Synthesis of Chiral Sulfoxides by A Cyclic Oxidation-Reduction Multi-Enzymatic Cascade Biocatalysis

Optically pure sulfoxides are valuable organosulfur compounds extensively employed in medicinal and organic synthesis. In this study, we present a biocatalytic oxidation-reduction cascade system designed for the preparation of enantiopure sulfoxides. The system involves the cooperation of a low-enantioselective chimeric oxidase SMO (styrene monooxygenase) with a high-enantioselective reductase MsrA (methionine sulfoxide reductase A), facilitating "non-selective oxidation and selective reduction" cycles for prochiral sulfide oxidation. The regeneration of requisite cofactors for MsrA and SMO was achieved via a cascade catalysis process involving three auxiliary enzymes, sustained by cost-effective D-glucose. Under the optimal reaction conditions, a series of heteroaryl alkyl, aryl alkyl and dialkyl sulfoxides in R configuration were synthesized through this "one-pot, one step" cascade reaction. The obtained compounds exhibited high yields of >90 % and demonstrated enantiomeric excess (ee) values exceeding 90 %. This study represents an unconventional and efficient biocatalytic way in utilizing the low-enantioselective oxidase for the synthesis of enantiopure sulfoxides.

Methionine biosynthetic genes and methionine sulfoxide reductase A are required for Rhizoctonia solani AG1-IA to cause sheath blight disease in rice

Rhizoctonia solani is a polyphagous necrotrophic fungal pathogen that causes sheath blight disease in rice. It deploys effector molecules as well as carbohydrate-active enzymes and enhances the production of reactive oxygen species for killing host tissues. Understanding R. solani ability to sustain growth under an oxidative-stress-enriched environment is important for developing disease control strategies. Here, we demonstrate that R. solani upregulates methionine biosynthetic genes, including Rs_MET13 during infection in rice, and double-stranded RNA-mediated silencing of these genes impairs the pathogen's ability to cause disease. Exogenous treatment with methionine restores the disease-causing ability of Rs_MET13-silenced R. solani and facilitates its growth on 10 mM H2O2-containing minimal-media. Notably, the Rs_MsrA gene that encodes methionine sulfoxide reductase A, an antioxidant enzyme involved in the repair of oxidative damage of methionine, is upregulated upon H2O2 treatment and also during infection in rice. Rs_MsrA-silenced R. solani is unable to cause disease, suggesting that it is important for the repair of oxidative damage in methionine during host colonization. We propose that spray-induced gene silencing of Rs_MsrA and designing of antagonistic molecules that block MsrA activity can be exploited as a drug target for effective control of sheath blight disease in rice.

Selenium, Selenoproteins, and Heart Failure: Current Knowledge and Future Perspective

PURPOSE OF REVIEW

(Mal-)nutrition of micronutrients, like selenium, has great impact on the human heart and improper micronutrient intake was observed in 30-50% of patients with heart failure. Low selenium levels have been reported in Europe and Asia and thought to be causal for Keshan disease. Selenium is an essential micronutrient that is needed for enzymatic activity of the 25 so-called selenoproteins, which have a broad range of activities. In this review, we aim to summarize the current evidence about selenium in heart failure and to provide insights about the potential mechanisms that can be modulated by selenoproteins.

RECENT FINDINGS

Suboptimal selenium levels (<100 μg/L) are prevalent in more than 70% of patients with heart failure and were associated with lower exercise capacity, lower quality of life, and worse prognosis. Small clinical trials assessing selenium supplementation in patients with HF showed improvement of clinical symptoms (NYHA class), left ventricular ejection fraction, and lipid profile, while governmental interventional programs in endemic areas have significantly decreased the incidence of Keshan disease. In addition, several selenoproteins are found impaired in suboptimal selenium conditions, potentially aggravating underlying mechanisms like oxidative stress, inflammation, and thyroid hormone insufficiency. While the current evidence is not sufficient to advocate selenium supplementation in patients with heart failure, there is a clear need for high level evidence to show whether treatment with selenium has a place in the contemporary treatment of patients with HF to improve meaningful clinical endpoints. Graphical summary summarizing the potential beneficial effects of the various selenoproteins, locally in cardiac tissues and systemically in the rest of the body. In short, several selenoproteins contribute in protecting the integrity of the mitochondria. By doing so, they contribute indirectly to reducing the oxidative stress as well as improving the functionality of immune cells, which are in particular vulnerable to oxidative stress. Several other selenoproteins are directly involved in antioxidative pathways, next to excreting anti-inflammatory effects. Similarly, some selenoproteins are located in the endoplasmic reticulum, playing roles in protein folding. With exception of the protection of the mitochondria and the reduction of oxidative stress, other effects are not yet investigated in cardiac tissues. The systemic effects of selenoproteins might not be limited to these mechanisms, but also may include modulation of endothelial function, protection skeletal muscles, in addition to thyroid metabolism.

