1
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Shi W, Sun S, Liu H, Meng Y, Ren K, Wang G, Liu M, Wu J, Zhang Y, Huang H, Shi M, Xu W, Ma Q, Sun B, Xu J. Guiding bar motif of thioredoxin reductase 1 modulates enzymatic activity and inhibitor binding by communicating with the co-factor FAD and regulating the flexible C-terminal redox motif. Redox Biol 2024; 70:103050. [PMID: 38277963 PMCID: PMC10840350 DOI: 10.1016/j.redox.2024.103050] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2023] [Revised: 01/05/2024] [Accepted: 01/17/2024] [Indexed: 01/28/2024] Open
Abstract
Thioredoxin reductase (TXNRD) is a selenoprotein that plays a crucial role in cellular antioxidant defense. Previously, a distinctive guiding bar motif was identified in TXNRD1, which influences the transfer of electrons. In this study, utilizing single amino acid substitution and Excitation-Emission Matrix (EEM) fluorescence spectrum analysis, we discovered that the guiding bar communicates with the FAD and modulates the electron flow of the enzyme. Differential Scanning Fluorimetry (DSF) analysis demonstrated that the aromatic amino acid in guiding bar is a stabilizer for TXNRD1. Kinetic analysis revealed that the guiding bar is vital for the disulfide reductase activity but hinders the selenocysteine-independent reduction activity of TXNRD1. Meanwhile, the guiding bar shields the selenocysteine residue of TXNRD1 from the attack of electrophilic reagents. We also found that the inhibition of TXNRD1 by caveolin-1 scaffolding domain (CSD) peptides and compound LCS3 did not bind to the guiding bar motif. In summary, the obtained results highlight new aspects of the guiding bar that restrict the flexibility of the C-terminal redox motif and govern the transition from antioxidant to pro-oxidant.
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Affiliation(s)
- Wuyang Shi
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Shibo Sun
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Haowen Liu
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Yao Meng
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Kangshuai Ren
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Guoying Wang
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Minghui Liu
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Jiaqi Wu
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Yue Zhang
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Huang Huang
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Meiyun Shi
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China
| | - Weiping Xu
- School of Ocean Science and Technology (OST) & Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), Dalian University of Technology, Panjin, 124221, China
| | - Qiang Ma
- Chinese Academy of Inspection and Quarantine, Beijing, 100176, China
| | - Bingbing Sun
- State Key Laboratory of Fine Chemicals, School of Chemical Engineering (CE), Dalian University of Technology, Dalian, 116023, China
| | - Jianqiang Xu
- School of Life and Pharmaceutical Sciences (LPS) & Panjin Institute of Industrial Technology (PIIT), Dalian University of Technology, Panjin, 124221, China.
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2
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Ste. Marie EJ, Hondal RJ. Application of alpha-methyl selenocysteine as a tool for the study of selenoproteins. Methods Enzymol 2022; 662:297-329. [DOI: 10.1016/bs.mie.2021.10.016] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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3
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Dereven'kov IA, Makarov SV, Molodtsov PA, Makarova AS. Studies on the reaction between reduced riboflavin and selenocystine. INT J CHEM KINET 2021. [DOI: 10.1002/kin.21430] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Affiliation(s)
| | - Sergei V. Makarov
- Ivanovo State University of Chemistry and Technology Ivanovo 153000 Russia
| | - Pavel A. Molodtsov
- Ivanovo State University of Chemistry and Technology Ivanovo 153000 Russia
| | - Anna S. Makarova
- G.A. Krestov Institute of Solution Chemistry of Russian Academy of Sciences Ivanovo 153045 Russia
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4
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Abstract
Protein semisynthesis-defined herein as the assembly of a protein from a combination of synthetic and recombinant fragments-is a burgeoning field of chemical biology that has impacted many areas in the life sciences. In this review, we provide a comprehensive survey of this area. We begin by discussing the various chemical and enzymatic methods now available for the manufacture of custom proteins containing noncoded elements. This section begins with a discussion of methods that are more chemical in origin and ends with those that employ biocatalysts. We also illustrate the commonalities that exist between these seemingly disparate methods and show how this is allowing for the development of integrated chemoenzymatic methods. This methodology discussion provides the technical foundation for the second part of the review where we cover the great many biological problems that have now been addressed using these tools. Finally, we end the piece with a short discussion on the frontiers of the field and the opportunities available for the future.
