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Parallel reaction pathways and noncovalent intermediates in thymidylate synthase revealed by experimental and computational tools. Proc Natl Acad Sci U S A 2018; 115:10311-10314. [PMID: 30249644 PMCID: PMC6187185 DOI: 10.1073/pnas.1811059115] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Thymidylate synthase was one of the most studied enzymes due to its critical role in molecular pathogenesis of cancer. Nevertheless, many atomistic details of its chemical mechanism remain unknown or debated, thereby imposing limits on design of novel mechanism-based anticancer therapeutics. Here, we report unprecedented isolation and characterization of a previously proposed intact noncovalent bisubstrate intermediate formed in the reaction catalyzed by thymidylate synthase. Free-energy surfaces of the bisubstrate intermediates interconversions computed with quantum mechanics/molecular mechanics (QM/MM) methods and experimental assessment of the corresponding kinetics indicate that the species is the most abundant productive intermediate along the reaction coordinate, whereas accumulation of the covalent bisubstrate species largely occurs in a parallel nonproductive pathway. Our findings not only substantiate relevance of the previously proposed noncovalent intermediate but also support potential implications of the overstabilized covalent intermediate in drug design targeting DNA biosynthesis.
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Kaiyawet N, Lonsdale R, Rungrotmongkol T, Mulholland AJ, Hannongbua S. High-level QM/MM calculations support the concerted mechanism for Michael addition and covalent complex formation in thymidylate synthase. J Chem Theory Comput 2015; 11:713-22. [PMID: 26579604 DOI: 10.1021/ct5005033] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Thymidylate synthase (TS) is a promising cancer target, due to its crucial function in thymine synthesis. It performs the reductive methylation of 2'-deoxyuridine-5'-phosphate (dUMP) to thymidine-5'-phosphate (dTMP), using N-5,10-methylene-5,6,7,8-tetrahydrofolate (mTHF) as a cofactor. After the formation of the dUMP/mTHF/TS noncovalent complex, and subsequent conformational activation, this complex has been proposed to react via nucleophilic attack (Michael addition) by Cys146, followed by methylene-bridge formation to generate the ternary covalent intermediate. Herein, QM/MM (B3LYP-D/6-31+G(d)-CHARMM27) methods are used to model the formation of the ternary covalent intermediate. A two-dimensional potential energy surface reveals that the methylene-bridged intermediate is formed via a concerted mechanism, as indicated by a single transition state on the minimum energy pathway and the absence of a stable enolate intermediate. A range of different QM methods (B3LYP, MP2 and SCS-MP2, and different basis sets) are tested for the calculation of the activation energy barrier for the formation of the methylene-bridged intermediate. We test convergence of the QM/MM results with respect to size of the QM region. Inclusion of Arg166, which interacts with the nucleophilic thiolate, in the QM region is important for reliable results; the MM model apparently does not reproduce energies for distortion of the guanidinium side chain correctly. The spin component scaled-Møller-Plessett perturbation theory (SCS-MP2) approach was shown to be in best agreement (within 1.1 kcal/mol) while the results obtained with MP2 and B3LYP also yielded acceptable values (deviating by less than 3 kcal/mol) compared with the barrier derived from experiment. Our results indicate that using a dispersion-corrected DFT method, or a QM method with an accurate treatment of electron correlation, increases the agreement between the calculated and experimental activation energy barriers, compared with the semiempirical AM1 method. These calculations provide important insight into the reaction mechanism of TS and may be useful in the design of new TS inhibitors.
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Affiliation(s)
| | - Richard Lonsdale
- Centre for Computational Chemistry, School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom
| | | | - Adrian J Mulholland
- Centre for Computational Chemistry, School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom
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Kaiyawet N, Rungrotmongkol T, Hannongbua S. Effect of halogen substitutions on dUMP to stability of thymidylate synthase/dUMP/mTHF ternary complex using molecular dynamics simulation. J Chem Inf Model 2013; 53:1315-23. [PMID: 23705822 DOI: 10.1021/ci400131y] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The stability of the thymidylate synthase (TS)/2-deoxyuridine-5-monophosphate (dUMP)/5,10-methylene-5,6,7,8-tetrahydrofolate (mTHF) ternary complex formation and Michael addition are considered as important steps that are involved in the inhibition mechanism of the anticancer prodrug 5-fluorouracil (5-FU). Here, the effect of three different halogen substitutions on the C-5 position of the dUMP (XdUMPs = FdUMP, CldUMP, and BrdUMP), the normal substrate, on the stability of the TS/dUMP and TS/dUMP/mTHF binary and ternary complexes, respectively, was investigated via molecular dynamics simulation. The simulated results revealed that the stability of all the systems was substantially increased by mTHF binding to the catalytic pocket. In the ternary complex, a much greater stabilization of the dUMP and XdUMPs through electrostatic interactions, including charge-charge and hydrogen bond interactions, was found compared to mTHF. An additional unique hydrogen bond between the substituted fluorine of FdUMP and the hydroxyl group of the TS Y94 residue was observed in both the binary and ternary complexes. The distance between the S(-) atom of the TS C146 residue and the C6 atom of dUMP, at <4 Å in all systems, suggested that a Michael addition with the formation of a S-C6 covalent bond potentially occurred, although the hydrogen atom on C6 of dUMP is substituted by a halogen atom. The MM/PBSA binding free energy revealed the significant role of the bridging waters around the ligands in the increased binding affinity (∼10 kcal/mol) of dUMP/XdUMP, either alone or together with mTHF, toward TS. The order of the averaged binding affinity in the ternary systems was found to be CldUMP ≈ FdUMP > dUMP > BrdUMP, suggesting that CldUMP could be a potent candidate TS inhibitor, the same as FdUMP (the metabolite form of 5-FU).