ABBREVIATIONS

DIO, iodothyronine deiodinase; GPx, glutathione peroxidase; MsrB2, methionine-R-sulfoxide reductase B2; SELENOK, selenoprotein K; SELENON, selenoprotein N; SELENOP, selenoprotein P; SELENOS, selenoprotein S; SELENOT, selenoprotein T; TXNRD, thioredoxin reductase.

A second human methionine sulfoxide reductase (hMSRB2) reducing methionine-R-sulfoxide displays a tissue expression pattern distinct from hMSRB1

Peptide methionine sulfoxide reductases are important enzymes in the defense against cellular oxidative stress as they reduce methionine sulfoxide, the product of methionine oxidation by physiologically relevant reactive oxygen species. Two distinct enzyme classes, MSRA and MSRB, have evolved for selectively reducing the two epimers, methionine-S-sulfoxide and methionine-R-sulfoxide. A new human MSR enzyme (hMSRB2) specifically reducing methionine-R-sulfoxide, which showed a conversion rate for peptide-bound methionine-S-sulfoxide similar to hMSRB1, was characterized with respect to its tissue expression. As previously found for hMSRB1, expression of hMSRB2 mRNA was weak in brain, but strong in heart and skeletal muscle. In contrast to hMSRB1, its expression was high in smooth muscle-containing organs (digestive system, bladder), lung and aorta, while hMSRB1 displayed a higher expression than hMSRB2 in liver and kidney.

Compartmentalization and regulation of mitochondrial function by methionine sulfoxide reductases in yeast

Elevated levels of reactive oxygen species can damage proteins. Sulfur-containing amino acid residues, cysteine and methionine, are particularly susceptible to such damage. Various enzymes evolved to protect proteins or repair oxidized residues, including methionine sulfoxide reductases MsrA and MsrB, which reduce methionine (S)-sulfoxide (Met-SO) and methionine (R)-sulfoxide (Met-RO) residues, respectively, back to methionine. Here, we show that MsrA and MsrB are involved in the regulation of mitochondrial function. Saccharomyces cerevisiae mutant cells lacking MsrA, MsrB, or both proteins had normal levels of mitochondria but lower levels of cytochrome c and fewer respiration-competent mitochondria. The growth of single MsrA or MsrB mutants on respiratory carbon sources was inhibited, and that of the double mutant was severely compromised, indicating impairment of mitochondrial function. Although MsrA and MsrB are thought to have similar roles in oxidative protein repair each targeting a diastereomer of methionine sulfoxide, their deletion resulted in different phenotypes. GFP fusions of MsrA and MsrB showed different localization patterns and primarily localized to cytoplasm and mitochondria, respectively. This finding agreed with compartment-specific enrichment of MsrA and MsrB activities. These results show that oxidative stress contributes to mitochondrial dysfunction through oxidation of methionine residues in proteins located in different cellular compartments.

Methionine sulfoxide reductase A expression is regulated by the DAF-16/FOXO pathway in Caenorhabditis elegans

The methionine sulfoxide reductase system has been implicated in aging and protection against oxidative stress. This conserved system reverses the oxidation of methionine residues within proteins. We analyzed one of the components of this system, the methionine sulfoxide reductase A gene, in Caenorhabditis elegans. We found that the msra-1 gene is expressed in most tissues, particularly in the intestine and the nervous system. Worms carrying a deletion of the msra-1 gene are more sensitive to oxidative stress, show chemotaxis and locomotory defects, and a 30% decrease in median survival. We established that msra-1 expression decreases during aging and is regulated by the DAF-16/FOXO3a transcription factor. The absence of this enzyme decreases median survival and affects oxidative stress resistance of long lived daf-2 worms. A similar effect of MSRA-1 absence in wild-type and daf-2 (where most antioxidant enzymes are activated) backgrounds, suggests that the lack of this member of the methionine repair system cannot be compensated by the general antioxidant response. Moreover, FOXO3a directly activates the human MsrA promoter in a cell culture system, implying that this could be a conserved mechanism of MsrA regulation. Our results suggest that repair of oxidative damage in proteins influences the rate at which tissues age. This repair mechanism, rather than the general decreased of radical oxygen species levels, could be one of the main determinants of organisms' lifespan.

