Structure of Mycobacterium tuberculosis methionine sulfoxide reductase A in complex with protein-bound methionine

Catalytic Mechanism of Mycobacterium tuberculosis Methionine Sulfoxide Reductase A

The oxidation of Met to methionine sulfoxide (MetSO) by oxidants such as hydrogen peroxide, hypochlorite, or peroxynitrite has profound effects on protein function. This modification can be reversed by methionine sulfoxide reductases (msr). In the context of pathogen infection, the reduction of oxidized proteins gains significance due to microbial oxidative damage generated by the immune system. For example, Mycobacterium tuberculosis (Mt) utilizes msrs (MtmsrA and MtmsrB) as part of the repair response to the host-induced oxidative stress. The absence of these enzymes makes Mycobacteria prone to increased susceptibility to cell death, pointing them out as potential therapeutic targets. This study provides a detailed characterization of the catalytic mechanism of MtmsrA using a comprehensive approach, including experimental techniques and theoretical methodologies. Confirming a ping-pong type enzymatic mechanism, we elucidate the catalytic parameters for sulfoxide and thioredoxin substrates (kcat/KM = 2656 ± 525 M-1 s-1 and 1.7 ± 0.8 × 106 M-1 s-1, respectively). Notably, the entropic nature of the activation process thermodynamics, representing ∼85% of the activation free energy at room temperature, is underscored. Furthermore, the current study questions the plausibility of a sulfurane intermediate, which may be a transition-state-like structure, suggesting the involvement of a conserved histidine residue as an acid-base catalyst in the MetSO reduction mechanism. This mechanistic insight not only advances our understanding of Mt antioxidant enzymes but also holds implications for future drug discovery and biotechnological applications.

A widespread group of large plasmids in methanotrophic Methanoperedens archaea

Anaerobic methanotrophic (ANME) archaea obtain energy from the breakdown of methane, yet their extrachromosomal genetic elements are little understood. Here we describe large plasmids associated with ANME archaea of the Methanoperedens genus in enrichment cultures and other natural anoxic environments. By manual curation we show that two of the plasmids are large (155,605 bp and 191,912 bp), circular, and may replicate bidirectionally. The plasmids occur in the same copy number as the main chromosome, and plasmid genes are actively transcribed. One of the plasmids encodes three tRNAs, ribosomal protein uL16 and elongation factor eEF2; these genes appear to be missing in the host Methanoperedens genome, suggesting an obligate interdependence between plasmid and host. Our work opens the way for the development of genetic vectors to shed light on the physiology and biochemistry of Methanoperedens, and potentially genetically edit them to enhance growth and accelerate methane oxidation rates.

Structural Insights into a Bifunctional Peptide Methionine Sulfoxide Reductase MsrA/B Fusion Protein from Helicobacter pylori

Methionine sulfoxide reductase (Msr) is a family of enzymes that reduces oxidized methionine and plays an important role in the survival of bacteria under oxidative stress conditions. MsrA and MsrB exist in a fusion protein form (MsrAB) in some pathogenic bacteria, such as Helicobacter pylori (Hp), Streptococcus pneumoniae, and Treponema denticola. To understand the fused form instead of the separated enzyme at the molecular level, we determined the crystal structure of HpMsrABC44S/C318S at 2.2 Å, which showed that a linker region (Hpiloop, 193-205) between two domains interacted with each HpMsrA or HpMsrB domain via three salt bridges (E193-K107, D197-R103, and K200-D339). Two acetate molecules in the active site pocket showed an sp2 planar electron density map in the crystal structure, which interacted with the conserved residues in fusion MsrABs from the pathogen. Biochemical and kinetic analyses revealed that Hpiloop is required to increase the catalytic efficiency of HpMsrAB. Two salt bridge mutants (D193A and E199A) were located at the entrance or tailgate of Hpiloop. Therefore, the linker region of the MsrAB fusion enzyme plays a key role in the structural stability and catalytic efficiency and provides a better understanding of why MsrAB exists in a fused form.

New insights into the molecular physiology of sulfoxide reduction in bacteria

Sulfoxides occur in biology as products of the S-oxygenation of small molecules as well as in peptides and proteins and their formation is often associated with oxidative stress and can affect biological function. In bacteria, sulfoxide damage can be reversed by different types of enzymes. Thioredoxin-dependent peptide methionine sulfoxide reductases (MSR proteins) repair oxidized methionine residues and are found in all Domains of life. In bacteria MSR proteins are often found in the cytoplasm but in some bacteria, including pathogenic Neisseria, Streptococci, and Haemophilus they are extracytoplasmic. Mutants lacking MSR proteins are often sensitive to oxidative stress and in pathogens exhibit decreased virulence as indicated by reduced survival in host cell or animal model systems. Molybdenum enzymes are also known to reduce S-oxides and traditionally their physiological role was considered to be in anaerobic respiration using dimethylsulfoxide (DMSO) as an electron acceptor. However, it now appears that some enzymes (MtsZ) of the DMSO reductase family of Mo enzymes use methionine sulfoxide as preferred physiological substrate and thus may be involved in scavenging/recycling of this amino acid. Similarly, an enzyme (MsrP/YedY) of the sulfite oxidase family of Mo enzymes has been shown to be involved in repair of methionine sulfoxides in periplasmic proteins. Again, some mutants deficient in Mo-dependent sulfoxide reductases exhibit reduced virulence, and there is evidence that these Mo enzymes and some MSR systems are induced by hypochlorite produced by the innate immune system. This review describes recent advances in the understanding of the molecular microbiology of MSR systems and the broadening of the role of Mo-dependent sulfoxide reductase to encompass functions beyond anaerobic respiration.

The Oxidized Protein Repair Enzymes Methionine Sulfoxide Reductases and Their Roles in Protecting against Oxidative Stress, in Ageing and in Regulating Protein Function

Cysteine and methionine residues are the amino acids most sensitive to oxidation by reactive oxygen species. However, in contrast to other amino acids, certain cysteine and methionine oxidation products can be reduced within proteins by dedicated enzymatic repair systems. Oxidation of cysteine first results in either the formation of a disulfide bridge or a sulfenic acid. Sulfenic acid can be converted to disulfide or sulfenamide or further oxidized to sulfinic acid. Disulfide can be easily reversed by different enzymatic systems such as the thioredoxin/thioredoxin reductase and the glutaredoxin/glutathione/glutathione reductase systems. Methionine side chains can also be oxidized by reactive oxygen species. Methionine oxidation, by the addition of an extra oxygen atom, leads to the generation of methionine sulfoxide. Enzymatically catalyzed reduction of methionine sulfoxide is achieved by either methionine sulfoxide reductase A or methionine sulfoxide reductase B, also referred as to the methionine sulfoxide reductases system. This oxidized protein repair system is further described in this review article in terms of its discovery and biologically relevant characteristics, and its important physiological roles in protecting against oxidative stress, in ageing and in regulating protein function.