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Affiliation(s)
| | - Tom W. Muir
- Department of Chemistry, Princeton University, Frick Laboratory, Princeton, New Jersey 08544, United States
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5
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Fernandes J, Hu X, Ryan Smith M, Go YM, Jones DP. Selenium at the redox interface of the genome, metabolome and exposome. Free Radic Biol Med 2018; 127:215-227. [PMID: 29883789 PMCID: PMC6168380 DOI: 10.1016/j.freeradbiomed.2018.06.002] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/06/2018] [Revised: 05/19/2018] [Accepted: 06/02/2018] [Indexed: 02/06/2023]
Abstract
Selenium (Se) is a redox-active environmental mineral that is converted to only a small number of metabolites and required for a relatively small number of mammalian enzymes. Despite this, dietary and environmental Se has extensive impact on every layer of omics space. This highlights a need for global network response structures to provide reference for targeted, hypothesis-driven Se research. In this review, we survey the Se research literature from the perspective of the responsive physical and chemical barrier between an organism (functional genome) and its environment (exposome), which we have previously termed the redox interface. Recent advances in metabolomics allow molecular phenotyping of the integrated genome-metabolome-exposome structure. Use of metabolomics with transcriptomics to map functional network responses to supplemental Se in mice revealed complex network responses linked to dyslipidemia and weight gain. Central metabolic hubs in the network structure in liver were not directly linked to transcripts for selenoproteins but were, instead, linked to transcripts for glucose transport and fatty acid β-oxidation. The experimental results confirm the survey of research literature in showing that Se interacts with the functional genome through a complex network response structure. The results imply that systematic application of data-driven integrated omics methods to models with controlled Se exposure could disentangle health benefits and risks from Se exposures and also serve more broadly as an experimental paradigm for exposome research.
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Affiliation(s)
- Jolyn Fernandes
- Department of Medicine, Emory University, Atlanta, GA 30322, United States
| | - Xin Hu
- Department of Medicine, Emory University, Atlanta, GA 30322, United States
| | - M Ryan Smith
- Department of Medicine, Emory University, Atlanta, GA 30322, United States
| | - Young-Mi Go
- Department of Medicine, Emory University, Atlanta, GA 30322, United States.
| | - Dean P Jones
- Department of Medicine, Emory University, Atlanta, GA 30322, United States.
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6
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Barber DR, Hondal RJ. Gain of function conferred by selenocysteine: catalytic enhancement of one-electron transfer reactions by thioredoxin reductase. Protein Sci 2018; 28:79-89. [PMID: 30052295 DOI: 10.1002/pro.3480] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2018] [Revised: 07/03/2018] [Accepted: 07/05/2018] [Indexed: 11/07/2022]
Abstract
Selenocysteine (Sec) is the 21st amino acid in the genetic code and it is present in a small number of proteins where it replaces the much more commonly used amino acid cysteine (Cys). It is both more complicated and bioenergetically costly to insert Sec into a protein in comparison to Cys, and this cost is most likely compensated by a gain of function to the enzyme/protein in which it is incorporated. Here we investigate one such gain of function, the enhancement of one-electron transfer catalysis. We compared the ability of Sec-containing mouse mitochondrial thioredoxin reductase (mTrxR2) to catalyze the reduction of bovine cytochrome c, ascorbyl radical, and dehydroascorbate in comparison to Cys-containing thioredoxin reductases from D. melanogaster (DmTrxR) and P. falciparum (PfTrxR). The Sec-containing mTrxR2 was able to reduce all three substrates, while the Cys-containing enzymes had little or no activity. In addition, we constructed Cys➔Sec mutants of DmTrxR and PfTrxR and found that this substitution resulted in a gain of function, as these mutant enzymes were now able to catalyze the reduction of these substrates. We also found that in the case of PfTrxR, reduction of cytochrome c was enhanced five-fold in a truncated PfTrxR in which the C-terminal redox center was removed. This shows that some of the ability of thioredoxin reductase to reduce this substrate comes from the flavin coenzyme. We also discuss a possible mechanism by which Sec-containing thioredoxin reductase reduces dehydroascorbate to ascorbate by two sequential, one-electron reductions, in part catalyzed by Sec.
