Patent Application
20170218348
The disclosure relates generally to chemical warfare. More specifically, the disclosure relates to detoxification of organophosphate nerve agents.
The G-type (sarin, cyclosarin, and soman) and V-type (VX and VR) organophosphonates are among the most toxic compounds known. The toxicity of these compounds is due to their ability to inactivate acetylcholine esterase, an enzyme required for proper nerve function.(1) Acetylcholine esterase breaks down the neurotransmitter acetylcholine into acetic acid and choline. Acetylcholine conducts a nerve impulse between the nerve and the muscle, stimulating the muscle. The organophosphonate binds to the hydroxyl group on a serine at a binding site on acetylcholine esterase, preventing acetylcholine from binding at that site. If acetylcholine esterase is inhibited by an organophosphonate, acetylcholine builds up at the synapses and neuromuscular junctions and the receptor is desensitized resulting in paralysis with an estimated lethal dermal exposure of about 6 milligrams of VX for an average human.
Contact with VX is about 200-fold more toxic than soman (GD) and 300-fold more toxic than sarin (GB).(2) The extreme potential for acute toxicity with VX is due, in part, to the low volatility of this compound, which allows it to persist indefinitely on common surfaces.(3) Methods currently utilized for the destruction of organophosphate nerve agents include high temperature incineration and treatment with strong base or concentrated bleach.(4,5) Medical treatment of VX toxicity is currently limited to the injection of atropine, which reduces neurological symptoms, and oximes, which can help to reactivate the inactivated acetylcholine esterase.(2) Butyrylcholine esterase, which is closely related to acetylcholine esterase, has proven effective in animal models as a stoichiometric scavenger of VX.(6) However, the large amount of enzyme required for treatment with a stoichiometric scavenger, and the limited supply of this protein, have prevented butyrylcholine esterase from being an effective antidote for medical use.(7,8)
Enzymatic hydrolysis of nerve agents provides numerous advantages over harsh physical or chemical methods of decontamination and could provide a catalytic antidote for medical use. Enzymes such as organophosphorus acid anhydrolase (OPAA), diisopropyl-fluorophospatase (DFPase), and human paraoxonase (PON1) can hydrolytically neutralize the various G-type agents, (9,10,11,12) but except for PON1, they have no activity against the V-type agents.(11)
The enzyme phosphotriesterase (PTE) is capable of hydrolyzing a wide variety of organophosphonates including both the G-type and V-type nerve agents.(13,14) A substrate for PTE, the insecticide paraoxon (
The high toxicity and environmental persistence of VX makes the development of novel decontamination methods particularly important. PTE is capable of hydrolyzing VX. The enzymatic efficiency of PTE for VX is more than 5-orders of magnitude lower than with paraoxon. For the hydrolysis of the G-type agents by PTE, the values of kcat/Km are between 104 and 105 M−1 s−1.(13)
The G- and V-type nerve agents all contain a chiral phosphorus center where the SP-enantiomers are significantly more toxic than the corresponding RP-enantiomers.(16,17) In general, wild-type PTE preferentially hydrolyzes the RP-enantiomers of these compounds. The overall selectivity depends on the relative size of the substituents attached to the phosphorus center, with larger differences in size resulting in greater stereoselectivity.(18)
Chiral chromophoric analogues of the G-type agents have been utilized to guide the evolution of PTE for the identification of variants that prefer the more toxic SP-enantiomers of sarin, cyclosarin, and soman.(13,16,18) The catalytic activity of PTE for the more toxic SP-enantiomer of cyclosarin (GF) has been increased by more than 4-orders of magnitude.(13) The catalytic efficiencies for the hydrolysis of the more toxic SP-enantiomers by the enhanced variants of PTE for the hydrolysis of GB, GD, and GF approach 106 M−1 s−1.(13)
Unfortunately, the activity of PTE against the V-type agents is about 3-orders of magnitude lower than that with the G-type agents (kcat/Km<103 M−1 s−1). (14, 19). The net rate of VX hydrolysis by PTE is thought to be limited more by the chemistry of the leaving group than by the stereochemistry of the phosphorus center.(13,14,20) The X-ray crystal structure of PTE shows that this enzyme folds as a distorted (β/α)8-barrel and that the bulk of the active site is formed from the 8 loops that connect the core β-strands to the subsequent α-helices.(21)
The twelve residues which make up the substrate binding site of PTE can be subdivided into three pockets that accommodate the small, large and leaving-group moieties of the substrate.(21)
The residues in the active site have been shown to be largely responsible for the observed substrate specificity.(22) Loop-7 is the largest of the loops that contribute to the substrate binding site, and is known to tolerate substantial sequence variation.(18,21,23) Previous attempts to evolve PTE for the hydrolysis of VX have utilized the insecticide demeton-S with modest success.(24,25)
It would be advantageous to have enzymes that could optimize the hydrolysis of organophosphate nerve agents, including a new analogue and mutation strategies to optimize PTE for the hydrolysis of G-agents and V-agents such as VX.
An embodiment of the disclosure is a synthetic amino acid sequence comprising the synthetic amino acid sequence of VQFL (SEQ ID NO: 2), capable of hydrolyzing organophosphates. In an embodiment, a synthetic DNA sequence encodes the synthetic amino acid sequence of SEQ ID NO: 2. In another embodiment, a synthetic cDNA sequence comprises the coding sequence of the synthetic DNA. In another embodiment, a plasmid comprises the synthetic DNA sequence.
An embodiment of the disclosure is a method of hydrolysis of an organophosphate nerve agent comprising contacting an organophosphate nerve agent with the synthetic amino acid sequence SEQ ID NO: 2; and hydrolyzing the organophosphate nerve agent. In an embodiment, the organophosphate is VX. In another embodiment, the organophosphate is VR. In yet another embodiment, the organophosphate is selected from the group consisting of GB, GD, and GF.
An embodiment of the disclosure is a system for detoxifying an organophosphate nerve agent comprising contacting the synthetic amino acid sequence of SEQ ID NO: 2 with an organophosphate nerve agent.
An embodiment of the disclosure is a kit for detoxifying an organophosphate nerve agent comprising the synthetic amino acid sequence of SEQ ID NO: 2.
An embodiment of the disclosure is the synthetic amino acid sequence of claim 1, further comprising mutations I106C (CVQFL (SEQ ID NO: 3)). In an embodiment, a synthetic DNA sequence encodes the synthetic amino acid sequence of SEQ ID NO: 3. In an embodiment, a synthetic cDNA sequence comprises the coding sequence of the synthetic DNA sequence. In another embodiment, a plasmid comprises the synthetic DNA sequence.
An embodiment of the disclosure is a method of hydrolysis of an organophosphate nerve agent comprising contacting an organophosphate nerve agent with the synthetic amino acid sequence of SEQ ID NO: 3; and hydrolyzing the organophosphate nerve agent. In an embodiment, the organophosphate is VX. In another embodiment, the organophosphate is VR. In yet another embodiment, the organophosphate is selected from the group consisting of GB, GD, and GF.
An embodiment of the disclosure is a system for detoxifying an organophosphate nerve agent comprising contacting the synthetic amino acid sequence of SEQ ID NO: 3 with an organophosphate nerve agent.
An embodiment of the disclosure is a kit for detoxifying an organophosphate nerve agent comprising the synthetic amino acid sequence of SEQ ID NO: 3.
An embodiment of the disclosure is the synthetic amino acid sequence of claim 1, further comprising mutations A80V, K185R, and I274N (VRN-VQFL (SEQ ID NO: 4)). In an embodiment, the synthetic DNA sequence encodes the synthetic amino acid sequence of SEQ ID NO: 4. In another embodiment, the synthetic cDNA sequence comprises the coding sequence of the synthetic DNA sequence. In yet another embodiment, a plasmid comprises the synthetic DNA sequence.
An embodiment of the disclosure is a method of hydrolysis of an organophosphate nerve agent comprising contacting an organophosphate nerve agent with the synthetic amino acid sequence of SEQ ID NO: 4; and hydrolyzing the organophosphate nerve agent. In an embodiment, the organophosphate is VX. In an embodiment, the organophosphate is VR. In an embodiment, the organophosphate is selected from the group consisting of GB, GD, and GF.
An embodiment of the disclosure is a system for detoxifying an organophosphate nerve agent comprising contacting the synthetic amino acid sequence of SEQ ID NO: 4 with an organophosphate nerve agent.
An embodiment of the kit for detoxifying an organophosphate nerve agent comprising the synthetic amino acid sequence of SEQ ID NO: 4.
An embodiment of the disclosure is the synthetic amino acid sequence comprising the synthetic amino acid sequence of L7ep-3a (SEQ ID NO: 5). In an embodiment, a synthetic DNA sequence encodes the synthetic amino acid sequence of SEQ ID NO: 5. In an embodiment, a synthetic cDNA sequence comprises the coding sequence of the synthetic DNA sequence. In yet another embodiment, a plasmid comprises the synthetic DNA sequence.
An embodiment of the disclosure is a method of hydrolysis of an organophosphate nerve agent comprising contacting an organophosphate nerve agent with the synthetic amino acid sequence of SEQ ID NO: 5; and hydrolyzing the organophosphate nerve agent. In an embodiment, the organophosphate is VX. In an embodiment, the organophosphate is VR. In another embodiment, the organophosphate is selected from the group consisting of GB, GD, and GF.
An embodiment of the disclosure is a system for detoxifying an organophosphate nerve agent comprising contacting the synthetic amino acid sequence of SEQ ID NO: 5 with an organophosphate nerve agent.
An embodiment of the disclosure is a kit for detoxifying an organophosphate nerve agent comprising the synthetic amino acid sequence of SEQ ID NO: 5.
An embodiment of the disclosure is a synthetic amino acid sequence comprising the synthetic amino acid sequence of L7ep-3a I106G (SEQ ID NO.: 6), capable of hydrolyzing organophosphates. In an embodiment, a synthetic DNA sequence encodes the synthetic amino acid sequence of SEQ ID NO.: 6. In another embodiment, a synthetic cDNA sequence comprising the coding sequence of the synthetic DNA sequence. In yet another embodiment, a plasmid comprising the synthetic DNA sequence.