Essential Role of the Linker Region in the Higher Catalytic Efficiency of a Bifunctional MsrA-MsrB Fusion Protein

Many bacteria, particularly pathogens, possess methionine sulfoxide reductase A (MsrA) and B (MsrB) as a fusion form (MsrAB). However, it is not clear why they possess a fusion MsrAB form rather than the separate enzymes that exist in most organisms. In this study, we performed biochemical and kinetic analyses of MsrAB from Treponema denticola (TdMsrAB), single-domain forms (TdMsrA and TdMsrB), and catalytic Cys mutants (TdMsrAB(C11S) and TdMsrAB(C285S)). We found that the catalytic efficiency of both MsrA and MsrB increased after fusion of the domains and that the linker region (iloop) that connects TdMsrA and TdMsrB is required for the higher catalytic efficiency of TdMsrAB. We also determined the crystal structure of TdMsrAB at 2.3 Å, showing that the iloop mainly interacts with TdMsrB via hydrogen bonds. Further kinetic analysis using the iloop mutants revealed that the iloop-TdMsrB interactions are critical to MsrB and MsrA activities. We also report the structure in which an oxidized form of dithiothreitol, an in vitro reductant for MsrA and MsrB, is present in the active site of TdMsrA. Collectively, the results of this study reveal an essential role of the iloop in maintaining the higher catalytic efficiency of the MsrAB fusion enzyme and provide a better understanding of why the MsrAB enzyme exists as a fused form.

A Methionine Residue Promotes Hyperoxidation of the Catalytic Cysteine of Mouse Methionine Sulfoxide Reductase A

Methionine sulfoxide reductase A (msrA) reduces methionine sulfoxide in proteins back to methionine. Its catalytic cysteine (Cys72-SH) has a low pKa that facilitates oxidation by methionine sulfoxide to cysteine sulfenic acid. If the catalytic cycle proceeds efficiently, the sulfenic acid is reduced back to cysteine at the expense of thioredoxin. However, the sulfenic acid is vulnerable to "irreversible" oxidation to cysteine sulfinic acid that inactivates msrA (hyperoxidation). We observed that human msrA is resistant to hyperoxidation while mouse msrA is readily hyperoxidized by micromolar concentrations of hydrogen peroxide. We investigated the basis of this difference in susceptibility to hyperoxidation and established that it is controlled by the presence or absence of a Met residue in the carboxyl-terminal domain of the enzyme, Met229. This residue is Val in human msrA, and when it was mutated to Met, human msrA became sensitive to hyperoxidation. Conversely, mouse msrA was rendered insensitive to hyperoxidation when Met229 was mutated to Val or one of five other residues. Positioning of the methionine at residue 229 is not critical, as hyperoxidation occurred as long as the methionine was located within the group of 14 carboxyl-terminal residues. The carboxyl domain of msrA is known to be flexible and to have access to the active site, and Met residues are known to form stable, noncovalent bonds with aromatic residues through interaction of the sulfur atom with the aromatic ring. We propose that Met229 forms such a bond with Trp74 at the active site, preventing formation of a protective sulfenylamide with Cys72 sulfenic acid. As a consequence, the sulfenic acid is available for facile, irreversible oxidation to cysteine sulfinic acid.

Evidence for the dimerization-mediated catalysis of methionine sulfoxide reductase A from Clostridium oremlandii

Clostridium oremlandii MsrA (CoMsrA) is a natively selenocysteine-containing methionine-S-sulfoxide reductase and classified into a 1-Cys type MsrA. CoMsrA exists as a monomer in solution. Herein, we report evidence that CoMsrA can undergo homodimerization during catalysis. The monomeric CoMsrA dimerizes in the presence of its substrate methionine sulfoxide via an intermolecular disulfide bond between catalytic Cys16 residues. The dimeric CoMsrA is resolved by the reductant glutaredoxin, suggesting the relevance of dimerization in catalysis. The dimerization reaction occurs in a concentration- and time-dependent manner. In addition, the occurrence of homodimer formation in the native selenoprotein CoMsrA is confirmed. We also determine the crystal structure of the dimeric CoMsrA, having the dimer interface around the two catalytic Cys16 residues. A central cone-shaped hole is present in the surface model of dimeric structure, and the two Cys16 residues constitute the base of the hole. Collectively, our biochemical and structural analyses suggest a novel dimerization-mediated mechanism for CoMsrA catalysis that is additionally involved in CoMsrA regeneration by glutaredoxin.

Corynebacterium diphtheriae methionine sulfoxide reductase a exploits a unique mycothiol redox relay mechanism

Methionine sulfoxide reductases are conserved enzymes that reduce oxidized methionines in proteins and play a pivotal role in cellular redox signaling. We have unraveled the redox relay mechanisms of methionine sulfoxide reductase A of the pathogen Corynebacterium diphtheriae (Cd-MsrA) and shown that this enzyme is coupled to two independent redox relay pathways. Steady-state kinetics combined with mass spectrometry of Cd-MsrA mutants give a view of the essential cysteine residues for catalysis. Cd-MsrA combines a nucleophilic cysteine sulfenylation reaction with an intramolecular disulfide bond cascade linked to the thioredoxin pathway. Within this cascade, the oxidative equivalents are transferred to the surface of the protein while releasing the reduced substrate. Alternatively, MsrA catalyzes methionine sulfoxide reduction linked to the mycothiol/mycoredoxin-1 pathway. After the nucleophilic cysteine sulfenylation reaction, MsrA forms a mixed disulfide with mycothiol, which is transferred via a thiol disulfide relay mechanism to a second cysteine for reduction by mycoredoxin-1. With x-ray crystallography, we visualize two essential intermediates of the thioredoxin relay mechanism and a cacodylate molecule mimicking the substrate interactions in the active site. The interplay of both redox pathways in redox signaling regulation forms the basis for further research into the oxidative stress response of this pathogen.

Essential role of the C-terminal helical domain in active site formation of selenoprotein MsrA from Clostridium oremlandii

We previously determined the crystal structures of 1-Cys type selenoprotein MsrA from Clostridium oremlandii (CoMsrA). The overall structure of CoMsrA is unusual, consisting of two domains, the N-terminal catalytic domain and the C-terminal distinct helical domain which is absent from other known MsrA structures. Deletion of the helical domain almost completely abolishes the catalytic activity of CoMsrA. In this study, we determined the crystal structure of the helical domain-deleted (ΔH-domain) form of CoMsrA at a resolution of 1.76 Å. The monomer structure is composed of the central rolled mixed β-sheet surrounded by α-helices. However, there are significant conformational changes in the N- and C-termini and loop regions of the ΔH-domain protein relative to the catalytic domain structure of full-length CoMsrA. The active site structure in the ΔH-domain protein completely collapses, thereby causing loss of catalytic activity of the protein. Interestingly, dimer structures are observed in the crystal formed by N-terminus swapping between two molecules. The ΔH-domain protein primarily exists as a dimer in solution, whereas the full-length CoMsrA exists as a monomer. Collectively, this study provides insight into the structural basis of the essential role of the helical domain of CoMsrA in its catalysis.

