Catalytic bioscavengers against organophosphorus agents: mechanistic issues of self-reactivating cholinesterases

© 2018 Elsevier B.V. Catalytic bioscavengers are the second-generation bioscavengers. These biopharmaceuticals are intended to degrade toxic organophosphorus agents on the skin for decontamination or in the bloodstream for pre-treatment and post-exposure treatment of organophosphate poisoning. Because catalytic degradation has to be fast, their catalytic efficiency has to be as high as possible (kcat/Km>106 M−1 min−1). Certain evolved mammalian paraoxonases and bacterial phosphotriesterases already fulfill this requirement. To be of interest, the catalytic activity of certain enzymes has to be increased by several orders of magnitude. This can be reached by computer-redesign or directed evolution existing enzymes, and alternatively, combinational strategies. The present paper focuses on the better understanding of catalytic mechanisms of cholinesterase inhibition, aging and reactivation and how this knowledge serves the rational design of novel catalytic bioscavengers based on cholinesterase structure

Emergence of catalytic bioscavengers against organophosphorus agents

© 2016 Elsevier Ireland LtdBioscavengers are an effective alternative approach for pre- and post-exposure treatments of nerve agent (NA) poisoning. Bioscavengers are natural or recombinant enzymes, reactive proteins, and antibodies that neutralize NAs before they reach their physiological targets. They are administered by injection (protein or gene delivery vector) and react with NAs in the bloodstream. Other ways of delivery can be used: inhalation for pulmonary delivery, topical creams for skin protection, etc. Operational bioscavengers must be producible at low cost, not susceptible to induce immune response and adverse effects, and stable in the bloodstream, upon storage, and under field conditions. First generation bioscavengers, cholinesterases and carboxylesterases, are stoichiometric bioscavengers. However, stoichiometric neutralization of NAs needs administration of huge doses of costly biopharmaceuticals. Second generation bioscavengers are catalytic bioscavengers. These are capable of detoxifying organophosphates regeneratively. By virtue of high turnover, much lower doses are needed for rapid neutralization of toxicants. The most promising catalytic bioscavengers are evolved mutants of phosphotriesterases (bacterial enzymes, mammalian paraoxonases), displaying enantiomeric preference for toxic NA isomers. However, engineering of cholinesterases, carboxylesterases, prolidases and other enzymes, e.g. phosphotriesterases-lactonases from extremophiles is of interest. In particular, association of cholinesterase mutants (not susceptible to age after phosphylation) with fast-reactivating oximes leads to pseudocatalytic bioscavengers. Thus, catalytic and pseudocatalytic bioscavengers are an improvement of bioscavenger-based medical countermeasures in terms of efficacy and cost

Slow-binding inhibition of cholinesterases, pharmacological and toxicological relevance

© 2016 Elsevier Inc. All rights reserved. Slow-binding inhibition (SBI) of enzymes is characterized by slow establishment of enzyme-inhibitor equilibrium. Cholinesterases (ChEs) display slow onset of inhibition with certain inhibitors. After a survey of SBI mechanisms, SBI of ChEs is examined. SBI results either from simple slow interaction, induced-fit, or slow conformational selection. In some cases, the slow equilibrium is followed by an irreversible chemical step. This later was observed for the interaction of ChEs with certain irreversible inhibitors. Because slow-binding inhibitors present pharmacological advantages over classical reversible inhibitors (e.g. long target-residence times, resulting in prolonged efficacy with minimal unwanted side effects), slow-binding inhibitors of ChEs are promising new drugs for treatment of Alzheimer disease, myasthenia, and neuroprotection. SBI is also of toxicological importance; it may play a role in mechanisms of resistance and protection against poisoning by irreversible agents

Understanding the non-catalytic behavior of human butyrylcholinesterase silent variants: Comparison of wild-type enzyme, catalytically active Ala328Cys mutant, and silent Ala328Asp variant