An embodiment of the disclosure is a method of hydrolysis of an organophosphate nerve agent comprising contacting an organophosphate nerve agent with the synthetic amino acid sequence of SEQ ID NO.: 6; and hydrolyzing the organophosphate nerve agent. In an embodiment, wherein the organophosphate is VX. In another embodiment, the organophosphate is VR. In yet another embodiment, the organophosphate is selected from the group consisting of GB, GD, and GF.
An embodiment of the disclosure is a system for detoxifying an organophosphate nerve agent comprising contacting the synthetic amino acid sequence of SEQ ID NO.: 6 with an organophosphate nerve agent.
An embodiment of the disclosure is a kit for detoxifying an organophosphate nerve agent comprising the synthetic amino acid sequence of SEQ ID NO.: 6.
An embodiment of the disclosure is a method of producing variants of phosphotriesterase, wherein the variants are capable of detoxifying an organophosphate nerve agent, comprising the steps of: obtaining a PTE gene; inserting the PTE gene into a vector; preparing a series of sequential mutational libraries wherein the PTE gene encodes a synthetic amino acid sequence of SEQ ID NO.: 6; expressing the variant as a protein; screening the variant for catalytic activity against one selected from the group consisting of DEVX, DMVX, DEVR, and OMVR to determine the hydrolytic activity; and selecting the variant for use in hydrolysis of an organophosphate nerve agent based upon its hydrolytic activity. In an embodiment, the variant synthetic amino acid sequence is at least 80% homogenous to the synthetic amino acid sequence of SEQ ID NO.: 6.
An embodiment of the disclosure is a method of producing variants of phosphotriesterase, wherein the variants are capable of detoxifying an organophosphate nerve agent, comprising the steps of: obtaining a PTE gene; inserting the PTE gene into a vector; preparing a series of sequential mutational libraries wherein the PTE gene encodes a synthetic amino acid sequence comprising the mutations I106C, F132V, H254Q, H257Y, A270V, L272M, I274N, and S308L (SEQ ID NO.: 5); expressing the variant as a protein; screening the variant for catalytic activity against one selected from the group consisting of DEVX, DMVX, DEVR, and OMVR to determine the hydrolytic activity; and selecting the variant for use in hydrolysis of an organophosphate nerve agent based upon its hydrolytic activity. In an embodiment, the variant comprising the mutations I106C, F132V, H254Q, H257Y, A270V, L272M, I274N, and S308L synthetic amino acid sequence is at least 80% homogenous to the synthetic amino acid sequence of SEQ ID NO.: 5)
The foregoing has outlined rather broadly the features of the present disclosure in order that the detailed description that follows can be better understood. Additional features and advantages of the disclosure will be described hereinafter, which form the subject of the claims.
In order that the manner in which the above-recited and other enhancements and objects of the disclosure are obtained, a more particular description of the disclosure briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the disclosure and are therefore not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through the use of the accompanying drawings in which:
The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the disclosure. In this regard, no attempt is made to show structural details of the disclosure in more detail than is necessary for the fundamental understanding of the disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the disclosure can be embodied in practice.
The following definitions and explanations are meant and intended to be controlling in any future construction unless clearly and unambiguously modified in the following examples or when application of the meaning renders any construction meaningless or essentially meaningless. In cases where the construction of the term would render it meaningless or essentially meaningless, the definition should be taken from Webster's Dictionary 3rd Edition.
As used herein the term, “wild-type” means and refers to the non-mutated version of a gene as it appears in nature.
As used herein the term, “enantiomer” means and refers to either of a pair of chemical compounds that have molecular structures that are nonsuperimposable mirror images.
As used herein the term, “G-agent” or “G-type” means and refers to nerve agents of the G (German) series. The series includes but is not limited to GA (tabun), GB (sarin), GF (cyclosarin) and GD (soman).
As used herein the term, “V-type” means and refers to nerve agents of the V series. The series includes but is not limited to VX, VE, V-gas, VG, VR, and VM.
Table 1. Mutations present in additional variants identified
Table 2. Activity of PTE Variants against VX Analogues DEVX and Compound 1 (RP-1 and SP-1) (see
Table 3. Activity of additional PTE Variants with DEVX
Table 4. Kinetic Parameters for PTE Variants with Paraoxon and Demeton-S
Table 5. Activity of PTE Variants with Racemic VX
Table 6. Identification of Mutants
Table 7. Activity of Wild-Type and Mutant Enzymes with Racemic G-Agents
Table 8. Kinetic Constants for Hydrolysis of GB, GD, and GF
Table 9. Values of kcat (s−1) for Wild-Type PTE and its Mutants
Table 10. Values of kcat/Km (M−1 s−1) for Wild-Type PTE and its Mutants
Table 11. Kinetic constants for PTE variants with APVR.
Table 12. Kinetic parameters for PTE variants with paraoxon and DEVX.
Table 13. Kinetic constants with the racemic nerve agents VX and VR.
Table 14. X-ray crystallography data for L7ep-3a and L7-ep-3a I106G.
Table 15. Kinetic constants for PTE variants with V-agent analogs.
The V-type organophosphorus nerve agents are among the most hazardous compounds known. Previous efforts to evolve the bacterial enzyme phosphotriesterase (PTE) for the hydrolytic decontamination of VX resulted in the variant L7ep-3a, which has a kcat value more than 2-orders of magnitude higher than wild-type PTE. Because of the relatively small size of the O-ethyl, methylphosphonate center in VX, stereoselectivity is not a major concern. However, the Russian V-agent, VR, contains a larger 0-isobutyl, methylphosphonate center making stereoselectivity a significant issue since the SP-enantiomer is more toxic than the RP-enantiomer. The three-dimensional structure of the L7ep-3a variant was determined to a resolution of 2.01 Å (PDB id: 4ZST). The active site of the L7ep-3a mutant has revealed a network of hydrogen bonding interactions between Asp-301, Tyr-257, Gln-254 and the hydroxide that bridges the two metal ions. A series of new analogs that mimic VX and VR has helped to identify critical structural features for the development of new enzyme variants that are further optimized for the catalytic detoxification of VR and VX. The best of these mutants has been shown to hydrolyze the more toxic SP-enantiomer of VR more than 600-fold faster than the wild-type phosphotriesterase. The organophosphorus nerve agents are among the most toxic compounds known. Compounds such as sarin, soman and VX are all chiral methyl phosphonates where the toxicity of the SP-enantiomer is much greater than for the RP-enantiomer.(33) Recent events have dramatically demonstrated the continuing importance of developing rapid and environmentally compatible methods for the decontamination of these compounds.(34) This situation is particularly true for the V-type nerve agents, where the lethal dose is approximately 6 mg/person, and these compounds have been shown to persist for long periods of time.(35, 36) Significant advances have been made in developing enzymatic methods of decontamination for the G-type and VX nerve agents using the bacterial enzyme phosphotriesterase (PTE).(37, 38) While wild-type PTE has reasonable activity against the G-type nerve agents (kcat/Km˜105 M−1 s−1), this enzyme preferentially hydrolyzes the less toxic RP-enantiomers.(39) Directed evolution of PTE to specifically target the G-type nerve agents has led to the identification of the variant H257Y/L303T (YT), which has proven highly efficient at the hydrolysis of the more toxic SP-enantiomer of sarin (GB), soman (GD), and cyclosarin (GF) with values of kcat/Km that exceed 106 M−1 s−1.(38)
Wild-type PTE exhibits little stereoselectivity against the relatively small phosphonate center of VX, but the elevated pKa of the thiol-leaving group provides a significant challenge for enzyme-catalyzed hydrolysis (kcat/Km˜102 M−1 s−1).(37) Mutation of residues contained within the active site of PTE resulted in the isolation of the variant H245Q/H257F (QF), which exhibited a 100-fold improvement for the hydrolysis of VX, relative to the wild-type enzyme (see Table 1 for identity of variants).(37) Additional active site variations resulted in the identification of the mutant CVQFL (QF+I106C/F132V/S308L) with a similar catalytic efficiency for the hydrolysis of VX, but a three-fold improvement in kcat. The best variant identified to date for the hydrolysis of VX is VRN-VQFL (QF+F132V/S308L+A80V/K185R/1274N). This mutant combines expression-enhancing mutations (A80V/K185R/1274N) with additional changes in the active site to achieve a kcat/Km of 7×104 M−1 s−1 for the hydrolysis of the SP-enantiomer of VX. (37, 40, 41) Expansion of the mutation strategy to targeted error-prone PCR, led to the identification of the variant L7ep3a (CVQFL+H257Y/A270V/L272M/I274N), which has a kcat value enhanced 152-fold relative to wild-type PTE.(37) How these combined mutations, some of which do not fall in the active site, are able to bring about such a dramatic improvement in catalytic ability is not clear.
In addition to VX, the V-agents include the Russian (VR) and Chinese versions. Exemplified by VR, these additional V-agents contain a smaller thiol leaving group, and a larger ester group attached to the phosphorus center (
PTE variants can be used to decontaminate areas, equipment, and personnel after they come in contact with V-type or G-type nerve agents. This is especially useful for military or homeland security applications. The decontamination occurs without exposing the area, equipment, or personnel to harsh chemicals. Mutations in the sequence of PTE have been made to increase the ability of PTE to hydrolyze, and thus decontaminate, an area, equipment, or personnel. PTE was subjected to directed evolution for the improvement of catalytic activity against selected compounds through the manipulation of active site residues. A series of sequential two-site mutational libraries encompassing twelve active site residues of PTE was created. The libraries were screened for catalytic activity against a new VX analogue (DEVX), which contains the same thiolate leaving group of VX coupled to a di-ethoxy phosphate core rather than the ethoxy, methylphosphonate core of VX. The catalytic activity with DEVX was enhanced 26-fold relative to wild-type PTE. Further improvements were facilitated by targeted error-prone PCR mutagenesis of Loop-7 and additional PTE variants were identified with up to a 78-fold increase in the rate of DEVX hydrolysis. The best mutant hydrolyzed the racemic nerve agent VX with a value of kcat/Km of 7×104 M−1 s−1; a 230-fold improvement relative to the wild-type PTE. The sequence of wild-type PTE (organophosphate-degrading protein (opd)), without the leader peptide residues (1-29), is found in
The P—S bond in VX is chemically more stable than the P—F bond found in the G-agents.(20) (
The QF variant of PTE shows improved hydrolysis against the SP-enantiomer of a chiral VX analogue.(18) Table 1 lists the amino acid changes present in the variants. This mutant was found to be significantly better for the hydrolysis of the P—S bond in DEVX and demeton-S than wild-type PTE. The synergistic mutations in this variant suggested that further improvements in catalytic activity could be facilitated by simultaneously mutating pairs of residues in the active site. The initial mutant libraries targeted pairs of residues in the active site that modulated the size and shape of the three substrate binding pockets. Sequential optimization of the active site residues resulted in an 18-fold improvement in catalytic activity against DEVX. Combining the best variant (VQFL) with expression enhancing mutations resulted in the variant VRN-VQFL. The VRN-VQFL variant exhibited a 26-fold improvement for the hydrolysis of DEVX.