Mechanism of 1-Cys type methionine sulfoxide reductase A regeneration by glutaredoxin

Glutaredoxin (Grx), a major redox regulator, can act as a reductant of methionine sulfoxide reductase A (MsrA). However, the biochemical mechanisms involved in MsrA activity regeneration by Grx remain largely unknown. In this study, we investigated the regeneration mechanism of 1-Cys type Clostridium oremlandii MsrA (cMsrA) lacking a resolving Cys residue in a Grx-dependent assay. Kinetic analysis showed that cMsrA could be reduced by both monothiol and dithiol Grxs as efficiently as by in vitro reductant dithiothreitol. Our data revealed that the catalytic Cys sulfenic acid intermediate is not glutathionylated in the presence of the substrate, and that Grx instead directly formed a complex with cMsrA. Mass spectrometry analysis identified a disulfide bond between the N-terminal catalytic Cys of the active site of Grx and the catalytic Cys of cMsrA. This mixed disulfide bond could be resolved by glutathione. Based on these findings, we propose a model for regeneration of 1-Cys type cMsrA by Grx that involves no glutathionylation on the catalytic Cys of cMsrA. This mechanism contrasts with that of the previously known 1-Cys type MsrB.

Methionine sulfoxide reductase: chemistry, substrate binding, recycling process and oxidase activity

Three classes of methionine sulfoxide reductases are known: MsrA and MsrB which are implicated stereo-selectively in the repair of protein oxidized on their methionine residues; and fRMsr, discovered more recently, which binds and reduces selectively free L-Met-R-O. It is now well established that the chemical mechanism of the reductase step passes through formation of a sulfenic acid intermediate. The oxidized catalytic cysteine can then be recycled by either Trx when a recycling cysteine is operative or a reductant like glutathione in the absence of recycling cysteine which is the case for 30% of the MsrBs. Recently, it was shown that a subclass of MsrAs with two recycling cysteines displays an oxidase activity. This reverse activity needs the accumulation of the sulfenic acid intermediate. The present review focuses on recent insights into the catalytic mechanism of action of the Msrs based on kinetic studies, theoretical chemistry investigations and new structural data. Major attention is placed on how the sulfenic acid intermediate can be formed and the oxidized catalytic cysteine returns back to its reduced form.

Multi-scale computational enzymology: enhancing our understanding of enzymatic catalysis

Elucidating the origin of enzymatic catalysis stands as one the great challenges of contemporary biochemistry and biophysics. The recent emergence of computational enzymology has enhanced our atomistic-level description of biocatalysis as well the kinetic and thermodynamic properties of their mechanisms. There exists a diversity of computational methods allowing the investigation of specific enzymatic properties. Small or large density functional theory models allow the comparison of a plethora of mechanistic reactive species and divergent catalytic pathways. Molecular docking can model different substrate conformations embedded within enzyme active sites and determine those with optimal binding affinities. Molecular dynamics simulations provide insights into the dynamics and roles of active site components as well as the interactions between substrate and enzymes. Hybrid quantum mechanical/molecular mechanical (QM/MM) can model reactions in active sites while considering steric and electrostatic contributions provided by the surrounding environment. Using previous studies done within our group, on OvoA, EgtB, ThrRS, LuxS and MsrA enzymatic systems, we will review how these methods can be used either independently or cooperatively to get insights into enzymatic catalysis.

The methionine sulfoxide reduction system: selenium utilization and methionine sulfoxide reductase enzymes and their functions

SIGNIFICANCE

Selenium is utilized in the methionine sulfoxide reduction system that occurs in most organisms. Methionine sulfoxide reductases (Msrs), MsrA and MsrB, are the enzymes responsible for this system. Msrs repair oxidatively damaged proteins, protect against oxidative stress, and regulate protein function, and have also been implicated in the aging process. Selenoprotein forms of Msrs containing selenocysteine (Sec) at the catalytic site are found in bacteria, algae, and animals.

RECENT ADVANCES

A selenoprotein MsrB1 knockout mouse has been developed. Significant progress in the biochemistry of Msrs has been made, which includes findings of a novel reducing system for Msrs and of an interesting reason for the use of Sec in the Msr system. The effects of mammalian MsrBs, including selenoprotein MsrB1 on fruit fly aging, have been investigated. Furthermore, it is evident that Msrs are involved in methionine metabolism and regulation of the trans-sulfuration pathway.

CRITICAL ISSUES

This article presents recent progress in the Msr field while focusing on the physiological roles of mammalian Msrs, functions of selenoprotein forms of Msrs, and their biochemistry.

FUTURE DIRECTIONS

A deeper understanding of the roles of Msrs in redox signaling, the aging process, and metabolism will be achieved. The identity of selenoproteome of Msrs will be sought along with characterization of the identified selenoprotein forms. Exploring new cellular targets and new functions of Msrs is also warranted.

Recharging oxidative protein repair: catalysis by methionine sulfoxide reductases towards their amino acid, protein, and model substrates

The sulfur-containing amino acid methionine (Met) in its free and amino acid residue forms can be readily oxidized to the R and S diastereomers of methionine sulfoxide (MetO). Methionine sulfoxide reductases A (MSRA) and B (MSRB) reduce MetO back to Met in a stereospecific manner, acting on the S and R forms, respectively. A third MSR type, fRMSR, reduces the R form of free MetO. MSRA and MSRB are spread across the three domains of life, whereas fRMSR is restricted to bacteria and unicellular eukaryotes. These enzymes protect against abiotic and biotic stresses and regulate lifespan. MSRs are thiol oxidoreductases containing catalytic redox-active cysteine or selenocysteine residues, which become oxidized by the substrate, requiring regeneration for the next catalytic cycle. These enzymes can be classified according to the number of redox-active cysteines (selenocysteines) and the strategies to regenerate their active forms by thioredoxin and glutaredoxin systems. For each MSR type, we review catalytic parameters for the reduction of free MetO, low molecular weight MetO-containing compounds, and oxidized proteins. Analysis of these data reinforces the concept that MSRAs reduce various types of MetO-containing substrates with similar efficiency, whereas MSRBs are specialized for the reduction of MetO in proteins.