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Affiliation(s)
- Nopporn Kaiyawet
- Computational Chemistry Unit Cell, Department of Chemistry, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Patumwan, Bangkok 10330, Thailand
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Mishanina TV, Koehn EM, Kohen A. Mechanisms and inhibition of uracil methylating enzymes. Bioorg Chem 2012; 43:37-43. [PMID: 22172597 PMCID: PMC3315608 DOI: 10.1016/j.bioorg.2011.11.005] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2011] [Revised: 11/21/2011] [Accepted: 11/21/2011] [Indexed: 10/14/2022]
Abstract
Uracil methylation is essential for survival of organisms and passage of information from generation to generation with high fidelity. Two alternative uridyl methylation enzymes, flavin-dependent thymidylate synthase and folate/FAD-dependent RNA methyltransferase, have joined the long-known classical enzymes, thymidylate synthase and SAM-dependent RNA methyltransferase. These alternative enzymes differ significantly from their classical counterparts in structure, cofactor requirements and chemical mechanism. This review covers the available structural and mechanistic knowledge of the classical and alternative enzymes in biological uracil methylation, and offers a possibility of using inhibitors specifically aiming at microbial thymidylate production as antimicrobial drugs.
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Affiliation(s)
- Tatiana V. Mishanina
- Department of Chemistry, The University of Iowa, E274 Chemistry Building, Iowa City, IA 52245, USA
| | - Eric M. Koehn
- Department of Chemistry, The University of Iowa, E274 Chemistry Building, Iowa City, IA 52245, USA
| | - Amnon Kohen
- Department of Chemistry, The University of Iowa, E274 Chemistry Building, Iowa City, IA 52245, USA
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Mishanina TV, Koehn EM, Conrad JA, Palfey BA, Lesley SA, Kohen A. Trapping of an intermediate in the reaction catalyzed by flavin-dependent thymidylate synthase. J Am Chem Soc 2012; 134:4442-8. [PMID: 22295882 DOI: 10.1021/ja2120822] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Thymidylate is a DNA nucleotide that is essential to all organisms and is synthesized by the enzyme thymidylate synthase (TSase). Several human pathogens rely on an alternative flavin-dependent thymidylate synthase (FDTS), which differs from the human TSase both in structure and molecular mechanism. It has recently been shown that FDTS catalysis does not rely on an enzymatic nucleophile and that the proposed reaction intermediates are not covalently bound to the enzyme during catalysis, an important distinction from the human TSase. Here we report the chemical trapping, isolation, and identification of a derivative of such an intermediate in the FDTS-catalyzed reaction. The chemically modified reaction intermediate is consistent with currently proposed FDTS mechanisms that do not involve an enzymatic nucleophile, and it has never been observed during any other TSase reaction. These findings establish the timing of the methylene transfer during FDTS catalysis. The presented methodology provides an important experimental tool for further studies of FDTS, which may assist efforts directed toward the rational design of inhibitors as leads for future antibiotics.
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Pandit UK. Biomolecular approach to the design of potential drugs. PURE APPL CHEM 2007. [DOI: 10.1351/pac200779122119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Abstract
The approach to drug design on the basis of molecular-level information on biological processes is being driven by the expanding knowledge of the details of molecular events in biological systems. We have directed attention to the design of potentially active compounds based on the aforementioned "biomolecular" concepts. Selected examples from our studies are discussed. This paper presents three case studies of approaches to the development of potential medicinal agents whose design has evolved from considerations of molecular mechanisms of processes in selected biological systems.