Additional strategies were used to further enhance the activity of PTE against the phosphorothiolate bond. Error-prone PCR is a useful technique for enzyme evolution, but the mutation frequencies are typically restricted to 1-3 base pair changes per gene because of the significant chance of introducing deleterious mutations. Substantial improvements in enzyme activity can require numerous amino acid changes, which are not typically achievable by error-prone PCR. Targeting error-prone PCR to only Loop-7 (residues 253-276) resulted in a mutation library with an average of 6 mutations per gene but still retained>20% active colonies. The hydrolysis of DEVX for one of the variants (L7ep-3) was improved to 36-fold over wild-type PTE due to 3 additional amino acid changes. The best variant identified (L7ep-2) was improved 63-fold for the hydrolysis of DEVX by 5 additional amino acid changes. Further optimization of L7ep-2 and L7ep-3 resulted in additional mutations that improved the activity to 78-fold (L7ep-2a) and 71-fold (L7ep-3a) over wild-type PTE, and achieved turnover numbers for the hydrolysis of the phosphorothiolate bond in excess of 100 s−1.
Hydrolysis of VX. A full kinetic characterization of wild-type and improved variants of PTE using racemic VX was conducted. Wild-type PTE exhibited low activity against VX, but there was a dramatic improvement with the QF mutant. The mutations in the large group pocket resulted in substantial improvements to kcat. The best variant identified (VRN-VQFL) against VX combines active site mutations in all three pockets and has a kcat/Km value that is increased 235-fold over wild-type PTE (
The Loop-7 optimized variants show good activity against VX, but did not demonstrate improved activity relative to the VRN-VQFL variant. The changes to Loop-7 resulted in substantial improvements in kcat but little change in the catalytic efficiency.
The L7ep-3a variant has a kcat of 137 s−1 for the hydrolysis of VX. This value is the highest ever reported for the enzymatic hydrolysis of VX.(11,14,24,25) Single concentration experiments with the H254R/H257L mutant of PTE showed an improvement of 10-fold against racemic VX.(24) Another variant exhibited a 26-fold improvement over wild-type PTE at 0.5 mM VX.(25) By contrast, the VRN-VQFL and L7ep-3 variants are improved by more than 200-fold in the value of kcat/Km. Human PON1 (paraoxonase) has been evolved in the laboratory for the hydrolysis of VX, but the reported value of kcat/Km for the best variant is 2.5×103 M−1 s−1, whereas the best PTE variant (VRN-VQFL) identified in this investigation has a value of kcat/Km of 7×104 M−1 s−1.(11)
Stereochemical Preferences of Active Site Mutants. The toxicity of the organophosphate nerve agents depends on the stereochemistry of the phosphorus center.(17) With VX, it is estimated that the SP-enantiomer is about 100-fold more toxic than the RP-enantiomer. The QF mutant prefers to hydrolyze the more toxic SP-enantiomer of VX by a factor of 12, relative to the RP-enantiomer, whereas the L7ep-2b mutant prefers to hydrolyze the RP-enantiomer by a factor of 12. The stereochemical preferences for the hydrolysis of VX are fully consistent with the stereoselective properties of these two mutants for the hydrolysis of SP-1 and RP-1, suggesting that the variants VRN-VQFL, L7ep-3, and L7ep-3a also prefer the SP-enantiomer of VX. While the modest selectivity prevented definitive assignment of the preferred enantiomer, complete neutralization of VX by the VRN-VQFL mutant via the hydrolysis of both enantiomers was demonstrated by 31P-NMR spectroscopy (
The reconstruction of PTE for the hydrolysis of VX has resulted in dramatic improvements in the values of kcat and kcat/Km, relative to the wild type enzyme. It is proposed that the increase in the catalytic constants has been achieved by an increase in the rate constant for cleavage of the P—S bond (k3) rather than changes in the formation of the ternary complex (k1, k2) or the rate constant for product release (k5) as illustrated in a minimal kinetic mechanism (Scheme 1). The thiol leaving group of VX has a higher pKa than the fluoride leaving group of the G-agents, and the p-nitrophenol group of paraoxon. It has been demonstrated with the wild-type PTE that the chemical step (k3) is rate limiting for substrates with leaving groups having pKa values higher than 8.(20) Disruption of the hydrogen bonded network from D301-H254-D233 reduced the rate of hydrolysis of substrates with leaving groups having low pKa values but increased the rate of hydrolysis of substrates with leaving groups of higher pKa values.(30) Introduction of a glutamine at residue position 254 (as in the initial QF mutant), which apparently cannot support the transport of a proton away from the active site, may now facilitate the protonation of the thiol group by Asp-301 as the phosphorothiolate bond is cleaved.
The turnover numbers for some slow substrates of PTE are thought to be reflective of the ability of the enzyme to align the substrate with the nucleophilic hydroxyl group attached to the binuclear metal center.(13) There is a strong likelihood that for some of the variants, subtle changes in the conformation of the active site will facilitate a better alignment between the substrate and attacking hydroxide, thereby achieving higher enzymatic rates of hydrolysis. In particular, the Loop-7 variants have been modified at residues that are somewhat distant from the active site, but are expected to bring about changes in the positioning of the Loop-7 α-helix.(13,28) This alignment effect would, of course, differ between the di-ethoxy phosphorus center of DEVX and the methylphosphonate core of VX (
The structure of the L7ep3a variant was determined. The structure aided in understanding the chemical mechanism for the enhancement of phosphorothiolate bond cleavage. With this new structural information, a series of PTE variants was created to incorporate changes in the active site of PTE that would more easily accommodate the O-isobutyl group of VR. To facilitate the further development of PTE for the hydrolysis of various V-agents, a new series of analogs were designed and synthesized. Enhanced variants of PTE have been identified that have more than a 600-fold improvement in the catalytic activity for the hydrolysis of the toxic SP-enantiomer of VR.
Most chemicals can be obtained from Sigma Chemical Company. The pfuTurbo DNA polymerase can be obtained from Agilent Technologies and the various restriction enzymes can be acquired from New England Biolabs. The two enantiomers of compound 1 (
Active Site Library Construction. The nucleotides in the gene for PTE were modified to replace the nucleotides encoding the leader peptide (amino acid residues 1-29) with nucleotides encoding a methionine. (
Construction of Targeted Error-Prone Library. To construct the Loop-7 error-prone library, a set of 30-bp primers corresponding to the DNA sequences upstream and downstream of Loop-7 (amino acid residues 253-276) were used to amplify the PTE gene in three fragments. The amino acid residue numbering of the sequence including the leader sequence has been retained. The DNA coding region of Loop-7 was amplified in an error-prone PCR reaction while the two remaining fragments were amplified using standard PCR techniques. The final gene was constructed using PCR overlap-extension, resulting in a gene library with errors only in the coding region for residues 253-276.
Optimization of Error-Prone Variants. The five residue positions (254, 265, 270, 272, and 276) identified in the best Loop-7 error prone variant and residues 257 and 274 were further optimized by construction of two two-site (254X/257X and 272X/274X) and three single-site libraries (265X, 270X, 276X). The amino acid residue numbering of the sequence including the leader sequence has been retained. Libraries were constructed via QuikChange mutagenesis using degenerate primers to allow all 20 amino acids at the positions of interest. Approximately 200 colonies from each single-site library were screened, and approximately 1200 colonies from each two-site library were screened. The two enhanced variants identified in the Loop-7 error prone library were used as the template for a second round of targeted error-prone PCR of Loop-7.
Library Screening. Plasmid libraries were transformed into BL21 (DE3) E. coli competent cells and grown on LB plates. For all library transformations, the amount of DNA was kept low (<10 ng) to avoid the potential complication of double transformants.(27) Single colonies were used to inoculate 0.75 mL cultures of Super Broth (32 g tryptone, 20 g yeast extract, 5 g NaCl, and 0.4 g NaOH in 1 L H2O) supplemented with 0.5 mM CoCl2 in a 96-well block format. Cultures were grown at 37° C. for 8 hours. The temperature was reduced to 30° C. and protein expression induced by addition of 1 mM IPTG. Following 16 hours of additional growth, the bacteria were harvested by diluting a portion of the culture in a 1:1 ratio with 50 mM HEPES pH 8.0, 100 μM CoCl2, 10% BugBuster® 10× (EMD Chemicals). Cultures were tested for activity against DEVX using a standard 250 μL assay that consisted of 50 mM HEPES, pH 8.0, 100 μM CoCl2, 0.3 mM 5,5′-dithiobis(2-nitro-benzoic acid)(DTNB) and 0.2-0.5 mM DEVX. The reactions were initiated by the addition of 10 μL of cell lysate. Reactions proceeded at room temperature until color was clearly visible (1-4 hours). Product formation was determined by the change in absorbance at 412 nm using a plate reader. The variant used as the starting template for each library was included as a control on each plate. To account for differential culture growth, the final change in absorbance was normalized using the OD600 for each culture compared to the average OD600 of controls. The colonies giving the best results were re-grown as 5 mL overnight cultures and the plasmids harvested and sequenced to identify the variants.