Electrostatics of cysteine residues in proteins: parameterization and validation of a simple model

One of the most popular and simple models for the calculation of pK(a) s from a protein structure is the semi-macroscopic electrostatic model MEAD. This model requires empirical parameters for each residue to calculate pK(a) s. Analysis of current, widely used empirical parameters for cysteine residues showed that they did not reproduce expected cysteine pK(a) s; thus, we set out to identify parameters consistent with the CHARMM27 force field that capture both the behavior of typical cysteines in proteins and the behavior of cysteines which have perturbed pK(a) s. The new parameters were validated in three ways: (1) calculation across a large set of typical cysteines in proteins (where the calculations are expected to reproduce expected ensemble behavior); (2) calculation across a set of perturbed cysteines in proteins (where the calculations are expected to reproduce the shifted ensemble behavior); and (3) comparison to experimentally determined pK(a) values (where the calculation should reproduce the pK(a) within experimental error). Both the general behavior of cysteines in proteins and the perturbed pK(a) in some proteins can be predicted reasonably well using the newly determined empirical parameters within the MEAD model for protein electrostatics. This study provides the first general analysis of the electrostatics of cysteines in proteins, with specific attention paid to capturing both the behavior of typical cysteines in a protein and the behavior of cysteines whose pK(a) should be shifted, and validation of force field parameters for cysteine residues.

Glutaredoxin serves as a reductant for methionine sulfoxide reductases with or without resolving cysteine

Methionine sulfoxide reductases A and B (MsrA and MsrB) have been known to be thioredoxin (Trx)-dependent enzymes that catalyze the reduction of methionine sulfoxide in a stereospecific manner. This work reports that glutaredoxin, another major thiol-disulfide oxidoreductase, can serve as a reductant for both MsrA and MsrB. Glutaredoxins efficiently reduced 1-Cys MsrA lacking a resolving Cys, which is not reducible by Trx. Glutaredoxins also reduced 3-Cys MsrA containing two resolving Cys. The glutaredoxin-dependent activity of the 3-Cys MsrA was comparable with the Trx-dependent activity. The kinetic data suggest that 1-Cys MsrA is more efficiently reduced by glutaredoxin than 3-Cys form. Also, glutaredoxins could function as a reductant for 1-Cys MsrB lacking a resolving Cys as previously reported. In contrast to the previous report, 2-Cys MsrB containing a resolving Cys was reducible by the glutaredoxins. Collectively, this study demonstrates that glutaredoxins reduce MsrAs and MsrBs with or without resolving Cys.

A low pKa cysteine at the active site of mouse methionine sulfoxide reductase A

Methionine sulfoxide reductase A is an essential enzyme in the antioxidant system which scavenges reactive oxygen species through cyclic oxidation and reduction of methionine and methionine sulfoxide. Recently it has also been shown to catalyze the reverse reaction, oxidizing methionine residues to methionine sulfoxide. A cysteine at the active site of the enzyme is essential for both reductase and oxidase activities. This cysteine has been reported to have a pK(a) of 9.5 in the absence of substrate, decreasing to 5.7 upon binding of substrate. Using three independent methods, we show that the pK(a) of the active site cysteine of mouse methionine sulfoxide reductase is 7.2 even in the absence of substrate. The primary mechanism by which the pK(a) is lowered is hydrogen bonding of the active site Cys-72 to protonated Glu-115. The low pK(a) renders the active site cysteine susceptible to oxidation to sulfenic acid by micromolar concentrations of hydrogen peroxide. This characteristic supports a role for methionine sulfoxide reductase in redox signaling.

Two highly homologous methionine sulfoxide reductase A from tomato (Solanum lycopersicum), exhibit distinct catalytic properties

E4, which is a fruit-ripening gene that is strongly induced by ethylene, has been reported to be a member of the methionine sulfoxide reductase A (MSRA) gene. In the present study, we determined for the first time the enzymatic activity and delineated the catalytic mechanism of the E4 protein via site-directed mutagenesis. The disulfide intermolecular cross-linking, kinetics parameter, thiol content titration analysis of wild-type and mutated E4 proteins revealed that the cysteine at position 37 (Cys-37) was the key catalytic residue, and Cys-194, but not Cys-180 served as the first recycling Cys in the thioredoxin (Trx)-dependent regeneration system. In addition, the SlMSRA2 protein, which was encoded by another MSRA gene, shared high similarity with the E4 protein and was truncated at the C-terminus. The wild-type and mutated SlMSRA2 enzymes had similar activities compared to the E4 protein using DTT as a reductant, but showed extremely low activities in the Trx-dependent reduction system. Our results indicated that E4 and SlMSRA2 proteins might exhibit distinct catalytic mechanisms.

Methionine sulfoxide reductase B (MsrB) of Mycobacterium smegmatis plays a limited role in resisting oxidative stress

Pathogenic mycobacteria including Mycobacterium tuberculosis resists phagocyte generated reactive oxygen intermediates (ROI) and this constitutes an important virulence mechanism. We have previously reported, using Mycobacterium smegmatis as a model to identify the bacterial components that resist intracellular ROI, that an antioxidant methionine sulfoxide reductase A (MsrA) plays a critical role in this process. In this study, we report the role of methionine sulfoxide reductase B (MsrB) in resistance to ROI by constructing a msrB mutant (MSDeltamsrB) and MsrA/B double mutant (MSDeltamsrA/B) strains of M. smegmatis and testing their survival in unactivated and interferon gamma activated mouse macrophages. WhilemsrB mutant exhibited significantly lower intracellular survival than its wild type counterpart, the survival rate seemed to be much higher than msrA mutant (MSDeltamsrA) strain. Further, the msrB mutant showed no sensitivity to oxidants in vitro. The msrA/B double mutant (MSDeltamsrA/B), on the other hand, exhibited a phenotype similar to that of msrA mutant in terms of both intracellular survival and sensitivity to oxidants. We conclude, therefore, that MsrB of M. smegmatis plays only a limited role in resisting intracellular and in vitro ROI.

Methionine sulfoxide reductase A is a stereospecific methionine oxidase

Methionine sulfoxide reductase A (MsrA) catalyzes the reduction of methionine sulfoxide to methionine and is specific for the S epimer of methionine sulfoxide. The enzyme participates in defense against oxidative stresses by reducing methionine sulfoxide residues in proteins back to methionine. Because oxidation of methionine residues is reversible, this covalent modification could also function as a mechanism for cellular regulation, provided there exists a stereospecific methionine oxidase. We show that MsrA itself is a stereospecific methionine oxidase, producing S-methionine sulfoxide as its product. MsrA catalyzes its own autooxidation as well as oxidation of free methionine and methionine residues in peptides and proteins. When functioning as a reductase, MsrA fully reverses the oxidations which it catalyzes.