Selenium and the methionine sulfoxide reductase system

Selenium is a chemical element participating in the synthesis of selenocysteine residues that play a pivotal role in the enzymatic activity efficiency of selenoproteines. The methionine sulfoxide reductase (Msr) system that reduces methionine sulfoxide (MetO) to methionine comprises the selenoprotein MsrB (MsrB1) and the non-selenoprotein MsrA, which reduce the R- and the S- forms of MetO, respectively. The effects of a selenium deficient (SD) diet, which was administrated to wild type (WT) and MsrA knockout mice (MsrA(-)/(-)), on the expression and function of Msr-related proteins are examined and discussed. Additionally, new data about the levels of selenium in brain, liver, and kidneys of WT and MsrA(-)/(-) mice are presented and discussed.

OsMSRA4.1 and OsMSRB1.1, two rice plastidial methionine sulfoxide reductases, are involved in abiotic stress responses

In proteins, methionine residues are especially sensitive to oxidation, leading to the formation of S- and R-methionine sulfoxide diastereoisomers, and these two methionine sulfoxides can be specifically reversed by two types of methionine sulfoxide reductases (MSRs), MSRA and MSRB. Previously, we have identified a gene encoding a putative MSR from NaCl-treated roots of Brazilian upland rice (Oryza sativa L. cv. IAPAR 9) via subtractive suppression hybridization (Wu et al. in Plant Sci 168:847-853, 2005). Blast database analysis indicated that at least four MSRA and three MSRB orthologs exist in rice, and two of them, OsMSRA4.1 and OsMSRB1.1, were selected for further functional analysis. Expression analysis showed that both OsMSRA4.1 and OsMSRB1.1 are constitutively expressed in all organs and can be induced by various stress conditions. Subcellular localization and in vitro activity assay revealed that both OsMSR proteins are targeted to the chloroplast and have MSR activity. Overexpression of either OsMSRA4.1 or OsMSRB1.1 in yeast enhanced cellular resistance to oxidative stress. In addition, OsMSRA4.1-overexpressing transgenic rice plants also showed enhanced viability under salt treatment. Our results provide genetic evidence of the involvement of OsMSRs in the plant stress responses.

OsMSRA4.1 and OsMSRB1.1, two rice plastidial methionine sulfoxide reductases, are involved in abiotic stress responses

In proteins, methionine residues are especially sensitive to oxidation, leading to the formation of S- and R-methionine sulfoxide diastereoisomers, and these two methionine sulfoxides can be specifically reversed by two types of methionine sulfoxide reductases (MSRs), MSRA and MSRB. Previously, we have identified a gene encoding a putative MSR from NaCl-treated roots of Brazilian upland rice (Oryza sativa L. cv. IAPAR 9) via subtractive suppression hybridization (Wu et al. in Plant Sci 168:847-853, 2005). Blast database analysis indicated that at least four MSRA and three MSRB orthologs exist in rice, and two of them, OsMSRA4.1 and OsMSRB1.1, were selected for further functional analysis. Expression analysis showed that both OsMSRA4.1 and OsMSRB1.1 are constitutively expressed in all organs and can be induced by various stress conditions. Subcellular localization and in vitro activity assay revealed that both OsMSR proteins are targeted to the chloroplast and have MSR activity. Overexpression of either OsMSRA4.1 or OsMSRB1.1 in yeast enhanced cellular resistance to oxidative stress. In addition, OsMSRA4.1-overexpressing transgenic rice plants also showed enhanced viability under salt treatment. Our results provide genetic evidence of the involvement of OsMSRs in the plant stress responses.