© 2016 Elsevier Ireland LtdConformational dynamics of wild-type human butyrylcholinesterase (BChE), two mutants of residue Ala328, the catalytically active Ala328Cys, and the catalytically inactive (silent) Ala328Asp, and their interactions with butyrylcholine were studied. The aim was to understand the molecular mechanisms by which point mutations may lead to silent BChE variant or alter catalytic activity. Importance of BChE natural variants is due to medical consequences, i.e. prolonged apnea, following administration of the myorelaxant esters, succinylcholine and mivacurium. Comparison of molecular dynamics (MD) simulations for the three model systems showed that: 1) the active mutant Ala328Cys mutant has some changes in configuration of catalytic residues, which do not prevent binding of butyrylcholine to the active site; 2) in the naturally-occurring silent variant Ala328Asp, the Asp328 carboxylate may either form a salt bridge with Lys339 or a H-bond with His438. In the first case, the Ω-loop swings off the gorge, disrupting the π-cation binding site and the catalytic triad. In the second case, binding of cationic substrates in the catalytic center is also impaired. MD simulations carried out in 0.15 M NaCl, close to physiological ionic strength conditions, favored the second situation. It was seen that Asp328 forms a H-bond with the catalytic triad His438, which in turn disrupts the catalytic machinery. Therefore, we concluded that the Ala328Asp variant is not catalytically active because of that dramatic event. Computational results, consistent with in vitro biochemical data and clinical observations, validate our MD approach

Water structure changes in oxime-mediated reactivation process of phosphorylated human acetylcholinesterase

c 2018 The Author(s). The role of water in oxime-mediated reactivation of phosphylated cholinesterases (ChEs) has been asked with recurrence. To investigate oximate water structure changes in this reaction, reactivation of paraoxon-inhibited human acetylcholinesterase (AChE) was performed by the oxime asoxime (HI-6) at different pH in the presence and absence of lyotropic salts: a neutral salt (NaCl), a strong chaotropic salt (LiSCN) and strong kosmotropic salts (ammonium sulphate and phosphate HPO42−). At the same time, molecular dynamic (MD) simulations of enzyme reactivation under the same conditions were performed over 100 ns. Reactivation kinetics showed that the low concentration of chaotropic salt up to 75 mM increased the percentage of reactivation of diethylphosphorylated AChE whereas kosmotropic salts lead only to a small decrease in reactivation. This indicates that water-breaker salt induces de-structuration of water molecules that are electrostricted around oximate ions. Desolvation of oximate favors nucleophilic attack on the phosphorus atom. Effects observed at high salt concentrations (>100 mM) result either from salting-out of the enzyme by kosmotropic salts (phosphate and ammonium sulphate) or denaturing action of chaotropic LiSCN. MDs simulations of diethylphosphorylated hAChE complex with HI-6 over 100 ns were performed in the presence of 100 mM (NH4)2SO4 and 50 mM LiSCN. In the presence of LiSCN, it was found that protein and water have a higher mobility, i.e. water is less organized, compared with the ammonium sulphate system. LiSCN favors protein solvation (hydrophobic hydration) and breakage of elelectrostricted water molecules around of oximate ion. As a result, more free water molecules participated to reaction steps accompanying oxime-mediated dephosphorylation

Understanding the non-catalytic behavior of human butyrylcholinesterase silent variants: Comparison of wild-type enzyme, catalytically active Ala328Cys mutant, and silent Ala328Asp variant