Redox proteomics of protein-bound methionine oxidation

We here present a new method to measure the degree of protein-bound methionine sulfoxide formation at a proteome-wide scale. In human Jurkat cells that were stressed with hydrogen peroxide, over 2000 oxidation-sensitive methionines in more than 1600 different proteins were mapped and their extent of oxidation was quantified. Meta-analysis of the sequences surrounding the oxidized methionine residues revealed a high preference for neighboring polar residues. Using synthetic methionine sulfoxide containing peptides designed according to the observed sequence preferences in the oxidized Jurkat proteome, we discovered that the substrate specificity of the cellular methionine sulfoxide reductases is a major determinant for the steady-state of methionine oxidation. This was supported by a structural modeling of the MsrA catalytic center. Finally, we applied our method onto a serum proteome from a mouse sepsis model and identified 35 in vivo methionine oxidation events in 27 different proteins.

Structural plasticity of the thioredoxin recognition site of yeast methionine S-sulfoxide reductase Mxr1

The methionine S-sulfoxide reductase MsrA catalyzes the reduction of methionine sulfoxide, a ubiquitous reaction depending on the thioredoxin system. To investigate interactions between MsrA and thioredoxin (Trx), we determined the crystal structures of yeast MsrA/Mxr1 in their reduced, oxidized, and Trx2-complexed forms, at 2.03, 1.90, and 2.70 Å, respectively. Comparative structure analysis revealed significant conformational changes of the three loops, which form a plastic "cushion" to harbor the electron donor Trx2. The flexible C-terminal loop enabled Mxr1 to access the methionine sulfoxide on various protein substrates. Moreover, the plasticity of the Trx binding site on Mxr1 provides structural insights into the recognition of diverse substrates by a universal catalytic motif of Trx.

Functional characterization of methionine sulfoxide reductase A from Trypanosoma spp

Methionine is an amino acid susceptible to being oxidized to methionine sulfoxide (MetSO). The reduction of MetSO to methionine is catalyzed by methionine sulfoxide reductase (MSR), an enzyme present in almost all organisms. In trypanosomatids, the study of antioxidant systems has been mainly focused on the involvement of trypanothione, a specific redox component in these organisms. However, no information is available concerning their mechanisms for repairing oxidized proteins, which would be relevant for the survival of these pathogens in the various stages of their life cycle. We report the molecular cloning of three genes encoding a putative A-type MSR in trypanosomatids. The genes were expressed in Escherichia coli, and the corresponding recombinant proteins were purified and functionally characterized. The enzymes were specific for L-Met(S)SO reduction, using Trypanosoma cruzi tryparedoxin I as the reducing substrate. Each enzyme migrated in electrophoresis with a particular profile reflecting the differences they exhibit in superficial charge. The in vivo presence of the enzymes was evidenced by immunological detection in replicative stages of T. cruzi and Trypanosoma brucei. The results support the occurrence of a metabolic pathway in Trypanosoma spp. involved in the critical function of repairing oxidized macromolecules.

Structural and kinetic analysis of an MsrA-MsrB fusion protein from Streptococcus pneumoniae

Methionine sulphoxide reductases (Msr) catalyse the reduction of oxidized methionine to methionine. These enzymes are divided into two classes, MsrA and MsrB, according to substrate specificity. Although most MsrA and MsrB exist as separate enzymes, in some bacteria these two enzymes are fused to form a single polypeptide (MsrAB). Here, we report the first crystal structure of MsrAB from Streptococcus pneumoniae (SpMsrAB) at 2.4 Å resolution. SpMsrAB consists of an N-terminal MsrA domain, a C-terminal MsrB domain and a linker. The linker is composed of 13 residues and contains one 3₁₀-helix and several hydrogen bonds interacting with both MsrA and MsrB domains. Interestingly, our structure includes the MsrB domain complexed with an SHMAEI hexa-peptide that is the N-terminal region of neighbouring MsrA domain. A kinetic analysis showed that the apparent K_m of SpMsrAB for the R-form-substrate was 20-fold lower than that for the S-form substrate, indicating that the MsrB domain had a much higher affinity for the substrate than the MsrA domain. Our study reveals the first structure of the MsrAB by providing insights into the formation of a disulphide bridge in the MsrB, the structure of the linker region, and the distinct structural nature of active site of each MsrA and MsrB domain.

Mycobacterium tuberculosis expresses methionine sulphoxide reductases A and B that protect from killing by nitrite and hypochlorite

Methionine sulphoxide reductases (Msr) reduce methionine sulphoxide to methionine and protect bacteria against reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI). Many organisms express both MsrA, active against methionine-(S)-sulphoxide, and MsrB, active against methionine-(R)-sulphoxide. Mycobacterium tuberculosis (Mtb) expresses MsrA, which protects DeltamsrA-Escherichia coli from ROI and RNI. However, the function of MsrA in Mtb has not been defined, and it is unknown whether Mtb expresses MsrB. We identified MsrB as the protein encoded by Rv2674 in Mtb and confirmed the distinct stereospecificities of recombinant Mtb MsrA and MsrB. We generated strains of Mtb deficient in MsrA, MsrB or both and complemented the mutants. Lysates of singly deficient strains displayed half as much Msr activity as wild type against N-acetyl methionine sulphoxide. However, in contrast to other bacteria, single mutants were no more vulnerable than wild type to killing by ROI/RNI. Only Mtb lacking both MsrA and MsrB was more readily killed by nitrite or hypochlorite. Thus, MsrA and MsrB contribute to the enzymatic defences of Mtb against ROI and RNI.

Functional analysis of free methionine-R-sulfoxide reductase from Saccharomyces cerevisiae

Methionine sulfoxide reductases (Msrs) are oxidoreductases that catalyze thiol-dependent reduction of oxidized methionines. MsrA and MsrB are the best known Msrs that repair methionine-S-sulfoxide (Met-S-SO) and methionine-R-sulfoxide (Met-R-SO) residues in proteins, respectively. In addition, an Escherichia coli enzyme specific for free Met-R-SO, designated fRMsr, was recently discovered. In this work, we carried out comparative genomic and experimental analyses to examine occurrence, evolution, and function of fRMsr. This protein is present in single copies and two mutually exclusive subtypes in about half of prokaryotes and unicellular eukaryotes but is missing in higher plants and animals. A Saccharomyces cerevisiae fRMsr homolog was found to reduce free Met-R-SO but not free Met-S-SO or dabsyl-Met-R-SO. fRMsr was responsible for growth of yeast cells on Met-R-SO, and the double fRMsr/MsrA mutant could not grow on a mixture of methionine sulfoxides. However, in the presence of methionine, even the triple fRMsr/MsrA/MsrB mutant was viable. In addition, fRMsr deletion strain showed an increased sensitivity to oxidative stress and a decreased life span, whereas overexpression of fRMsr conferred higher resistance to oxidants. Molecular modeling and cysteine residue targeting by thioredoxin pointed to Cys(101) as catalytic and Cys(125) as resolving residues in yeast fRMsr. These residues as well as a third Cys, resolving Cys(91), clustered in the structure, and each was required for the catalytic activity of the enzyme. The data show that fRMsr is the main enzyme responsible for the reduction of free Met-R-SO in S. cerevisiae.