Genome-wide association scan meta-analysis identifies three Loci influencing adiposity and fat distribution

To identify genetic loci influencing central obesity and fat distribution, we performed a meta-analysis of 16 genome-wide association studies (GWAS, N = 38,580) informative for adult waist circumference (WC) and waist-hip ratio (WHR). We selected 26 SNPs for follow-up, for which the evidence of association with measures of central adiposity (WC and/or WHR) was strong and disproportionate to that for overall adiposity or height. Follow-up studies in a maximum of 70,689 individuals identified two loci strongly associated with measures of central adiposity; these map near TFAP2B (WC, P = 1.9x10(-11)) and MSRA (WC, P = 8.9x10(-9)). A third locus, near LYPLAL1, was associated with WHR in women only (P = 2.6x10(-8)). The variants near TFAP2B appear to influence central adiposity through an effect on overall obesity/fat-mass, whereas LYPLAL1 displays a strong female-only association with fat distribution. By focusing on anthropometric measures of central obesity and fat distribution, we have identified three loci implicated in the regulation of human adiposity.

Methionine sulphoxide reductases protect iron-sulphur clusters from oxidative inactivation in yeast

Methionine residues and iron-sulphur (FeS) clusters are primary targets of reactive oxygen species in the proteins of micro-organisms. Here, we show that methionine redox modifications help to preserve essential FeS cluster activities in yeast. Mutants defective for the highly conserved methionine sulphoxide reductases (MSRs; which re-reduce oxidized methionines) are sensitive to many pro-oxidants, but here exhibited an unexpected copper resistance. This phenotype was mimicked by methionine sulphoxide supplementation. Microarray analyses highlighted several Cu and Fe homeostasis genes that were upregulated in the mxrDelta double mutant, which lacks both of the yeast MSRs. Of the upregulated genes, the Cu-binding Fe transporter Fet3p proved to be required for the Cu-resistance phenotype. FET3 is known to be regulated by the Aft1 transcription factor, which responds to low mitochondrial FeS-cluster status. Here, constitutive Aft1p expression in the wild-type reproduced the Cu-resistance phenotype, and FeS-cluster functions were found to be defective in the mxrDelta mutant. Genetic perturbation of FeS activity also mimicked FET3-dependent Cu resistance. 55Fe-labelling studies showed that FeS clusters are turned over more rapidly in the mxrDelta mutant than the wild-type, consistent with elevated oxidative targeting of the clusters in MSR-deficient cells. The potential underlying molecular mechanisms of this targeting are discussed. Moreover, the results indicate an important new role for cellular MSR enzymes in helping to protect the essential function of FeS clusters in aerobic settings.

1H, 15N and 13C NMR assignments of mouse methionine sulfoxide reductase B2

A recombinant mouse methionine-r-sulfoxide reductase 2 (MsrB2 Delta S) isotopically labeled with (15)N and (15)N/(13)C was generated. We report here the (1)H, (15)N, and (13)C NMR assignments of the reduced form of this protein.

Retinoic acid regulates the human methionine sulfoxide reductase A (MSRA) gene via two distinct promoters

MSRAs (methionine sulfoxide reductases A) are enzymes that reverse the effects of oxidative damage by reducing methionine sulfoxide back to methionine and recovering protein function. In this study we demonstrate that the transcriptional regulation of the human MSRA gene is complex and driven by two distinct promoters. Both promoters demonstrate high expression in human brain and kidney tissues. The upstream (promoter 1) regulates the msrA1 transcript that codes for the mitochondrial form of MSRA and is highly active in a broad range of cell lines. The downstream promoter (promoter 2) regulates the msrA2/3 transcripts that code for the cytosolic/nuclear forms of MSRA and is generally less active. Promoter 2 contains a 65 bp putative enhancer region that is very active in the retinal pigment epithelium-derived D407 cell line. Both promoters are partially regulated by all-trans retinoic acid via RARA and other RARs.

Mammals reduce methionine-S-sulfoxide with MsrA and are unable to reduce methionine-R-sulfoxide, and this function can be restored with a yeast reductase

Methionine is an essential amino acid in mammals at the junction of methylation, protein synthesis, and sulfur pathways. However, this amino acid is highly susceptible to oxidation, resulting in a mixture of methionine-S-sulfoxide and methionine-R-sulfoxide. Whether methionine is quantitatively regenerated from these compounds is unknown. Here we report that SK-Hep1 hepatocytes grew on methionine-S-sulfoxide and consumed this compound by import and methionine-S-sulfoxide reductase (MsrA)-dependent reduction, but methionine-R-sulfoxide reductases were not involved in this process, and methionine-R-sulfoxide could not be used by the cells. However, SK-Hep1 cells expressing a yeast free methionine-R-sulfoxide reductase proliferated in the presence of either sulfoxide, reduced them, and showed increased resistance to oxidative stress. Only methionine-R-sulfoxide was detected in the plasma of wild type mice, but both sulfoxides were in the plasma of MsrA knock-out mice. These results show that mammals can support methionine metabolism by reduction of methionine-S-sulfoxide, that this process is dependent on MsrA, that mammals are inherently deficient in the reduction of methionine-R-sulfoxide, and that expression of yeast free methionine-R-sulfoxide reductase can fully compensate for this deficiency.

