphinates, phosphates, phosphine oxides, and phosphonamidates is demonstrated with (31)P, (13)C, and (1)H NMR spectroscopy. The chiral differentiation provided a rapid and convenient method for measuring the enantiomeric purity of these phosphorus compounds.