A natural carbohydrate substrate for Mycobacterium tuberculosis methionine sulfoxide reductase A

Enzymatic reduction of the methylsulfinylxylofuranosyl (MSX) groups in lipoarabinomannan provides proof of the absolute configuration of MSX and a possible biochemical mechanism for oxidative protection in Mycobacterium tuberculosis.

Comparison of the thiol-dependent antioxidant systems in the ectomycorrhizal Laccaria bicolor and the saprotrophic Phanerochaete chrysosporium

Sequencing of the Laccaria bicolor and Phanerochaete chrysosporium genomes, together with the availability of many fungal genomes, allow careful comparison to be made of these two basidiomycetes, which possess a different way of life (either symbiotic or saprophytic), with other fungi. Central to the antioxidant systems are superoxide dismutases, catalases and thiol-dependent peroxidases (Tpx). The two reducing systems (thioredoxin (Trx) and glutathione/glutaredoxin (Grx)) are of particular importance against oxidative insults, both for detoxification, through the regeneration of thiol-peroxidases, and for developmental, physiological and signalling processes. Among those thiol-dependent antioxidant systems, special emphasis is given to the redoxin and methionine sulfoxide reductase (Msr) multigenic families. The genes coding for these enzymes were identified in the L. bicolor and P. chrysosporium genomes, were correctly annotated, and the gene content, organization and distribution were compared with other fungi. Expression of the Laccaria genes was also compiled from microarray data. A complete classification, based essentially on gene structure, on phylogenetic and sequence analysis, and on existing experimental data, was proposed. Comparison of the gene content of fungi from all phyla did not show huge differences for multigenic families in the reactive oxygen species (ROS) detoxification network, although some protein subgroups were absent in some fungi.

The methionine sulfoxide reductases: Catalysis and substrate specificities

Oxidation of Met residues in proteins leads to the formation of methionine sulfoxides (MetSO). Methionine sulfoxide reductases (Msr) are ubiquitous enzymes, which catalyze the reduction of the sulfoxide function of the oxidized methionine residues. In vivo, the role of Msrs is described as essential in protecting cells against oxidative damages and to play a role in infection of cells by pathogenic bacteria. There exist two structurally-unrelated classes of Msrs, called MsrA and MsrB, with opposite stereoselectivity towards the S and R isomers of the sulfoxide function, respectively. Both Msrs present a similar three-step catalytic mechanism. The first step, called the reductase step, leads to the formation of a sulfenic acid on the catalytic Cys with the concomitant release of Met. In recent years, significant efforts have been made to characterize structural and molecular factors involved in the catalysis, in particular of the reductase step, and in structural specificities.

A structural analysis of the catalytic mechanism of methionine sulfoxide reductase A from Neisseria meningitidis

The methionine sulfoxide reductases (Msrs) are thioredoxin-dependent oxidoreductases that catalyse the reduction of the sulfoxide function of the oxidized methionine residues. These enzymes have been shown to regulate the life span of a wide range of microbial and animal species and to play the role of physiological virulence determinant of some bacterial pathogens. Two structurally unrelated classes of Msrs exist, MsrA and MsrB, with opposite stereoselectivity towards the R and S isomers of the sulfoxide function, respectively. Both Msrs share a similar three-step chemical mechanism including (1) the formation of a sulfenic acid intermediate on the catalytic Cys with the concomitant release of the product-methionine, (2) the formation of an intramonomeric disulfide bridge between the catalytic and the regenerating Cys and (3) the reduction of the disulfide bridge by thioredoxin or its homologues. In this study, four structures of the MsrA domain of the PilB protein from Neisseria meningitidis, representative of four catalytic intermediates of the MsrA catalytic cycle, were determined by X-ray crystallography: the free reduced form, the Michaelis-like complex, the sulfenic acid intermediate and the disulfide oxidized forms. They reveal a conserved overall structure up to the formation of the sulfenic acid intermediate, while a large conformational switch is observed in the oxidized form. The results are discussed in relation to those proposed from enzymatic, NMR and theoretical chemistry studies. In particular, the substrate specificity and binding, the catalytic scenario of the reductase step and the relevance and role of the large conformational change observed in the oxidized form are discussed.

Methionine sulfoxide reductases and virulence of bacterial pathogens

Oxidation of methionine (Met) residues in proteins by reactive oxygen species and reactive nitrogen intermediates results in altered protein structures, which subsequently affect their functions. Oxidized Met (Met-O) residues are reduced to Met by the methionine sulfoxide reductase (Msr) system, which includes mainly MsrA and MsrB. MsrA and MsrB show no sequence and structural identity with each other but both reduce methionine sulfoxides. MsrA is specific to the reduction of methionine-S-sulfoxide, whereas MsrB is specific to the reduction of methionine-R-sulfoxide. Genes encoding the enzymes MsrA and MsrB exist in most living organisms including bacteria. In recent times, absence of these enzymes has been implicated in the virulence of bacterial pathogens. In particular, pathogens deficient in Msr have been reported to have reduced ability to adhere with eukaryotic cells, to survive inside hosts and to resist in vitro oxidative stress. Bacterial proteins that are susceptible to Met oxidation, in the absence of Msr, have also been identified. This review discusses the current knowledge on the role of Msr in bacterial virulence.

Methionine sulfoxide reductases: selenoprotein forms and roles in antioxidant protein repair in mammals

Msrs (methionine sulfoxide reductases), MsrA and MsrB, are repair enzymes that reduce methionine sulfoxide residues in oxidatively damaged proteins to methionine residues in a stereospecific manner. These enzymes protect cells from oxidative stress and have been implicated in delaying the aging process and progression of neurodegenerative diseases. In recent years, significant efforts have been made to explore the catalytic properties and physiological functions of these enzymes. In the current review, we present recent progress in this area, with the focus on mammalian MsrA and MsrBs including their roles in disease, evolution and function of selenoprotein forms of MsrA and MsrB, and the biochemistry of these enzymes.