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Affiliation(s)
- Drew R Barber
- Department of Biochemistry, 89 Beaumont Ave, Given Building Room B413, Burlington, Vermont, 05405
| | - Robert J Hondal
- Department of Biochemistry, 89 Beaumont Ave, Given Building Room B413, Burlington, Vermont, 05405
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7
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O'Keefe JP, Dustin CM, Barber D, Snider GW, Hondal RJ. A "Seleno Effect" Differentiates the Roles of Redox Active Cysteine Residues in Plasmodium falciparum Thioredoxin Reductase. Biochemistry 2018; 57:1767-1778. [PMID: 29485860 DOI: 10.1021/acs.biochem.8b00004] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Here, we introduce the concept of the "seleno effect" in the study of oxidoreductases that catalyze thiol/disulfide exchange reactions. In these reactions, selenium can replace sulfur as a nucleophile, electrophile, or leaving group, and the resulting change in rate (the seleno effect) is defined as kS/ kSe. In solution, selenium accelerates the rate of thiol/disulfide exchange regardless of its chemical role (e.g., nucleophile or electrophile). Here we show that this is not the case for enzyme catalyzed reactions and that the magnitude of the seleno effect can differentiate the role of each sulfur atom of a disulfide bond between that of an electrophile or leaving group. We used selenium for sulfur substitution to study the thiol/disulfide exchange step that occurs between the N-terminal redox center and the C-terminal disulfide-containing β-hairpin motif of Plasmodium falciparum thioredoxin reductase (PfTrxR), which has the sequence Gly-Cys535-Gly-Gly-Gly-Lys-Cys540-Gly. We assayed a truncated PfTrxR enzyme missing this C-terminal tail for disulfide-reductase activity using synthetic peptide substrates in which either Cys535 or Cys540 was replaced with selenocysteine (Sec). The results show that substitution of Cys535 with Sec resulted in a nearly 9-fold decrease in the rate of reduction, while substitution of Cys540 resulted in a 1.5-fold increase in the rate of reduction. We also produced full-length, semisynthetic enzymes in which Sec replaced either of these two Cys residues and observed similar results using E. coli thioredoxin as the substrate. In this assay, the observed seleno effect ( kS/ kSe) for the C535U mutant was 7.4, and that for the C540U mutant was 0.2.
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Affiliation(s)
- John P O'Keefe
- Department of Biochemistry , University of Vermont , 89 Beaumont Ave, Given Building Room B413 , Burlington , Vermont 05405 , United States
| | - Christopher M Dustin
- Department of Biochemistry , University of Vermont , 89 Beaumont Ave, Given Building Room B413 , Burlington , Vermont 05405 , United States
| | - Drew Barber
- Department of Biochemistry , University of Vermont , 89 Beaumont Ave, Given Building Room B413 , Burlington , Vermont 05405 , United States
| | - Gregg W Snider
- Department of Biochemistry , University of Vermont , 89 Beaumont Ave, Given Building Room B413 , Burlington , Vermont 05405 , United States
| | - Robert J Hondal
- Department of Biochemistry , University of Vermont , 89 Beaumont Ave, Given Building Room B413 , Burlington , Vermont 05405 , United States
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8
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Maia LB, Moura I, Moura JJ. Molybdenum and tungsten-containing formate dehydrogenases: Aiming to inspire a catalyst for carbon dioxide utilization. Inorganica Chim Acta 2017. [DOI: 10.1016/j.ica.2016.07.010] [Citation(s) in RCA: 71] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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9