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Affiliation(s)
- Upendra K. Pandit
- Van't Hoff Institute of Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands
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Abstract
The three-dimensional structures of tryptophan synthase, carbamoyl phosphate synthetase, glutamine phosphoribosylpyrophosphate amidotransferase, and asparagine synthetase have revealed the relative locations of multiple active sites within these proteins. In all of these polyfunctional enzymes, a product formed from the catalytic reaction at one active site is a substrate for an enzymatic reaction at a distal active site. Reaction intermediates are translocated from one active site to the next through the participation of an intermolecular tunnel. The tunnel in tryptophan synthase is approximately 25 A in length, whereas the tunnel in carbamoyl phosphate synthetase is nearly 100 A long. Kinetic studies have demonstrated that the individual reactions are coordinated through allosteric coupling of one active site with another. The participation of these molecular tunnels is thought to protect reactive intermediates from coming in contact with the external medium.
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Affiliation(s)
- X Huang
- Wyeth-Ayerst Research, 401 North Middleton Road, Pearl River, New York 10965, USA. [corrected]
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Huang W, Santi DV. Isolation of a covalent steady-state intermediate in glutamate 60 mutants of thymidylate synthase. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(18)31695-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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Vega E, Rood GA, de Waard ER, Pandit UK. Reduction of 5-uracilylmethylenepyridinium salts by thiols. A model of the reduction step of the thymidylate synthase reaction. Tetrahedron 1991. [DOI: 10.1016/s0040-4020(01)87105-2] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Matthews DA, Villafranca JE, Janson CA, Smith WW, Welsh K, Freer S. Stereochemical mechanism of action for thymidylate synthase based on the X-ray structure of the covalent inhibitory ternary complex with 5-fluoro-2'-deoxyuridylate and 5,10-methylenetetrahydrofolate. J Mol Biol 1990; 214:937-48. [PMID: 2201779 DOI: 10.1016/0022-2836(90)90347-o] [Citation(s) in RCA: 92] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
The structure of the Escherichia coli thymidylate synthase (TS) covalent inhibitory ternary complex consisting of enzyme, 5-fluoro-2'-deoxyuridylate (FdUMP) and 5,10-methylene tetrahydrofolate (CH2-H4PteGlu) has been determined at 2.5 A resolution using difference Fourier methods. This complex is believed to be a stable structural analog of a true catalytic intermediate. Knowledge of its three-dimensional structure and that for the apo enzyme, also reported here, suggests for the first time how TS may activate dUMP and CH2-H4PteGlu leading to formation of the intermediate and offers additional support for the hypothesis that the substrate and cofactor are linked by a methylene bridge between C-5 of the substrate nucleotide and N-5 of the cofactor. By correlating these structural results with the known stereospecificity of the TS-catalyzed reaction it can be inferred that the catalytic intermediate, once formed, must undergo a conformational isomerization before eliminating across the bond linking C-5 of dUMP to C-11 of the cofactor. The elimination itself may be catalyzed by proton transfer to the cofactor's 5 nitrogen from invariant Asp169 buried deep in the TS active site. The juxtaposition of Asp169 and bound tetrahydrofolate in TS is remarkably reminiscent of binding geometry found in dihydrofolate reductase where a similarly conserved carboxyl group serves as a general acid for protonating the corresponding pyrazine ring nitrogen of dihydrofolate.
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Affiliation(s)
- D A Matthews
- Agouron Pharmaceuticals, Inc. La Jolla, CA 92037
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Cisneros RJ, Dunlap RB. Characterization of the parameters affecting covalent binding stoichiometry in binary and ternary complexes of thymidylate synthase. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1039:149-56. [PMID: 2114176 DOI: 10.1016/0167-4838(90)90180-n] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Covalent binding stoichiometries for both the enzyme:5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) binary complex and the enzyme:FdUMP:5,10-methylenetetrahydrofolate (inhibitory ternary) complex at equilibrium were measured by the trichloroacetic acid precipitation assay and shown to be a function of temperature, time, pH, salt concentration, buffer composition and thiol concentration. Incubation at 37 degrees C yielded the maximum covalent binding ratio (mol FdUMP/mol enzyme) for the latter binary (0.7) and ternary (1.7) complexes. In most buffers studied, the maximum covalent binding ratio (1.5-1.7) for the inhibitory ternary complex occurred over a broad pH range (4.5-8.0), while the optimum covalent binding ratio for binary complex was observed at a much narrower region centered between pH 5.5-6.5. In the presence of increasing concentrations of phosphate buffer, the maximum binding ratio for the covalent binary complex decreased from 0.63 in the absence of phosphate to 0.1 in the presence of 225 mM phosphate, while that for the inhibitory ternary complex was unchanged. When a ternary complex was formed with enzyme, FdUMP and (+/-)-tetrahydrofolate in the absence of phosphate, the FdUMP:enzyme covalent binding ratio was 1.8, while in the presence of 75 mM phosphate, the binding ratio was only 1.0. When exogenous thiol was removed by centrifugal column chromatography, the maximum binding stoichiometry of the resulting inhibitory ternary complex was 1.7 and was independent of added thiol over a 2 h incubation period at 37 degrees C. When extensive dialysis at 5 degrees C was used to remove the thiol, the maximum binding stoichiometry of the resulting inhibitory ternary complex was found to be dependent on both the concentration of added thiol and the time of incubation at 37 degrees C and did not exceed a value of 1.0.