Transcriptional characterization of the antioxidant response of Mycobacterium tuberculosis in vivo and during adaptation to hypoxia in vitro

Transcriptional profiling of antioxidant genes of Mycobacterium tuberculosis was performed by real-time RT-PCR during mouse lung infection and during adaptation to gradual oxygen depletion in vitro. M. tuberculosis genes involved in major detoxification pathways of oxidative stress were not up-regulated during chronic mouse lung infection, which is established in response to expression of host adaptive immunity. This result suggests that a major function of bacterial antioxidant enzymes is to protect from oxidants generated during the early, acute phase of infection. In vivo transcription profiles of bacterial antioxidant enzymes differed from those seen under adaptation to low oxygen in vitro, indicating differences between growth arrest in vivo and that induced by hypoxia in vitro.

Characterization of the Amino Acids Involved in Substrate Specificity of Methionine Sulfoxide Reductase A

Methionine sulfoxide reductases (Msrs) are ubiquitous enzymes that catalyze the thioredoxin-dependent reduction of methionine sulfoxide (MetSO) back to methionine. In vivo, Msrs are essential in protecting cells against oxidative damages on proteins and in the virulence of some bacteria. There exists two structurally unrelated classes of Msrs. MsrAs are stereo-specific toward the S epimer on the sulfur of the sulfoxide, whereas MsrBs are specific toward the R isomer. Both classes of Msrs display a similar catalytic mechanism of sulfoxide reduction by thiols via the sulfenic acid chemistry and a better affinity for protein-bound MetSO than for free MetSO. Recently, the role of the amino acids implicated in the catalysis of the reductase step of Neisseria meningitidis MsrA was determined. In the present study, the invariant amino acids potentially involved in substrate binding, i.e. Phe-52, Trp-53, Asp-129, His-186, Tyr-189, and Tyr-197, were substituted. The catalytic parameters under steady-state conditions and of the reductase step of the mutated MsrAs were determined and compared with those of the wild type. Altogether, the results support the presence of at least two binding subsites. The first one, whose contribution is major in the efficiency of the reductase step and in which the epsilon-methyl group of MetSO binds, is the hydrophobic pocket formed by Phe-52 and Trp-53, the position of the indole ring being stabilized by interactions with His-186 and Tyr-189. The second subsite composed of Asp-129 and Tyr-197 contributes to the binding of the main chain of the substrate but to a lesser extent.

Solution Structure and Backbone Dynamics of the Reduced Form and an Oxidized Form of E. coli Methionine Sulfoxide Reductase A (MsrA): Structural Insight of the MsrA Catalytic Cycle

Methionine sulfoxide reductases (Msr) reduce methionine sulfoxide (MetSO)-containing proteins, back to methionine (Met). MsrAs are stereospecific for the S epimer whereas MsrBs reduce the R epimer of MetSO. Although structurally unrelated, the Msrs characterized so far display a similar catalytic mechanism with formation of a sulfenic intermediate on the catalytic cysteine and a concomitant release of Met, followed by formation of at least one intramolecular disulfide bond (between the catalytic and a recycling cysteine), which is then reduced by thioredoxin. In the case of the MsrA from Escherichia coli, two disulfide bonds are formed, i.e. first between the catalytic Cys51 and the recycling Cys198 and then between Cys198 and the second recycling Cys206. Three crystal structures including E. coli and Mycobacterium tuberculosis MsrAs, which, for the latter, possesses only the unique recycling Cys198, have been solved so far. In these structures, the distances between the cysteine residues involved in the catalytic mechanism are too large to allow formation of the intramolecular disulfide bonds. Here structural and dynamical NMR studies of the reduced wild-type and the oxidized (Cys51-Cys198) forms of C86S/C206S MsrA from E. coli have been carried out. The mapping of MetSO substrate-bound C51A MsrA has also been performed. The data support (1) a conformational switch occurring subsequently to sulfenic acid formation and/or Met release that would be a prerequisite to form the Cys51-Cys198 bond and, (2) a high mobility of the C-terminal part of the Cys51-Cys198 oxidized form that would favor formation of the second Cys198-Cys206 disulfide bond.

Functional and Structural Aspects of Poplar Cytosolic and Plastidial Type A Methionine Sulfoxide Reductases

The genome of Populus trichocarpa contains five methionine sulfoxide reductase A genes. Here, both cytosolic (cMsrA) and plastidial (pMsrA) poplar MsrAs were analyzed. The two recombinant enzymes are active in the reduction of methionine sulfoxide with either dithiothreitol or poplar thioredoxin as a reductant. In both enzymes, five cysteines, at positions 46, 81, 100, 196, and 202, are conserved. Biochemical and enzymatic analyses of the cysteine-mutated MsrAs support a catalytic mechanism involving three cysteines at positions 46, 196, and 202. Cys(46) is the catalytic cysteine, and the two C-terminal cysteines, Cys(196) and Cys(202), are implicated in the thioredoxin-dependent recycling mechanism. Inspection of the pMsrA x-ray three-dimensional structure, which has been determined in this study, strongly suggests that contrary to bacterial and Bos taurus MsrAs, which also contain three essential Cys, the last C-terminal Cys(202), but not Cys(196), is the first recycling cysteine that forms a disulfide bond with the catalytic Cys(46). Then Cys(202) forms a disulfide bond with the second recycling cysteine Cys(196) that is preferentially reduced by thioredoxin. In agreement with this assumption, Cys(202) is located closer to Cys(46) compared with Cys(196) and is included in a (202)CYG(204) signature specific for most plant MsrAs. The tyrosine residue corresponds to the one described to be involved in substrate binding in bacterial and B. taurus MsrAs. In these MsrAs, the tyrosine residue belongs to a similar signature as found in plant MsrAs but with the first C-terminal cysteine instead of the last C-terminal cysteine.

Catalytic advantages provided by selenocysteine in methionine-S-sulfoxide reductases

Methionine sulfoxide reductases are key enzymes that repair oxidatively damaged proteins. Two distinct stereospecific enzyme families are responsible for this function: MsrA (methionine-S-sulfoxide reductase) and MsrB (methionine-R-sulfoxide reductase). In the present study, we identified multiple selenoprotein MsrA sequences in organisms from bacteria to animals. We characterized the selenocysteine (Sec)-containing Chlamydomonas MsrA and found that this protein exhibited 10-50-fold higher activity than either its cysteine (Cys) mutant form or the natural mouse Cys-containing MsrA, making this selenoenzyme the most efficient MsrA known. We also generated a selenoprotein form of mouse MsrA and found that the presence of Sec increased the activity of this enzyme when a resolving Cys was mutated in the protein. These data suggest that the presence of Sec improves the reduction of methionine sulfoxide by MsrAs. However, the oxidized selenoprotein could not always be efficiently reduced to regenerate the active enzyme. Overall, this study demonstrates that sporadically evolved Sec-containing forms of methionine sulfoxide reductases reflect catalytic advantages provided by Sec in these and likely other thiol-dependent oxidoreductases.