Kinetic Measurements. All assays with DEVX, paraoxon, SP-1, RP-1, and demeton-S were 250 μL in total volume and followed for 15 minutes in a 96-well plate reader at 30° C. Assays with VX were conducted in a volume of 500 μL in 1 mL cuvettes. DEVX, demeton-S, and VX assays monitored the release of the product thiol at 412 nm (Δε412=14,150 M−1 cm−1) by the inclusion of DTNB in the reaction mixture (50 mM HEPES, pH 8.0, 100 μM CoCl2, and 0.3 mM DTNB). Assays with paraoxon and compound 1 (
v/E
t
=k
cat(A)/(Km+A) (Equation 1)
Stereoselective Hydrolysis of Racemic VX. Low initial concentrations (19 to 160 μM) of racemic VX were hydrolyzed by variants of PTE in a solution containing 0.1 mM CoCl2, 0.3 mM DTNB, and 50 mM Hepes, pH 8.0. The reactions were followed to completion and the fraction of VX hydrolyzed plotted as a function of time. The time courses were fit to equations 2 and 3 where F is the fraction of substrate hydrolyzed, a and b are the magnitudes of the exponential phases, t is time, and k1 and k2 are the rate constants for each phase.
F=a(1−e−k
F=a(1−e−k
To identify which one of the two enantiomers of VX was preferentially hydrolyzed by the PTE variants, one gram of racemic VX was hydrolyzed in a 400 mL reaction mixture containing 50 mM bis-tris-propane (pH 8.0), 100 μM CoCl2, and 36 nM of the QF mutant (H254Q/H257F) at 33° C. The reaction was monitored by determining the concentration of the thiol product with DTNB. When the reaction was approximately 50% complete, the remaining VX was extracted with 200 mL of ethyl acetate. The volume of the extract was reduced to approximately 2 mL by rotary evaporation at 41° C. The unreacted VX was analyzed with a polarimeter and observed to rotate plane polarized light in a positive direction (+0.055° to +0.075°) which corresponds to an enantiomeric preference for hydrolysis of the SP-enantiomer of VX by the QF mutant.(17)
Construction and Screening of Active Site Libraries. The variant QF (H254Q/H257F) was previously identified as being improved against the chiral centers in VX and VR.(18) Testing the catalytic activity of this mutant with the VX analogue, DEVX, revealed that this variant has an enhanced activity for the hydrolysis of the phosphorothiolate bond, relative to wild-type PTE. The amino acid residue numbering of the sequence including the leader sequence has been retained. The variant QF then served as the starting point for the construction of the F306X/Y309X and W131X/F132X double-substitution protein libraries. Screening 920 colonies from the F306X/Y309X library with DEVX failed to identify any variant that was improved relative to the QF parent. From the W131X/F132X library, a total of 1100 colonies were screened with DEVX and the two best mutants were identified as LQF (QF+F132L) and VQF (QF+F132V). The LQF variant served as the starting template for the 254X/257X library. Approximately 1650 colonies were screened from this library with DEVX, but none proved to be better for the hydrolysis of DEVX.
The 271X/308X library was created using sequential QuikChange procedures; first at position 271 then at position 308 using the LQF template. Approximately 2200 colonies from this library were screened and the best variant was LQFL (LQF+S308L). Incorporation of the new mutation (S308L) into the previously identified VQF variant further enhanced the catalytic activity. The variant VQFL (VQF+S308L) was utilized as the parent for the 106X/303X library. Approximately 1100 colonies were screened with DEVX and the best variant identified was CVQFL (VQFL+I106C). The variant CVQFL was carried forward in the construction of the 60X/317X library. Nearly 1500 colonies from this library were screened with DEVX, but improved variants were not detected.
A number of mutations are known to improve protein expression levels for PTE, including A80V, K185R, and I274N.(28,29) These mutations do not typically result in significant changes in the kinetic constants for a given substrate, but they dramatically improve the amount of enzyme produced per liter of cell culture. Adding these expression-enhancing mutations to VQFL resulted in an additional variant, VRN-VQFL (A80V/K185R/I274N+VQFL) with a 26-fold improvement in the value of kcat/Km, relative to the wild-type PTE. The inclusion of two additional expression-enhancing mutations (D208G/R319S) resulted in a decrease in catalytic activity.(29) Kinetic constants for the PTE variants with DEVX as the target substrate are presented in Table 2. Kinetic constants for additional variants are provided in Table 3.
Construction of Targeted Error-Prone Library. The CVQFL variant was used as the parent for the construction of an error-prone library with an average of six mutations per gene, targeted exclusively to Loop-7 of PTE (residues 253-276). The amino acid residue numbering of the sequence including the leader sequence has been retained. Approximately 4000 colonies from this library were screened with DEVX and a total of 12 variants were identified as being more active than the parent, CVQFL. The values of kcat/Km for the best variants, L7ep-1, L7ep-2 and L7ep-3, were improved 27-, 63-, and 36-fold, respectively, for the hydrolysis of DEVX.
The variant L7ep-2 (CVQFL+H254R/N265D/A270D/L272M/S276T) has 5 amino acid changes to the sequence of Loop-7, relative to the parent. These five sites, and residue positions 257 and 274, were subjected to further optimization. Two, two-site libraries (R254X/F257X, and M272X/I274X) and three, single-site libraries (D265X, D270X, and T276X) were constructed to ensure that the optimum amino acid residue is represented at each position. Screening the libraries with DEVX revealed no improvements at residue positions 254, 257, 265, or 270, but numerous improved combinations were identified in the 272X/274X library. One of these variants, L7ep-2a (L7ep-2+I274T), has a kcat of 135 s−1 and a kcat/Km 78-fold improved over wild type enzyme. To further optimize the L7ep-3 variant (CVQFL+H257Y/A270V/L272M), Loop-7 was subjected to a second round of targeted error-prone PCR. Screening with DEVX identified two variants, L7ep-3a (L7ep-3+I274N) and L7ep-3b (L7ep-3+A270D) that were substantially improved over the parent enzyme. Similar experiments were attempted using L7ep-2 as the starting template, but no improved variants were identified. The kinetic constants for the hydrolysis of DEVX by these mutants are presented in Table 2 and Table 3.
Stereoselectivity of PTE Variants for Chiral VX Analogues. In this investigation there was no attempt to include a stereochemical preference in the screening of mutant libraries. Wild-type PTE is known to have a slight preference for the SP-enantiomer of the VX chiral center.(18) To determine that there were no perturbations in stereo selectivity, the variants with improved catalytic activity against DEVX were analyzed using the chromophoric analogues RP-1 and SP-1, and the results are presented in Table 2. With the exception of L7ep-2, L7ep-2a, and L7ep-2b, the evolved variants have values of kcat/Km that are greater for the SP-enantiomer than for the RP-enantiomer.
Enzymatic Specificity. To assess changes in substrate specificity, the enzyme variants were tested with paraoxon and demeton-S as alternative substrates. The results are provided in Table 4. The variants from the active site evolution experiments maintain a high enzymatic efficiency for paraoxon, although it is reduced nearly an order of magnitude from wild-type enzyme. The catalytic activity using demeton-S did not show any significant improvement for most of the variants tested. The exceptions are L7ep-2, L7ep-2a, and L7ep-2b, which have increased kcat and decreased Km values for demeton-S.
aStandard errors from fits of the data fit to equation 1 are less than 10% of the stated values
Hydrolysis of Racemic VX. Wild-type PTE and selected variants were characterized using racemic VX as a substrate and the results are presented in Table 5. Wild-type PTE has a low kcat (0.9 s−1) and relatively high Km, resulting in a diminished kcat/Km for the hydrolysis of VX. The variant QF dramatically improves both kcat and Km values resulting in a 100-fold increase in kcat/Km. The VRN-VQFL variant had the highest value of kcat/Km that was increased more than 230-fold over wild-type enzyme. VRN-VQFL includes the following mutations A80V, K185R, I274N, F132V, H254Q, H257F, and S308L. The L7ep-3a had the highest kcat and was increased more than 150-fold, relative to wild-type PTE. The QF mutant was shown to preferentially hydrolyze the SP-enantiomer of VX by polarimetry.
aStandard errors from fits of the data to equation 1.
bIdentity of preferred enantiomer was not determined for variants with less than a 10-fold preference.
cND = not determined.
Stereoselective hydrolysis of racemic VX by the PTE mutants was evaluated by analyzing the time courses for the complete hydrolysis of VX at concentrations below the Michaelis constant. The presence of stereoselectivity in these time courses is manifested as the appearance of two exponential phases as observed with the QF variant where the ratio of rate constants is 12:1 (
The CVQFL variant exhibited no selectivity (
Construction of Active Site libraries by Overlap Extension. Mutagenic primers contained an NNS codon at the position of interest and extended 15 bp to either side of this codon. The PTE gene was amplified in three segments using standard PCR techniques (10 ng template and 125 ng each primer in a 50 μL reaction using pfuTurbo polymerase). The first segment extended from the 5′ end of the gene to 15 bp beyond the first mutagenic position. The second segment extended from 15 bp upstream of the first mutagenic position to 15 bp downstream of the second mutagenic position. The third segment extended from 15 bp upstream of the second mutagenic site to the 3′ end of the gene. The 5′ and 3′ primers included NdeI and EcoRI restriction sites respectively. A second PCR reaction was performed using the generated segments as the template DNA. The three fragments were combined in equimolar ratio (500 ng total) and amplified for 30 cycles with pfuTurbo using the primers for the 5′ and 3′ ends of the PTE gene. The overlaps between the fragments allowed for the formation of a single product corresponding to the size of the complete PTE gene. The product and vector were then digested with NdeI and EcoRI, gel purified and ligated together.
Construction of Error Prone Library. Primer pairs used to amplify Loop-7 corresponding to the DNA sequence for residues 242-252 and 277-287. The reaction contained 20 ng template (CVQFL variant), 1 μM forward and reverse primers, 0.35 mM dATP, 0.4 mM dCTP, 0.2 mM dGTP, 1.35 mM dTTP, 1 mM MgCl2, 1× GoGreen Taq Buffer (Promega, Madison Wis.) 1.5 mM MnCl2 and 1 μL Go Taq in 50 μL reaction. Thermocycler program was 2 min initial denaturation at 95° C., followed by 30 cycles of 95° C. for 45 s, 60° C. for 1 min, 72° C. for 3 min, and a final elongation at 72° C. for 10 min. The remaining portions of the PTE gene were amplified using standard PCR techniques with the reverse primers for the Loop-7 fragment and the 5′ and 3′ end primer, resulting in three overlapping fragments. The final gene product was constructed by the overlap extension technique as described above. The mutated gene was digested with NdeI and EcoRI and ligated into pET 20 b (40 μL reaction containing 60 ng vector DNA, 3× molecular excess of PTE gene product, 4 μL T4 DNA ligase buffer and 2 μL T4 DNA ligase (NEB). Sequencing confirmed an average of 6 base pair changes per gene in loop-7. The identities of mutants from this library are given in Table 3.