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Affiliation(s)
- R J Cisneros
- Department of Chemistry, University of South Carolina, Columbia 29208
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Cisneros RJ, Dunlap RB. Development of a trichloroacetic acid precipitation assay for covalent adducts of thymidylate synthase. Anal Biochem 1990; 186:202-8. [PMID: 2363490 DOI: 10.1016/0003-2697(90)90067-j] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
The use of trichloroacetic acid as a protein precipitant and denaturant in the quantitative measurement of covalent complexes of thymidylate synthase is described. Enzyme inactivated with N[3H]ethylmaleimide and inhibitory ternary complex (formed with native enzyme, 5-[6-3H]fluoro-2'-deoxyuridylate, and methylenetetrahydrofolate) served as reagents which were used to establish the conditions under which trichloroacetic acid precipitation, washing, and solubilization steps provided quantitative results. The ternary complex formed by dihydrofolate reductase with [3H]methotrexate and NADPH was used as a control to assess whether tight, but noncovalent, enzyme:ligand complexes survived trichloroacetic acid precipitation. The fact that no counts above background were detected in the pellet of precipitated protein demonstrated that the noncovalent complexes were completely dissociated by this treatment. The dynamic range of linear response for the inhibitory ternary complex of thymidylate synthase spanned five orders of magnitude, and the assay detected levels of enzyme as low as 10 fmol, a value which was essentially limited by the specific radioactivity of 5-[6-3H]fluoro-2'-deoxyuridylate. The ability of the enzyme to bind 5-[6-3H]fluoro-2'-deoxyuridylate specifically, as measured by the trichloroacetic acid assay, generated a specific binding value of 13.4 nmol of enzyme/mg protein (assuming a binding ratio of 1.5 for the inhibitory ternary complex). Specific binding values were compared to specific activity values (obtained from the spectrophotometric assay) at each stage of purification of the enzyme from Lactobacillus casei and were found to give parallel results. The characteristics of the trichloracetic acid assay procedure, which exclusively detects covalent enzyme-ligand adducts, are compared to those for other ligand binding assays for thymidylate synthase.
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Affiliation(s)
- R J Cisneros
- Department of Chemistry, University of South Carolina, Columbia 29208
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Dev IK, Yates BB, Leong J, Dallas WS. Functional role of cysteine-146 in Escherichia coli thymidylate synthase. Proc Natl Acad Sci U S A 1988; 85:1472-6. [PMID: 3278315 PMCID: PMC279793 DOI: 10.1073/pnas.85.5.1472] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Analysis of mutant Escherichia coli thymidylate synthases (EC 2.1.1.45) with various amino acids substituted for cysteine at position 146 revealed the cysteine to be involved in the binding of 2'-deoxyuridylate as well as initiating the catalytic process. The substitution of a serine or alanine residue at position 146 did not appreciably alter the binding affinity for 2'-deoxyuridylate but the serine mutant enzyme was less active by a factor of 5000, whereas the alanine mutant enzyme was catalytically inactive. In contrast, the substitution of a glycine or threonine at position 146 created inactive enzymes with higher 2'-deoxyuridylate dissociation constants. The dissociation constant values for 2'-deoxyuridylate were used to estimate the overall contribution of the side chain of the amino acid at position 146 to substrate binding. The results suggested that the side chains of cysteine, alanine, and serine make nonspecific but effective van der Waals contacts with 2'-deoxyuridylate, thereby contributing about 0.82 kcal.mol-1 (1 cal = 4.184 J) to the apparent binding energy of the substrate.
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Affiliation(s)
- I K Dev
- Department of Microbiology, Wellcome Research Laboratories, Research Triangle Park, NC 27709
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Garvey EP, Santi DV. A stable binary complex between Leishmania major thymidylate synthase and the substrate deoxyuridylate. A slow-binding interaction. J Biol Chem 1987. [DOI: 10.1016/s0021-9258(18)48048-9] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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Models of folate cofactors 17. Methylation of 1,3-dimethyl-6-methylamino-uracil by a 5,10-CH2-H4folate model. the first mimic of the overall dTMP synthase reaction. Tetrahedron 1987. [DOI: 10.1016/s0040-4020(01)81684-7] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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Evidence for the existence of covalent nucleotide-thymidylate synthase complexes, identification of site of attachment, and enhancement by folates. J Biol Chem 1986. [DOI: 10.1016/s0021-9258(18)67155-8] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
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