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Maia LB, Fonseca L, Moura I, Moura JJG. Reduction of Carbon Dioxide by a Molybdenum-Containing Formate Dehydrogenase: A Kinetic and Mechanistic Study. J Am Chem Soc 2016; 138:8834-46. [PMID: 27348246 DOI: 10.1021/jacs.6b03941] [Citation(s) in RCA: 75] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Carbon dioxide accumulation is a major concern for the ecosystems, but its abundance and low cost make it an interesting source for the production of chemical feedstocks and fuels. However, the thermodynamic and kinetic stability of the carbon dioxide molecule makes its activation a challenging task. Studying the chemistry used by nature to functionalize carbon dioxide should be helpful for the development of new efficient (bio)catalysts for atmospheric carbon dioxide utilization. In this work, the ability of Desulfovibrio desulfuricans formate dehydrogenase (Dd FDH) to reduce carbon dioxide was kinetically and mechanistically characterized. The Dd FDH is suggested to be purified in an inactive form that has to be activated through a reduction-dependent mechanism. A kinetic model of a hysteretic enzyme is proposed to interpret and predict the progress curves of the Dd FDH-catalyzed reactions (initial lag phase and subsequent faster phase). Once activated, Dd FDH is able to efficiently catalyze, not only the formate oxidation (kcat of 543 s(-1), Km of 57.1 μM), but also the carbon dioxide reduction (kcat of 46.6 s(-1), Km of 15.7 μM), in an overall reaction that is thermodynamically and kinetically reversible. Noteworthy, both Dd FDH-catalyzed formate oxidation and carbon dioxide reduction are completely inactivated by cyanide. Current FDH reaction mechanistic proposals are discussed and a different mechanism is here suggested: formate oxidation and carbon dioxide reduction are proposed to proceed through hydride transfer and the sulfo group of the oxidized and reduced molybdenum center, Mo(6+)═S and Mo(4+)-SH, are suggested to be the direct hydride acceptor and donor, respectively.
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Affiliation(s)
- Luisa B Maia
- UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal
| | - Luis Fonseca
- UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal
| | - Isabel Moura
- UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal
| | - José J G Moura
- UCIBIO, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa , 2829-516 Caparica, Portugal
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10
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Xu J, Eriksson SE, Cebula M, Sandalova T, Hedström E, Pader I, Cheng Q, Myers CR, Antholine WE, Nagy P, Hellman U, Selivanova G, Lindqvist Y, Arnér ESJ. The conserved Trp114 residue of thioredoxin reductase 1 has a redox sensor-like function triggering oligomerization and crosslinking upon oxidative stress related to cell death. Cell Death Dis 2015; 6:e1616. [PMID: 25611390 PMCID: PMC4669772 DOI: 10.1038/cddis.2014.574] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2014] [Revised: 11/19/2014] [Accepted: 12/03/2014] [Indexed: 12/19/2022]
Abstract
The selenoprotein thioredoxin reductase 1 (TrxR1) has several key roles in cellular redox systems and reductive pathways. Here we discovered that an evolutionarily conserved and surface-exposed tryptophan residue of the enzyme (Trp114) is excessively reactive to oxidation and exerts regulatory functions. The results indicate that it serves as an electron relay communicating with the FAD moiety of the enzyme, and, when oxidized, it facilitates oligomerization of TrxR1 into tetramers and higher multimers of dimers. A covalent link can also be formed between two oxidized Trp114 residues of two subunits from two separate TrxR1 dimers, as found both in cell extracts and in a crystal structure of tetrameric TrxR1. Formation of covalently linked TrxR1 subunits became exaggerated in cells on treatment with the pro-oxidant p53-reactivating anticancer compound RITA, in direct correlation with triggering of a cell death that could be prevented by antioxidant treatment. These results collectively suggest that Trp114 of TrxR1 serves a function reminiscent of an irreversible sensor for excessive oxidation, thereby presenting a previously unrecognized level of regulation of TrxR1 function in relation to cellular redox state and cell death induction.