Enzyme Expression and Purification. BL21 (DE3) cells containing plasmid with wild-type or variant PTE were grown for ˜8 hours in 5 mL LB broth. 1 L cultures of Terrific Broth (12 g Tryptone, 24 g yeast extract, 4 mL glycerol, 2.3 g KH2PO4, 12.5 g K2HPO4 in 1 L H20) supplemented with 1.0 mM CoCl2 were inoculated with 1 mL of the growing culture. Cells were grown overnight at 30° C. with shaking. Protein expression was induced by addition of 1.0 mM IPTG and expression proceeded for an additional 24 hours. Cells were harvested by centrifugation at 11,000 g for 10 minutes. Cell pellets were stored at −80° C. prior to use. Cells from 1 L of culture were resuspended in 100 mL purification buffer (50 mM HEPES (pH 8.5), 100 μM CoCl2). Cellular lysis was achieved by sonication on ice for a total of 20 minutes using a medium power setting. Cell debris was removed by centrifugation at 18,500 g for 10 minutes. Protamine sulfate (0.45 g in 20 mL purification buffer) was added dropwise and incubated for 20 minutes to remove nucleic acids. Precipitated materials were removed by centrifugation at 18,500 g for 10 minutes. Supernatant was brought to 60% saturation with ammonium sulfate and stirred in the cold for 30 minutes to precipitate PTE. Protein was removed from the supernatant by centrifugation at 18,500 g for 20 minutes. The supernatant was decanted and the pellet re-dissolved in 5 mL purification buffer. Up to 5 mL of the protein solution was loaded on a Superdex 200 (16/60) preparatory size exclusion column on a GE Health Care (Piscataway, N.J.) AKTA FLPC system. Peak fractions were collected and assayed for activity against paraoxon. Fractions with the most activity were further purified using a gravity-fed DEAE column pre-equilibrated in purification buffer.
Synthesis of DEVX. DEVX was made by the reaction of diethylchlorophosphate with N,N-diisopropylaminoethanthiol. 1.5 grams of N,N-diisopropylaminoethanthiol was added to 100 mL diethyl ether and cooled in a dry ice acetone bath and purged with N2 gas. To this mixture, 7.5 ml of a 2.5 M solution of butyryl lithium in hexane was added. 1.5 g of diethylchlorophosphate was mixed with 30 mL diethyl ether in a separate flask purged with N2 and cooled in a dry ice acetone bath. The cooled diethylchlorophosphate solution was then added to the thiol solution and the reaction stirred at room temperature for 3 hr. The reaction was then brought to 400 mL with ethyl ether and extracted with water to remove side products. Product was then extracted into the aqueous phase with 0.5 M HCl and ethyl acetate. The aqueous phase was neutralized with sodium bicarbonate and extracted with chloroform. The organic phase was dried over MgSO4 filtered and evaporated yielding the desired product as a pure oil.
1H NMR (300 MHz, CDCl3): 4.05-4.174 (4H, m, OCH2CH3), 3.20-2.50 (6H, m, SCH2CH2N(CH)2),
1.41-1.36 (6H, t, J=6.9 Hz, OCH2CH3), 1.05-1.03 (12H, d, J=4.8 Hz, CH(CH3)2
31P NMR (121.4 MHz CDCl3): 29.77 ppm.
Purification of racemic VX. VX samples were Chemical Agent Standard Analytical Reference Material (CASARM) and were of the highest purity available, typically 99.9+/−5.4 weight % by oxidation—reduction titration, traceable to National Institute of Standards and Technology through 0.1 N iodine solution SRM 136e. However, as received, the VX gave high background readings at 412 nm at the concentrations required for kinetic analysis and therefore required further purification as follows: 80.1 μL neat VX was added to 120 μL isopropyl alcohol (to aid in dissolution), then added to 800 μL of 3 mM DTNB in 50 mM HEPES, pH 8.0. To this solution was added approximately 1 gram of Dowex® 1×4 chloride form beads (Sigma-Aldrich) and agitated gently for several minutes until the beads turned red. The VX was subsequently decanted and added to more beads until essentially all the yellow color was removed to the beads and the VX solution was almost colorless. A standard curve was then generated using 6, 30, 60 and 96 μM dilutions of VX, reacted to completion enzymatically. VX concentration in the bead-treated solution was determined by linear regression analysis using the standard curve from the direct dilutions of VX.
NMR Data Acquisition: All spectra were recorded on non-spinning samples at 25±2° C. with a Varian Unity INOVA 600 spectrometer (600 MHz 1H operating frequency) fitted with a triple resonance, z-gradient probe. Routine 1H free induction decay (FID) data sets of 16,384 complex points were collected as summations of eight or 16 acquisitions recorded with 10 ppm spectral windows, 90° pulse widths of 12 μsec, and 2 sec relaxation delays before archiving to computer disk. FID data sets were apodized with a line broadening factor of 0.3 Hz before Fourier transformation into spectra, manual phase correction into pure absorption mode, and chemical shift referencing to external tetramethylsilane.
31P FID data sets of 65,536 complex points were collected as summations of 32 acquisitions using 100 ppm spectral windows and 90° pulse widths of 30 μsec. All 31P data acquisitions incorporated inverse-gated 1H decoupling (decoupling only during FID acquisition) with a low power composite pulse sequence to increase signal-to-noise ratios without signal enhancements from 1H-31P nuclear Overhauser effects.(31) Spin-lattice relaxation times (T1) for the VX 31P signal and that for the O-ethyl methylphosphonate (EMP) hydrolysis product were measured with the inversion recovery pulse sequence [180°-τ-90°-acquisition] incorporating nine randomized T delays. For quantitative 31P spectra, data sets were collected with relaxation delays>5T1 for all 31P signals in the spectra (˜12 sec) to allow complete signal relaxation, and the spectrometer carrier frequency was centered between the VX substrate signal (ca. 57 ppm) and that of the O-ethyl,methylphosphonate (EMP) hydrolysis product (ca. 23 ppm) to minimize off-resonance effects. The 31P{1H} (1H decoupled, 31P observe) data sets were apodized with a 5 Hz line broadening factor before Fourier transformation into spectra and manual phase correction into pure absorption mode. 31P chemical shift values in spectra were referenced to external 85% phosphoric acid at −0.73 ppm.(32)
NMR Observation of Enzymatic Hydrolysis of VX: The enzymatic hydrolysis of VX in the presence of PTE enzymes was observed by using NMR spectroscopy to follow VX disappearance, or the appearance of its EMP hydrolysis product, over time. Enzymatic reactions were initiated by adding 0.1-25.0 μL of a single enzyme solution to a 1 mL aliquot of a racemic VX solution and briefly mixing before transferring to a NMR sample tube. This was immediately placed into the NMR spectrometer, and quantitative 31P{1H} FID data sets were acquired at 7.5 min time intervals over 20-75 min. Enzymatic hydrolysis rates were calculated directly from the integral values of the quantitative 31P{1H} signals, and included subtraction of the measured spontaneous rate (˜55 mole hr−1) determined in separate experiments. The VX signal intensity decreases throughout the entire time course of the experiment until 75.0 min., where ≧99% of the intensity has disappeared (
Oligonucleotide pairs that contained the mutated codons at the specified sites were used as primers to amplify the genes for the wild-type enzyme and the following mutant enzymes: QF, YT, and GWT. The identities of the mutants are listed in Table 6. The mutations were added to each template sequentially to make the following mutant proteins: RN, QFRN, YTRN, GWT-d1, and GWT-d2.
The multisite partially randomized PTE library was constructed by combining five separate segments of the gene for PTE as illustrated in
Mutant libraries were constructed using GWT-d1 as the starting template to identify more active PTE variants for the hydrolysis of SP-5. Nine amino acid residues in the substrate binding pocket were considered as potential “hot spots” for the construction of these PTE libraries. The single substitution library M317X was constructed first, followed by four double substitution libraries (W131X/F132X, F306X/Y309X, S308X/Y309X, and I106X/Y308X). Approximately 60 colonies from the M317X library and around 550 colonies from each of the double-substitution libraries were picked and subsequently screened with SP-5. The variants of GWT with catalytic activities higher than background from the first round of screening were isolated and then rescreened with the same substrate. (Table 6). No improvement in the hydrolysis of SP-5 (
The GWT-f1 mutant was partially randomized at six sites simultaneously. The total library contained 1.9×105 potential variants. Eight colonies from this library were selected to verify that the targeted sites were randomized. The PTE/GpdQ-pETDuet plasmid library was transformed into E. coli BL21(DE3) cells. Approximately 5.8×105 CFU were plated on phosphate-free minimal medium with 1 mM Sp-5 as the sole phosphorus source. The colonies that contained beneficial mutations for the hydrolysis of SP-5 were identified as being larger in size than a background colony of the parent GWT-f1 mutant. Approximately 30 of these colonies were selected for growth in 96-well blocks and subsequently assayed for catalytic activity with SP-5. The screening of the partially randomized multisite library with SP-5 is shown in
In
The GWT-f4 mutant served as the template for error-prone PCR (epPCR). Random mutagenesis of the GWT-f4 gene was conducted using the Mutazyme II DNA polymerase. Ten colonies from this library were selected to establish an average mutation rate of ˜1.5 mutations/1000 bp. The epPCR generated PTE/GpdQ-pETDuet library was transformed into E. coli BL21(DE3). Approximately 6×105 CFU were plated on phosphate-free minimal medium plates with 1 mM SP-5 as the sole phosphorus source. Colonies larger in size than the parental strain (GWT-f4) were assayed with SP-5, and the results are shown in
In
The outline for the discovery of PTE variants with enhanced activity for the hydrolysis of the SPSC-4 and SP-5 (
The structural modifications to GWT within the substrate binding pocket, surface and dimer interface have substantially enhanced substrate binding and catalytic turnover. The changes in the kcat/Km values of the SP-enantiomers of the organophosphonates are depicted in
Kinetic constants for sarin (GB), soman (GD), and cyclosarin (GF) were determined by monitoring the release of free fluoride at 25° C. in 50 mM bis-tris-propane buffer (pH 7.2) using a fluoride electrode. Stereospecificity for hydrolysis of nerve agents was obtained by following the complete hydrolysis of 0.5 mM racemic mixtures of GB or GF. Reactions were conducted in 50 mM bis-tris-propane buffer (pH 7.2) and followed by the release of fluoride.