Methionine sulfoxide reductase A protects dopaminergic cells from Parkinson's disease-related insults

Parkinson's disease (PD) is a neurologic disorder characterized by dopaminergic cell death in the substantia nigra. PD pathogenesis involves mitochondrial dysfunction, proteasome impairment, and alpha-synuclein aggregation, insults that may be especially toxic to oxidatively stressed cells including dopaminergic neurons. The enzyme methionine sulfoxide reductase A (MsrA) plays a critical role in the antioxidant response by repairing methionine-oxidized proteins and by participating in cycles of methionine oxidation and reduction that have the net effect of consuming reactive oxygen species. Here, we show that MsrA suppresses dopaminergic cell death and protein aggregation induced by the complex I inhibitor rotenone or mutant alpha-synuclein, but not by the proteasome inhibitor MG132. By comparing the effects of MsrA and the small-molecule antioxidants N-acetylcysteine and vitamin E, we provide evidence that MsrA protects against PD-related stresses primarily via methionine sulfoxide repair rather than by scavenging reactive oxygen species. We also demonstrate that MsrA efficiently reduces oxidized methionine residues in recombinant alpha-synuclein. These findings suggest that enhancing MsrA function may be a reasonable therapeutic strategy in PD.

MsrA knockout mouse exhibits abnormal behavior and brain dopamine levels

Oxidative stress can cause methionine oxidation that has been implicated in various proteins malfunctions, if not adequately reduced by the methionine sulfoxide reductase system. Recent evidence has found oxidized methionine residues in neurodegenerative conditions. Previously, we have described elevated levels of brain pathologies and an abnormal walking pattern in the methionine sulfoxide reductase A knockout (MsrA(-/-)) mouse. Here we show that MsrA(-/-) mice have compromised complex task learning capabilities relative to wild-type mice. Likewise, MsrA(-/-) mice exhibit lower locomotor activity and altered gait that exacerbated with age. Furthermore, MsrA(-/-) mice were less responsive to amphetamine treatment. Consequently, brain dopamine levels were determined. Surprisingly, relative to wild-type mice, MsrA(-/-) brains contained significantly higher levels of dopamine up to 12 months of age, while lower levels of dopamine were observed at 16 months of age. Moreover, striatal regions of MsrA(-/-) mice showed an increase of dopamine release parallel to observed dopamine levels. Similarly, the expression pattern of tyrosine hydroxylase activating protein correlated with the age-dependent dopamine levels. Thus, it is suggested that dopamine regulation and signaling pathways are impaired in MsrA(-/-) mice, which may contribute to their abnormal behavior. These observations may be relevant to age-related neurological diseases associated with oxidative stress.

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.

Myeloperoxidase: an inflammatory enzyme for generating dysfunctional high density lipoprotein

PURPOSE OF REVIEW

Evidence indicates that high density lipoprotein (HDL) is cardioprotective and that several mechanisms are involved. One important pathway is a membrane-associated ATP-binding cassette transporter, ABCA1, that clears cholesterol from macrophage foam cells. Anti-inflammatory and antioxidant properties also might contribute to HDL's ability to inhibit atherosclerosis.

RECENT FINDINGS

Myeloperoxidase targets HDL for oxidation, raising the possibility that the enzyme provides a specific mechanism for generating dysfunctional HDL in humans. Myeloperoxidase-dependent oxidation of apolipoprotein A-I, the major protein in HDL, blocks HDL's ability to remove excess cholesterol from cells by the ABCA1 pathway. Analysis of mutated forms of apoA-I and oxidized apoA-I treated with methionine sulfoxide reductase implicate oxidation of specific tyrosine and methionine residues in impairing the ABCA1 transport activity of apoA-I. The crystal structure of lipid-free apoA-I suggests that such oxidative damage might disrupt negatively charged regions on the protein's surface or alter its remodeling, resulting in conformations that fail to interact with ABCA1.