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Affiliation(s)
- J Xu
- Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - S E Eriksson
- Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - M Cebula
- Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - T Sandalova
- Division of Molecular Structural Biology, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - E Hedström
- Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - I Pader
- Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - Q Cheng
- Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - C R Myers
- Department of Pharmacology and Toxicology, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA
| | - W E Antholine
- Department of Biophysics, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA
| | - P Nagy
- Department of Molecular Immunology and Toxicology, National Institute of Oncology, Rath György ut 7-91, 1122, Budapest, Hungary
| | - U Hellman
- Ludwig Institutet for Cancer Research Ltd., Uppsala University BMC, SE-75 124 Uppsala, Sweden
| | - G Selivanova
- Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - Y Lindqvist
- Division of Molecular Structural Biology, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
| | - E S J Arnér
- Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-17177 Stockholm, Sweden
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11
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Snider GW, Dustin CM, Ruggles EL, Hondal RJ. A mechanistic investigation of the C-terminal redox motif of thioredoxin reductase from Plasmodium falciparum. Biochemistry 2014; 53:601-9. [PMID: 24400600 PMCID: PMC3957191 DOI: 10.1021/bi400931k] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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High-molecular
mass thioredoxin reductases (TRs) are pyridine nucleotide
disulfide oxidoreductases that catalyze the reduction of the disulfide
bond of thioredoxin (Trx). Trx is responsible for reducing multiple
protein disulfide targets in the cell. TRs utilize reduced β-nicotinamide
adenine dinucleotide phosphate to reduce a bound flavin prosthetic
group, which in turn reduces an N-terminal redox center that has the
conserved sequence CICVNVGCCT, where CIC is denoted as the interchange thiol while the thiol involved in
charge-transfer complexation is denoted as CCT. The reduced
N-terminal redox center reduces a C-terminal redox center on the opposite
subunit of the head-to-tail homodimer, the C-terminal redox center
that catalyzes the reduction of the Trx-disulfide. Variations in the
amino acid sequence of the C-terminal redox center differentiate high-molecular
mass TRs into different types. Type Ia TRs have tetrapeptide C-terminal
redox centers of with a GCUG sequence, where U is the rare amino acid
selenocysteine (Sec), while the tetrapeptide sequence in type Ib TRs
has its Sec residue replaced with a conventional cysteine (Cys) residue
and can use small polar amino acids such as serine and threonine in
place of the flanking glycine residues. The TR from Plasmodium
falciparum (PfTR) is similar in structure and mechanism to
type Ia and type Ib TRs except that the C-terminal redox center is
different in its amino acid sequence. The C-terminal redox center
of PfTR has the sequence G534CGGGKCG541, and
we classify it as a type II high-molecular mass TR. The oxidized type
II redox motif will form a 20-membered disulfide ring, whereas the
absence of spacer amino acids in the type I motif results in the formation
of a rare eight-membered ring. We used site-directed mutagenesis and
protein semisynthesis to investigate features of the distinctive type
II C-terminal redox motif that help it perform catalysis. Deletion
of Gly541 reduces thioredoxin reductase activity by ∼50-fold,
most likely because of disruption of an important hydrogen bond between
the amide NH group of Gly541 and the carbonyl of Gly534 that helps
to stabilize
the β–turn−β motif. Alterations of the 20-membered
disulfide ring either by amino acid deletion or by substitution resulted
in impaired catalytic activity. Subtle changes in the ring structure
and size caused by using semisynthesis to substitute homocysteine
for cysteine also caused significant reductions in catalytic activity,
demonstrating the importance of the disulfide ring’s geometry
in making the C-terminal redox center reactive for thiol–disulfide
exchange. The data suggested to us that the transfer of electrons
from the N-terminal redox center to the C-terminal redox center may
be rate-limiting. We propose that the transfer of electrons from the
N-terminal redox center in PfTR to the type II C-terminal disulfide
is accelerated by the use of an “electrophilic activation”
mechanism. In this mechanism, the type II C-terminal disulfide is
polarized, making the sulfur atom of Cys540 electron deficient, highly
electrophilic, and activated for thiol–disulfide exchange with
the N-terminal redox center. This hypothesis was investigated by constructing
chimeric PfTR mutant enzymes containing C-terminal type I sequences
GCCG and GCUG, respectively. The PfTR-GCCG chimera had 500-fold less
thioredoxin reductase activity than the native enzyme but still reduced
selenocystine and lipoic acid efficiently. The PfTR-GCUG chimera had
higher catalytic activity than the native enzyme with Trx, selenocystine,
and lipoic acid as substrates. The results suggested to us that (i)
Sec in the mutant enzyme accelerated the rate of thiol–disulfide
exchange between the N- and C-terminal redox centers, (ii) the type
II redox center evolved for efficient catalysis utilizing Cys instead
of Sec, and (iii) the type II redox center of PfTR is partly responsible
for substrate recognition of the cognate PfTrx substrate relative
to noncognate thioredoxins.
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Affiliation(s)
- Gregg W Snider
- Department of Biochemistry, University of Vermont, College of Medicine , 89 Beaumont Avenue, Given Building Room B413, Burlington, Vermont 05405, United States
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