© 2016 Elsevier Ireland Ltd.Conformational dynamics of wild-type human butyrylcholinesterase (BChE), two mutants of residue Ala328, the catalytically active Ala328Cys, and the catalytically inactive (silent) Ala328Asp, and their interactions with butyrylcholine were studied. The aim was to understand the molecular mechanisms by which point mutations may lead to silent BChE variant or alter catalytic activity. Importance of BChE natural variants is due to medical consequences, i.e. prolonged apnea, following administration of the myorelaxant esters, succinylcholine and mivacurium.Comparison of molecular dynamics (MD) simulations for the three model systems showed that: 1) the active mutant Ala328Cys mutant has some changes in configuration of catalytic residues, which do not prevent binding of butyrylcholine to the active site; 2) in the naturally-occurring silent variant Ala328Asp, the Asp328 carboxylate may either form a salt bridge with Lys339 or a H-bond with His438. In the first case, the Ω-loop swings off the gorge, disrupting the π-cation binding site and the catalytic triad. In the second case, binding of cationic substrates in the catalytic center is also impaired. MD simulations carried out in 0.15 M NaCl, close to physiological ionic strength conditions, favored the second situation. It was seen that Asp328 forms a H-bond with the catalytic triad His438, which in turn disrupts the catalytic machinery. Therefore, we concluded that the Ala328Asp variant is not catalytically active because of that dramatic event. Computational results, consistent with in vitro biochemical data and clinical observations, validate our MD approach

Computational Exploration of Reactivity of 6-Methyluracil/Imidazole-2-Carbaldehyde Oxime Conjugate

© 2016, Springer Science+Business Media New York.Molecular docking and ab intio quantum mechanical calculations were used to assess the nucleophilic reactivity of conjugates of 6-methyluracil and imidazole-2-carbaldehyde oxime. Minimum energy profiles for oxime group rotation and proton transfer were calculated for isolated conjugate. Results indicated that proton transfer and activation are possible. Results suggests that the compound can be active itself, reacting with esters in a way, similar to enzymatic histidine-containing catalytic triad. Thus, this compound is of potential interest for direct scavenging organophosphorus inhibitors of cholinesterases and/or as co-reagent in cholinesterase-based pseudocatalytic bioscavengers

Role of Acetylcholinesterase in β-Amyloid Aggregation Studied by Accelerated Molecular Dynamics

© 2016, Springer Science+Business Media New York.Mechanisms of Alzheimer’s disease development are still under investigation. It was shown that acetylcholinesterase promotes aggregation of β-amyloid. Accelerated molecular dynamics simulations were performed to investigate molecular mechanisms of this process. Results showed that Aβ is strongly attracted to the surface of acetylcholinesterase and forms stable complexes. It was hypothesized that acetylcholinesterase serves as a nucleation center for propagation of β-amyloid aggregation

Molecular polymorphism of human enzymes as the basis of individual sensitivity to drugs. Supercomputer-assisted modeling as a tool for analysis of structural changes and enzymatic activity of proteins

© 2016, Springer Science+Business Media New York.The nature of individual sensitivity to drugs associated with molecular polymorphism of human enzymes is discussed. The influence of molecular polymorphism on the activity of key human esterases, in particular, cholinesterases and carboxylesterase, responsible for hydrolytic metabolism of ester-containing drugs, is analyzed. A method was developed, which involves supercomputer-assisted modeling as a tool for assessment of molecular mechanism of the impact of point mutations on the catalytic activity of enzymes. This work is a part of a study aimed at elaboration of the concept and methods of personalized medicine

Macrocyclic derivatives of 6-methyluracil as ligands of the peripheral anionic site of acetylcholinesterase

© the Partner Organisations 2014. Novel pyrimidinophanes possessing two o-nitrobenzylethyldialkylammonium heads bridging with different spacers were prepared. Pyrimidinophanes 2a, 2b and 3 are reversible inhibitors of cholinesterases. They show a very good selectivity for human acetylcholinesterase (AChE), with an inhibitory power 100-200 times higher than for human butyrylcholinesterase (BChE). Docking simulations indicate specific binding of pyrimidinophanes 2a and 4 onto the peripheral anionic site of AChE. Other compounds bind to the active center of AChE as well as to the peripheral anionic site. These compounds are dual binding site inhibitors. Pyrimidinophane 2b and its acyclic counterpart 1 were tested in the animal model of myasthenia gravis and may be considered as valuable candidates for the treatment of pathological muscle weakness syndromes. This journal i