Screening and Kinetic Analysis of Wild-Type and Mutant Enzymes on GB, GD, and GF. For the purposes of initial screening, specific activities were determined with 3 mM substrate (Table 7). The YT mutant was very active with all three substrates, and the YT-RN mutant had higher activity than the wild type with GD. Therefore, kinetic constants were determined for these enzyme-substrate combinations (Table 8). The stereospecificity of variants was determined by following the fluoride released during the complete hydrolysis of GB or GF using the 0.5 mM racemic substrate. Substantial differences in the rates for the individual enantiomers result in biphasic curves. The specificity of GWT toward GF is known from polarimetry experiments,(9) while the stereopreference of the remaining variants was determined by the ability of the variant to complement the slow phase in the GWT-catalyzed reaction (Table 7). YT has the same stereopreference as GWT for GF and has previously been shown to have the same preference as GWT for GD.(5) The stereopreference for the variants toward GB was determined by the ability to complement the slow phase in the YT-catalyzed reaction (Table 7). Adding mutations to the variants YT and YT-RN will likely provide variants that have improved activity with G-agents over wild-type PTE. Mutations at various locations within the sequence of the YT mutant will be added and their activity measured to identify variants that exhibit high activity with G-agents. Such variants would be useful in the decontamination of people, items, and locations contaminated with one or more G-agents. In an embodiment, further improvements in catalytic activity could be gained by simultaneously mutating pairs of residues in the active site. In an embodiment, error-prone PCR could be utilized to obtain mutations within PTE. In an embodiment, mutations within the active site of PTE could provide increased catalytic activity toward a G-agent. In an embodiment, methods disclosed regarding mutating PTE and determining activity with V-agents can also be utilized with mutating PTE and determining activity with G-agents.
The kinetic parameters of the purified wild-type PTE and its mutants with the entire set of chiral organophosphonate compounds shown in
Materials. Growth media and antibiotics were procured from Research Products Incorporated. DNA polymerase was obtained from Agilent. Other supplies for the molecular biology experiments were acquired from New England Biolabs. DEVX and N,N-diisopropylaminoethanethiol were synthesized as previously reported.(37, 44) The individual enantiomers of p-acetophenyl VR (APVR) were synthesized as previously reported.(39) Samples of VX and VR were Chemical Agent Standard Analytical Reference Material (CASARM) of the highest purity available, and were further purified as described previously.(37) Unless otherwise noted, all other chemicals were purchased from Sigma Aldrich. The organophosphorus nerve agents used in this investigation are highly toxic and should be used with the proper safety precautions. PTE: phosphotriesterase; DEVX: O,O-diethyl-VX; DMVX: O,O-dimethyl-VX; DEVR: O,O-diethyl-VR; and OMVR: O-methyl-VR.
Synthesis of Dimethyl VX. Dimethyl VX (DMVX) was made by the reaction of dimethyl chlorophosphate with N,N-diisopropylaminoethanethiol. N,N-diisopropylaminoethanethiol (1.5 grams; 9.3 mmol) was added to 100 mL of diethyl ether and allowed to cool in a dry ice/acetone bath before being purged with N2. To this mixture was added 7.5 mL (2.0 equivalents) of a 2.5 M solution of butyl lithium in hexanes and the reaction allowed to come to room temperature before re-cooling in a dry ice/acetone bath. Dimethyl chlorophosphate (2.0 g; 1.5 equivalents) was mixed with 30 mL of diethyl ether in a separate flask and cooled. The dimethyl chlorophosphate solution was then added to the thiol solution and the reaction stirred at room temperature for 3 hours. The reaction was brought to 400 mL with diethyl ether and extracted with water. The product was extracted into the aqueous phase with 0.5 M HCl. The aqueous phase was neutralized with sodium bicarbonate and extracted with dichloromethane. The organic phase was dried over Na2SO4, filtered, and evaporated to dryness. The product was further purified by silica gel chromatography. The product was dissolved in dichloromethane and eluted from the column using using a 0-5% step gradient of methanol in dichloromethane. Fractions containing the desired product were combined and the solvent evaporated to provide the product as an oil. Overall isolated yield was 8%. 1H NMR (300 MHz, CDCl3): 3.84-3.77 (6H, d, J=12.6 Hz, OCH3), 3.08-2.62 (6H, m, SCH2CH2N(CH)2), 1.05-0.98 (12H, d, J=6.9 Hz, CH(CH3)2. 31P NMR (121.4 MHz, CDCl3): 32.74 ppm.
Synthesis of Diethyl VR. Diethyl VR (DEVR) was made by the reaction of diethyl chlorophosphate with N,N-diethylaminoethanethiol. N,N-diethylaminoethanethiol was prepared from the hydrochloride salt by dissolving the compound in a saturated NaHCO3 solution and extraction with diethyl ether. The organic phase was dried over Na2SO4 and evaporated in vacuo at room temperature. The remaining oil was distilled (50° C.) under high vacuum and recovered as a pure liquid in a dry ice cooled trap. N,N-diethylaminoethanethiol (1.1 grams; 8.35 mmol) was added to 100 mL of diethyl ether and cooled in a dry ice/acetone bath before being purged with N2. To this mixture was added 10 mL (3.0 equivalents) of a 2.5 M solution of butyl lithium in hexanes and the reaction allowed to come to room temperature before re-cooling in a dry ice/acetone bath. Diethyl chlorophosphate (2.9 g; 2 equivalents) was mixed with 30 mL of diethyl ether in a separate flask, purged with N2, and then cooled in a dry ice/acetone bath. The cooled diethyl chlorophosphate solution was added to the thiol solution and the reaction stirred at room temperature for 3 hours. The reaction was brought to 400 mL with diethyl ether and extracted with water. The product was extracted into the aqueous phase with 0.5 M HCl. The aqueous phase was neutralized with sodium bicarbonate and extracted with dichloromethane. The organic phase was dried over Na2SO4, filtered, and then evaporated, yielding the product as an oil. Further purification was conducted using silica gel chromatography as described above. Overall yield of the isolated product was 7%. 1H NMR (300 MHz, CDCl3): 4.27-4.10 (4H, m, OCH2CH3), 2.99-2.52 (8H, m, SCH2CH2N(CH2)2), 1.43-1.34 (6H, t, J=7.5 Hz, OCH2CH3), 1.11-1.01 (6H, t, J=7.2 Hz, CH2CH3). 31P NMR (121.4 MHz, CDCl3): 28.50 ppm.
Synthesis of O-methyl VR. O-methyl VR (OMVR) was synthesized by the reaction of methyl isobutyl chlorophosphate with N,N-diethylaminoethanethiol. N,N-diethylaminoethanethiol was prepared from the hydrochloride salt as described above. Methyl isobutyl chlorophosphate was prepared by dissolving 750 μL (8.4 mmol) of isobutanol in 50 mL of diethyl ether. The atmosphere was purged with N2 and then the mixture was chilled in a dry ice/acetone bath. A total of 3.3 mL (1.0 equivalent) of 2.5 M butyl lithium in hexanes and 1.5 g (1.0 equivalents) of methyl dichlorophosphate was added, and the reaction stirred for three hours at room temperature.
In a separate flask, 1.2 g (1.0 equivalents) of N,N-diethylaminoethanethiol was dissolved in 50 mL of diethyl ether and chilled in a dry ice/acetone bath. A total of 5 mL (1.5 equivalents) of 2.5 M butyl lithium was added and the reaction warmed to room temperature. The methyl isobutyl chlorophosphate and the thiol solutions were chilled in a dry ice/acetone bath and combined. The reaction was allowed to proceed at room temperature for 3 hours. The reaction was then brought to 400 mL with diethyl ether and washed with water. The product was extracted with 0.5 M HCl, neutralized with sodium bicarbonate, and then extracted with dichloromethane. The organic phase was dried over Na2SO4 and evaporated to yield the product as an oil. Overall yield of final product was 15%. 1H NMR (300 MHz, CDCl3): 3.92-3.72 (5H, m, OCH2CH(CH3)2, OCH3), 2.95-2.45 (8H, m, SCH2CH2N(CH2)2), 2.04-1.88 (1H, OCH2CH(CH3)2), 1.06-0.97 (6H, t, J=7.0 Hz, NCH2CH3), 0.97-0.90 (6H, d, J=6.8 Hz, OCH2CH(CH3)2. 31P NMR (121.4 MHz CDCl3): 30.30 ppm.
Mutagenesis, Expression and Enzyme Purification. The gene for PTE was cloned into the expression vector pET 20b between the NdeI and EcoRI restriction sites as previously described.(39) The new variants of PTE were generated by introducing the mutations I106C, I106G, and L308S into the appropriate templates by site directed mutagenesis using the Quick Change (Agilent) protocol. DNA sequencing at the Gene Technologies Laboratory at Texas A&M University verified the specific mutations. The proteins were expressed and purified as previously described. (37) Briefly, the variants were freshly transformed into E. coli BL21 (DE3) cells by electroporation, and single colonies used to inoculate 5.0 mL cultures of LB medium. After 8 hours of growth at 37° C., 1.0 mL of this culture was used to inoculate 1 L cultures of Terrific Broth supplemented with 1.0 mM CoCl2. The bacterial cultures were grown at 30° C. IPTG was added to a final concentration of 1.0 mM after 24 hours and growth continued for 40 hours. The cells were harvested by centrifugation and stored at −80° C. prior to purification. Cells were resuspended in 100 mL of purification buffer (50 mM HEPES, pH 8.5, with 100 μM CoCl2) and then lysed by sonication. The cell debris was cleared by centrifugation, nucleic acids were precipitated by protamine sulfate (0.45 g in 20 mL purification buffer per liter of culture), and then removed by centrifugation. The PTE mutants were precipitated with ammonium sulfate (60% saturation) and recovered by centrifugation. The pellet was resuspended in ˜5 mL of purification buffer, filtered (0.45 μm) and loaded onto a GE Superdex 200 (16/60) preparatory size exclusion column using a BioRad NCG FPLC system. Fractions with catalytic activity for the hydrolysis of paraoxon were pooled and then eluted from a 3.0 g (dry weight) DEAE Sephadex A25 resin that was pre-equilibrated in purification buffer. Protein purity was verified by SDS-PAGE.