Characterization of the Amino Acids from Neisseria meningitidis MsrA Involved in the Chemical Catalysis of the Methionine Sulfoxide Reduction Step

Methionine sulfoxide reductases (Msrs) are ubiquitous enzymes that reduce protein-bound methionine sulfoxide back to Met in the presence of thioredoxin. In vivo, the role of the Msrs is described as essential in protecting cells against oxidative damages and as playing a role in infection of cells by pathogenic bacteria. There exist two structurally unrelated classes of Msrs, called MsrA and MsrB, specific for the S and the R epimer of the sulfoxide function of methionine sulfoxide, respectively. Both Msrs present a similar catalytic mechanism, which implies, as a first step, a reductase step that leads to the formation of a sulfenic acid on the catalytic cysteine and a concomitant release of a mole of Met. The reductase step has been previously shown to be efficient and not rate-limiting. In the present study, the amino acids involved in the catalysis of the reductase step of the Neisseria meningitidis MsrA have been characterized. The invariant Glu-94 and to a lesser extent Tyr-82 and Tyr-134 are shown to play a major role in the stabilization of the sulfurane transition state and indirectly in the decrease of the pK(app) of the catalytic Cys-51. A scenario of the reductase step is proposed in which the substrate binds to the active site with its sulfoxide function largely polarized via interactions with Glu-94, Tyr-82, and Tyr-134 and participates via the positive or partially positive charge borne by the sulfur of the sulfoxide in the stabilization of the catalytic Cys.

Insights into the role of the metal binding site in methionine-R-sulfoxide reductases B

Methionine sulfoxide reductases B (MsrBs) catalyze the reduction of methionine-R-sulfoxide via a three-step chemical mechanism including a reductase step, formation of an intradisulfide bond followed by a thioredoxin recycling process. Fifty percent of the MsrBs, including the Escherichia coli enzyme, possess a metal binding site composed of two CXXC motifs of unknown function. It is located on the opposite side of the active site. The overexpressed E. coli MsrB tightly binds one atom of zinc/iron. Substitution of the cysteines of E. coli MsrB results in complete loss of bound metal and reductase activity, and leads to a low-structured conformation of the protein as shown by CD, fluorescence, and DSC experiments. Introduction of the two CXXC motifs in Neisseria meningitidis MsrB domain leads to a MsrB that tightly binds one atom of zinc/iron, shows a strongly increased thermal stability and displays a reductase activity similar to that of the wild-type but lacking thioredoxin recycling activity. These results demonstrate the stabilizing effect of the metal and the existence of a preformed metal binding site in the nonbound metal MsrB. The data also indicate that metal binding to N. meningitidis MsrB induces subtle structural modifications, which prevent formation of a competent binary complex between oxidized MsrB and reduced thioredoxin but not between reduced MsrB and substrate. The fact that the E. coli and the N. meningitidis MsrBs exhibit a similar thermal stability suggests the existence of other structural factors in the nonbound metal MsrBs that compensate the metal bound stabilizing effect.

Different catalytic mechanisms in mammalian selenocysteine- and cysteine-containing methionine-R-sulfoxide reductases

Selenocysteine (Sec) is found in active sites of several oxidoreductases in which this residue is essential for catalytic activity. However, many selenoproteins have fully functional orthologs, wherein cysteine (Cys) occupies the position of Sec. The reason why some enzymes evolve into selenoproteins if the Cys versions may be sufficient is not understood. Among three mammalian methionine-R-sulfoxide reductases (MsrBs), MsrB1 is a Sec-containing protein, whereas MsrB2 and MsrB3 contain Cys in the active site, making these enzymes an excellent system for addressing the question of why Sec is used in biological systems. In this study, we found that residues, which are uniquely conserved in Cys-containing MsrBs and which are critical for enzyme activity in MsrB2 and MsrB3, were not required for MsrB1, but increased the activity of its Cys mutant. Conversely, selenoprotein MsrB1 had a unique resolving Cys reversibly engaged in the selenenylsulfide bond. However, this Cys was not necessary for activities of either MsrB2, MsrB3, or the Cys mutant of MsrB1. We prepared Sec-containing forms of MsrB2 and MsrB3 and found that they were more than 100-fold more active than the natural Cys forms. However, these selenoproteins could not be reduced by the physiological electron donor, thioredoxin. Yet, insertion of the resolving Cys, which was conserved in MsrB1, into the selenoprotein form of MsrB3 restored the thioredoxin-dependent activity of this enzyme. These data revealed differences in catalytic mechanisms between selenoprotein MsrB1 and non-selenoproteins MsrB2 and MsrB3, and identified catalytic advantages and disadvantages of Sec- and Cys-containing proteins. The data also suggested that Sec- and Cys-containing oxidoreductases require distinct sets of active-site features that maximize their catalytic efficiencies and provide strategies for protein design with improved catalytic properties.

Altering cysteine-containing residues in a family of oxidoreductases reveals the role of selenocysteine in influencing the catalytic mechanism.

The Arabidopsis plastidic methionine sulfoxide reductase B proteins. Sequence and activity characteristics, comparison of the expression with plastidic methionine sulfoxide reductase A, and induction by photooxidative stress

Two types of methionine (Met) sulfoxide reductases (Msr) catalyze the reduction of Met sulfoxide (MetSO) back to Met. MsrA, well characterized in plants, exhibits an activity restricted to the Met-S-SO-enantiomer. Recently, a new type of Msr enzyme, called MsrB, has been identified in various organisms and shown to catalytically reduce the R-enantiomer of MetSO. In plants, very little information is available about MsrB and we focused our attention on Arabidopsis (Arabidopsis thaliana) MsrB proteins. Searching Arabidopsis genome databases, we have identified nine open reading frames encoding proteins closely related to MsrB proteins from bacteria and animal cells. We then analyzed the activity and abundance of the two chloroplastic MsrB proteins, MsrB1 and MsrB2. Both enzymes exhibit an absolute R-stereospecificity for MetSO and a higher catalytic efficiency when using protein-bound MetSO as a substrate than when using free MetSO. Interestingly, we observed that MsrB2 is reduced by thioredoxin, whereas MsrB1 is not. This feature of MsrB1 could result from the lack of the catalytical cysteine (Cys) corresponding to Cys-63 in Escherichia coli MsrB that is involved in the regeneration of Cys-117 through the formation of an intramolecular disulfide bridge followed by thioredoxin reduction. We investigated the abundance of plastidial MsrA and B in response to abiotic (water stress, photooxidative treatment) and biotic (rust fungus) stresses and we observed that MsrA and B protein levels increase in response to the photooxidative treatment. The possible role of plastidic MsrB in the tolerance to oxidative damage is discussed.