Generation and Characterization of New PTE Variants. The PTE variants QF, CVQFL, VRN-VQFL and L7ep-3a were previously shown as having substantially improved activity for the hydrolysis of VX.(37) The stereoselectivity of these mutants for the chiral center contained in VR was determined using the isolated enantiomers of APVR (Table 11). With the exception of QF, the VX-optimized variants of PTE prefer to hydrolyze the RP-enantiomer of the chiral center for VR. The crystal structure of L7ep-3a suggests that the mutations I106C and S308L affect the size of the small-group pocket. In order for the SP-enantiomer of VR to productively bind in the active site, the larger isobutyl group must be positioned in the small-group pocket of PTE. In an effort to maximize the hydrolysis of SP-VR, a series of variants was created to change the substitutions made to residues 106 and 308 in the variants previously optimized for the hydrolysis of VX.
1Errors from curve fitting are less than 10% with the exception of CVQFL-I106G/L308S, which has an error of 20% due to the high Km value.
2The ratio is kcat/Km values for fast enantiomer and slow enantiomer, with the preferred enantiomer identified.
Enzymatic Activity. Catalytic activity with paraoxon was followed by monitoring the release of p-nitrophenol at 400 nm (ΔE400=17,000 M−1 cm−1) in 250 μL reaction volumes containing 50 mM CHES, pH 9.0, 100 μM CoCl2, and 0-1.0 mM paraoxon. Activity with APVR utilized the same reaction conditions as paraoxon with the release of the leaving group monitored at 294 nm (Δ E294=7,710 M−1 cm−1). Activity with DEVX, DMVX, DEVR, and OMVR was measured in 250 μL reactions containing 50 mM HEPES, pH 8.0, 100 μM CoCl2, 0.3 mM DTNB and 0-1.0 mM substrate. The release of the thiol leaving group was followed by inclusion of DTNB in the assay mixture (Δ E412=14,150 M−1 s−1). All assays were initiated by the addition of the appropriately diluted enzyme and monitored in a 96-well format using a Molecular Devices SpectraMax 364 Plus plate reader. Reactions were monitored for 15 minutes at 30° C. and the linear portion of the time course was used to calculate the initial rate. Kinetic constants were determined by fitting the data to the Michaelis-Menten equation.(45) When saturation could not be observed, the data was fit to a linear equation and the slope taken as kcat/Km.
The catalytic activities of these variants were first characterized against the insecticide paraoxon and the V-agent analog DEVX (Table 12), and then the stereochemical preferences were determined using APVR (Table 11).
Inclusion of the I106C mutation substantially reduced the preference for the hydrolysis of the RP-enantiomer, while it tended to have relatively little effect on the hydrolysis of DEVX. For example, the CVQFL variant has a 2.5-fold preference for the RP-enantiomer of the VR chiral center, but CVQFL-C106I has a 35-fold preference for the RP-enantiomer. Despite this large difference in stereoselectivity, both of these variants hydrolyze DEVX with a kcat/Km of ˜2×104 M−1 s−1. The removal of the S308L mutation from the VX-optimized variants also results in a substantial diminished preference toward the RP-enantiomer, but in most cases it also resulted in diminished activity against DEVX. VRN-VQFL prefers the RP-enantiomer of APVR by 20-fold. The variant VRN-VQFL L308S has a preference of 1.2-fold, but the activity against DEVX was also diminished 2-fold. When glycine was substituted at position 106, the stereochemical preference shifted to the SP-enantiomer of APVR. L7ep-3a prefers the RP-enantiomer by 1.5-fold, but L7ep-3a I106G prefers the SP-enantiomer by 6.5-fold. Unfortunately, the I106G mutation reduced the activity against DEVX by more than an order of magnitude in these variants.
Stereoselelctive Hydrolysis of Racemic VX and VR. Low initial concentrations (19 to 160 μM) of racemic VX and VR were hydrolyzed by variants of PTE in a solution containing 0.1 mM CoCl2, 0.3 mM DTNB, and 50 mM Bis-Tris-propane, pH 8.0. The reactions were followed to completion and the fraction hydrolyzed plotted as a function of time. The time courses were fit to equations 1 and 2 where F is the fraction hydrolyzed, a and b are the magnitudes of the exponential phases, t is time, and k1 and k2 are the rate constants for each phase.(39)
F=a(1−e−k1t) (1)
F=a(1−e−k1t)+b(1−e−k1t) (2)
Stereochemical preferences were determined using the previously described complementation method.(37) Briefly, variants with large stereoselective preferences were placed in a reaction with mutants of PTE of known stereoselective preferences under conditions where each variant alone would exhibit a similar rate for the first exponential phase. If the variants prefer the same enantiomer, the resulting time course will exhibit two distinct phases. If the variants have the opposite enantiomers as the preferential substrate, the time courses will exhibit a single exponential phase.
Catalytic Activity with VX and VR. The most promising new variants were tested with racemic VX and VR. The assays were conducted for the complete hydrolysis of a single low concentration of these agents to enable the observation of exponential time courses, corresponding to the hydrolysis of each enantiomer contained within the racemic mixture. The kinetic constants are presented in Table 13. For enzyme variants with large stereochemical preferences, the identity of the preferred enantiomer was determined by the ability of the variant to complement the slow phase of a variant of known preference. Wild-type PTE is known to prefer to hydrolyze the RP-enantiomer of the VR chiral center, while QF is known to prefer the SP-enantiomer of VX.(37, 39) None of the variants tested, with the exception of QF, exhibited large stereochemical preferences for hydrolysis of the two enantiomers of VX. Removal of the S308L mutation (VRN-VQFL L308S) resulted in a 2-fold reduction in catalytic activity for the faster enantiomer of VX and a 5-fold reduction in the rate of hydrolysis for the slower enantiomer. Introduction of the I106G mutation (VRN-VQFL-I106G) led to the complete hydrolysis of racemic VX without detectable selectivity, but at a rate 6-fold slower than VRN-VQFL had for the slower enantiomer.
1Errors from curve fitting are less than 5%.
2The ratio is kcat/Km values for fast enantiomer and slow enantiomer. If the preferred enantiomer is not listed it has not been determined.
With racemic VR, the VRN-VQFL variant exhibited a 20-fold enhancement in the rate of hydrolysis compared to the wild-type enzyme, but this mutant was found to only hydrolyze the relatively nontoxic RP-enantiomer of VR. Removal of the S308L mutation (VRN-VQFL L308S) enabled the hydrolysis of both enantiomers of VR, with complete loss of stereoselectivity. This represents a 64-fold improvement toward the hydrolysis of the toxic SP-enantiomer compared to wild-type PTE. The introduction of the I106G mutation in the VRN-VQFL-I106G/L308S variant resulted in a strong preference for the hydrolysis of the SP-enantiomer relative to the RP-enantiomer. VRN-VQFL-I106G, which has both the I106G and S308L mutations, has much less stereochemical preference, but the kinetic data for the hydrolysis of the two enantiomers of APVR indicate that the stereochemical preference is for the SP-enantiomer. The best variant identified for the hydrolysis of VR was L7ep-3a I106G, which has a kcat/Km of 2.6×103 M−1 s−1 for the faster enantiomer. While the stereoselectivity was not sufficient to determine the stereochemical preference with VR directly, the kinetic data with the two enantiomers of APVR has identified the preferred enantiomer as the highly toxic SP-enatiomer. The L7ep-3a I106G mutant thus has a 620-fold enhanced rate of hydrolysis of SP-VR relative to wild-type PTE.
X-ray Crystallography. The PTE mutants L7ep-3a and L7ep-3a I106G were crystallized at 18° C. using the vapor diffusion method. In the crystallization experiments, 1.0 μL of protein (10 mg/mL with 1.0 mM CoCl2) was mixed with 1.0 μL of the precipitant solution (100 mM sodium cacodylate pH 5.5-7.0, 0.2 M magnesium acetate, 15-30% PEG 8000), and then equilibrated against 500 μL of the same precipitant solution using Intelliplates. Protein crystals appeared within a week and grew to maximum dimensions (200 μm×15 μm×15 μm) after 21 days. Prior to data collection, the crystals were soaked for 30 seconds in a cryoprotectant solution of the mother liquor containing 30% ethylene glycol and then frozen in liquid nitrogen. Diffraction data were collected locally at 120 K on an R-AXIS IV detector with Cu Kα X-rays produced from a rotating anode generator. X-ray data reduction and scaling were performed with HKL2000.(46) Structures of the PTE mutants L7ep-3a and L7ep-3aG were determined by molecular replacement using the coordinates of wild type PTE (PDB id: 1DPM) as the search model. The structures were built using COOT and refined with simulated annealing, B-factor randomization, and coordinate shaking using PHENIX.(47, 48) Later stages of refinement were also done in PHENIX using individual coordinate, anisotropic B-factor, and occupancy optimization. The PTE mutant structures were refined with Rwork/Rfree values of 13.5-21.5% with excellent geometry (Table 14).
Three-Dimensional Structure of the PTE Mutant L7ep-3a. The PTE variant L7ep-3a has the highest reported kcat for the hydrolysis of the nerve agent VX.(37) In an effort to elucidate the mechanism by which the activity of this mutant is enhanced, the enzyme was crystallized and the structure determined to a resolution of 2.01 Å by X-ray diffraction methods (PDB id: 4ZST).
The binding site for the substrate in wild-type PTE is divided into the large-group pocket (His-254, His-257, Leu-271, and Met-317), the leaving-group pocket (Trp-131, Phe-132, Phe-306, and Tyr-309), and the small-group pocket (Gly-60, Ile-106, Lue-303 and Ser-308).(50) The mutations H257Y and S308L, coupled with the shifting of the Loop-7 α-helix, have induced significant changes in the substrate binding pockets in the active site of the L7ep-3a mutant. The side-chain of Tyr-309 is repositioned so that the phenolic group now extends into the active site rather than towards Loop-7, as previously observed in the structure of wild-type PTE (
The reorientation of Tyr-309, along with the substitution of a tyrosine for His-257 and the repositioning of Leu-271 into the active site, has dramatically compressed the size of the large-group and leaving-group pockets. The leaving-group pocket is also constricted by the presence of the S308L mutation, which adds bulk to both the leaving-group and small-group pockets. However, the apparent contraction of the leaving-group pocket is partially relieved by the F132V mutation. Similarly, the I106C mutation opens additional space to the small-group pocket.
Three-Dimensional Structure of L7ep-3a I106G. In an effort to understand the physical basis for the observed improvement in the catalytic activity of L7ep-3a I106G, the three-dimensional structure was solved by X-ray crystallography (PDB id: 4ZSU). The core structure is very similar to wild-type PTE (Cα RMSD=0.66 Å) and the loop structure matches that observed with L7ep-3a.
Computational Docking of High Energy Intermediates. The pentavalent high-energy reaction intermediate states for the hydrolysis of VX and VR were computationally docked into the three-dimensional structures of wild-type PTE (PDB id: 1DPM), H254Q/H257F (PDB id: 2OQL), L7ep-3a (PDB id: 4ZST), and L7ep-3aG (PDB id: 4ZSU). The high energy intermediate states for the hydrolysis of the RP- and SP-enantiomers were manually generated as trigonal bipyrimidal structures with the attacking hydroxyl group protonated and the original phosphoryl oxygen substituent carrying a full negative charge using ChemBio Ultra 14.0. Computational docking was done using the program AutoDock Vina.(49) The appropriateness of the docked poses was evaluated by the value of the distance of the attacking hydroxyl group from the α- and β-metal ions, the distance of the phosphoryl oxygen from the β-metal and the orientation of the side ester substituents into the large and small group pockets contained in the active site of PTE.
Evaluation of New V-agent Analogs. The majority of research targeting the catalytic hydrolysis of organophosphorus nerve agents must be done using analogs for both regulatory and safety reasons. Intrinsic to the use of substrate analogs is the imperfect representation of catalytic activity with the authentic nerve agent. To address which structural factors of the VR and VX analogs are most significant, the compounds DMVX, DEVR and OMVR were synthesized and analyzed as substrates for the optimized mutants of PTE and compared with the ability of these mutants to hydrolyze the most toxic forms of VR and VX. The catalytic activity using these compounds was determined with a series of variants for which the hydrolysis of authentic nerve agents was available (Table 15). Wild-type PTE has a much lower catalytic activity with DMVX (kcat/Km=1.9×101 M−1 s−1) than was observed with DEVX (kcat/Km 1.2×103 M−1 s−1). The best variant with DMVX was L7ep-3a, which has a kcat=28 s−1 and a kcat/Km=1.3×104 M−1 s−1. These values are 1,300- and 680-fold better, respectively, than wild-type PTE. Wild-type PTE exhibited better catalytic activity with DEVR (kcat/Km=5.6×102 M−1 s−1). L7ep-3a also had the best catalytic activity with this analog (kcat/Km=1.5×104 M−1 s−1). With the exception of wild-type PTE (kcat/Km=6.8×101 M−1 s−1), racemic OMVR gave the least activity for most of the variants tested. The combination of poor solubility of this compound and high Km values limited analysis to the determination of kcat/Km for most variants. The best mutant for the hydrolysis of OMVR was L7ep-3a with a kcat/Km=5.8×102 M−1 s−1. However, given the switch in stereochemical preference, it is highly likely that L7ep-3a I106G has the best catalytic activity with the RP-enantiomer of OMVR (which corresponds to the same relative stereochemistry as the SP-enantiomer of VR.
1Errors from curve fitting were less than 10% except for kcat and Km for L7ep3aG with DEVR.
2kcat/Km was determined from linear fit at low concentration of racemic substrate.
Comparison of Substrate Analogs. The VX analog DEVX was successfully used to identify PTE variants for the hydrolysis of VX, but it over-estimated the activity of the wild-type enzyme and failed to detect a 100-fold increase in the catalytic activity with the QF variant.(37) DEVX was initially synthesized to mimic the O-ethyl substituent in VX, but the bulk of the diethyl phosphorus center is larger than the volume of the methylphosphonate moiety of VX. DMVX also contains an achiral phosphorus center, but the volume of the dimethyl center is more representative of authentic VX. The catalytic activity of the PTE variants with DMVX was surprisingly low, but this analog captures the much lower catalytic activity of wild-type PTE for the hydrolysis of VX and the substantial increase in activity with the QF variant. The catalytic properties for the hydrolysis of DMVX are also able to predict the high kcat for the hydrolysis of VX by the L7ep-3a variant. While DEVX was intended to ensure accommodation of the larger O-ethyl substituent, the data for DMVX suggests that for mimicking VX, the overall size of the phosphorus center is more important. While the asymmetry of the phosphonate center is obviously a major contributor to the catalytic activity using VX as a substrate, the dimethyl center provided a reasonable prediction of the catalytic activity.
The compound DEVR was synthesized to test the significance of the diethylamino vs. diisopropylamino groups contained within the VR and VX leaving groups, respectively. The smaller leaving group of DEVR resulted in 2-fold less activity compared to DEVX for wild-type PTE. Similar differences were obtained for most of the other variants. L7ep-3a I106G was the only variant where the catalytic activity with DEVR was less than 10% of the catalytic activity with DEVX. The relative activity between variants was similar with either leaving group but the reduced activity, especially manifested in the value of kcat, suggests that the interactions of the enzyme with the leaving group maybe partially responsible for aligning the phosphorus center for nucleophilic attack. The smaller leaving group probably contributes to the lower enzymatic activity observed with VR, but comparison to the catalytic activity with DEVR also highlights the dominance of the phosphorus center. The introduction of I106G in the L7ep-3a variant resulted in a 23-fold loss of activity with DEVR, but the activity with authentic VR was improved.
In an attempt to better reflect the asymmetric phosphorus center of VR, the analog OMVR was synthesized. The PTE variants exhibited the least activity with the analog OMVR, which employs a slightly larger but asymmetric phosphorus center and the authentic leaving group of VR. Kinetic constants with racemic OMVR are about an order of magnitude smaller than is obtained with authentic VR, but the relative activity between variants is much more consistent than is seen with the other analogs. VRN-VQFL and L7ep-3a both have substantially better activity than wild-type PTE or QF for both OMVR and authentic VR. Introduction of I106G or removal of S308L in the VRN-VQFL variant resulted in somewhat diminished rates for both authentic VR and OMVR.
Active Site Hydrogen Bonding Network. The kcat values for wild-type PTE with phosphorothiolate substrates are approximately 104-fold lower than that with its best substrates.(37, 51) In the hydrolysis of substrates such as paraoxon there is no need to protonate the leaving group.(52)
In the proposed reaction mechanism for wild-type PTE, the proton from the attacking nucleophilic water is passed to Asp-301, and, in turn, to His-254 (
Docking of Substrates in the Active Site. In an effort to gain insight into the altered activity of L7ep-3a and L7ep-3a I106G, the substrates VX and VR were docked into the active site using the program AutoDock Vina. Computational docking was conducted using the trigonal bipyrimidal intermediates formed during the hydrolysis of both enantiomers of these compounds.(55) Productive poses were assessed by the placement of the attacking hydroxyl group between the two metal ions and the orientation of the phosphoryl oxygen toward the β-metal.(56) For the wild-type and QF variants both enantiomers of VX and VR could be reasonably docked into the active site (data not shown). However, for L7ep-3a and L7ep-3a I106G, only the SP-enantiomer of VR could be reasonable positioned in the active site. For both of these variants, the constriction of the active site and the repositioning of Tyr-309 appear to play important roles in the activity against the V-agents (
The mutation F132V provides the extra room to accommodate the isopropyl amino group of VX. This effect is not as obvious with the smaller substituent contained within the leaving group in VR. The changes in the active site also enable the sulfur and the nitrogen of the leaving group to potentially hydrogen bond with the side chain phenol of Tyr-309, suggesting that this interaction may be partially responsible for the dramatically improved activity. The additional space in the small-group pocket due to the I106G mutation accommodates the isobutyl group in SP-VR, while the combination of the H257Y mutation and repositioning of Tyr-309 make binding of the RP-enantiomer more difficult.
The determination of the three-dimensional structure of the L7ep-3a mutant has led to a greater understanding of the underlying mechanisms by which the hydrolysis of V-agents is enhanced.
The determination of this structure has directly led to the rational construction of the new L7ep-3a I106G mutant, which is enhanced 620-fold for the hydrolysis of the toxic SP-enantiomer of VR, relative to the wild-type enzyme. Previous work with the insecticide demeton-S demonstrated the importance of the leaving group on the catalytic activity of PTE, and our results with the new analogs of VR and VX has further demonstrated that even small changes in the leaving group can have dramatic effect on the activity of the enzyme.(37) The crystal structures of L7ep-3a and L7ep-3a I106G have provided a physical basis for these observations. The computational docking results have suggested how the remodeled active site is able to exploit hydrogen bonding interactions with Tyr-309. The disruption of the proton shuttle, along with new hydrogen bonds to Asp-301, are apparently able to enhance the attack of the bridging hydroxide on phosphorothiolate substrates. The initial data with the new racemic analog OMVR suggests that the combined asymmetric phosphorus center and the authentic leaving group of VR will allow for much more accurate predictions of the activity against VR and enable the more rapid development of new variants that are more fully optimized for the hydrolysis of VR.
All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this disclosure have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations can be applied to the compositions and methods and in the steps or in the sequence of steps of the methods described herein without departing from the concept, spirit and scope of the disclosure. More specifically, it will be apparent that certain agents which are both chemically and physiologically related can be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the disclosure as defined by the appended claims.
This invention was made with government support under GM068550 awarded by National Institutes of Health and under HDTRA1-14-1-0004 awarded by the Defense Threat Reduction Agency. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2015/036745 | 6/19/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62015156 | Jun 2014 | US |