SUMMARY

Oxidation of HDL by myeloperoxidase may represent a specific molecular mechanism for converting the cardioprotective lipoprotein into a dysfunctional form, raising the possibility that the enzyme represents a potential therapeutic target for preventing vascular disease in humans. Moreover, oxidized HDL might prove useful as a blood marker for clinically significant cardiovascular disease in humans.

[Antioxidant gene polymorphism and longevity]

On sample of 1534 ethnic Tatars catalase (CAT, -262C/T), glutathione peroxidase 1 (GPX1, L198P) and methionine sulfoxide reductase A (MSRA, -402C/T) gene polymorphisms in connection with lifespan were studied. On each separate polymorphic marker nonlinear character of alleles and genotypes frequencies changes with age was revealed. Combinations of genotypes CAT*C/*T-GPX1 *L/*L were significant for elderly and senile age.

Overexpression, purification, and preliminary X-ray crystallographic studies of methionine sulfoxide reductase B from Bacillus subtilis

The peptide methionine sulfoxide reductases Msrs) are enzymes that catalyze the reduction of methionine sulfoxide back to methionine. Because of two enantiomers of methionine sulfoxide (S and R forms), this reduction reaction is carried out by two structurally unrelated classes of enzymes, MsrA (E.C. 1.8.4.11) and MsrB (E.C. 1.8.4.12). Whereas MsrA has been well characterized structurally and functionally, little information on MsrB is available. The recombinant MsrB from Bacillus subtilis has been purified and crystallized by the hanging-drop vapor-diffusion method, and the functional and structural features of MsrB have been elucidated. The crystals belong to the trigonal space group P3, with unit-cell parameters a=b=136.096, c=61.918 , and diffracted to 2.5 resolution using a synchrotron-radiation source at Pohang Light Source. The asymmetric unit contains six subunits of MsrB with a crystal volume per protein mass (VM) of 3.37 A3 Da(-1) and a solvent content of 63.5%.

Role of the methionine sulfoxide reductase MsrB3 in cold acclimation in Arabidopsis

Methionine residues of proteins are a major target for oxidation by reactive oxygen species (ROS), which are generated in response to a variety of stress conditions. Methionine sulfoxide (MetO) reductases are present in most organisms and play protective roles in the cellular response to oxidative stress, reducing oxidized MetO back to Met. Previously, an Arabidopsis MetO reductase, MsrB3, was identified as a cold-responsive protein. Here we report that MsrB3 functions in the process of cold acclimation, thus contributing to cold tolerance. In contrast to normal, wild-type plants, msrb3 mutant plants lost the ability to become tolerant to freezing temperatures following cold pre-treatment. Furthermore, when exposed to low temperature, msrb3 plants exhibited a larger increase in MetO and H(2)O(2) content and electrolyte leakage compared with wild-type and MsrB3 transgenic plants. It is also shown that MsrB3 is localized at the endoplasmic reticulum (ER). We propose that MsrB3 plays an important role in cold tolerance by eliminating MetO and ROS that accumulate at the ER during cold acclimation.

Characterization of the Amino Acids from Neisseria meningitidis Methionine Sulfoxide Reductase B Involved in the Chemical Catalysis and Substrate Specificity of the Reductase Step

Methionine sulfoxide reductases (Msrs) are antioxidant repair enzymes that catalyze the thioredoxin-dependent reduction of methionine sulfoxide back to methionine. The Msr family is composed of two structurally unrelated classes of enzymes named MsrA and MsrB, which display opposite stereoselectivities toward the S and R isomers of the sulfoxide function, respectively. Both classes of Msr share a similar three-step chemical mechanism involving first a reductase step that leads to the formation of a sulfenic acid intermediate. In this study, the invariant amino acids of Neisseria meningitidis MsrB involved in the reductase step catalysis and in substrate binding have been characterized by the structure-function relationship approach. Altogether the results show the following: 1) formation of the MsrB-substrate complex leads to an activation of the catalytic Cys-117 characterized by a decreased pKapp of approximately 2.7 pH units; 2) the catalytic active MsrB form is the Cys-117-/His-103+ species with a pKapp of 6.6 and 8.3, respectively; 3) His-103 and to a lesser extent His-100, Asn-119, and Thr-26 (via a water molecule) participate in the stabilization of the polarized form of the sulfoxide function and of the transition state; and 4) Trp-65 is essential for the catalytic efficiency of the reductase step by optimizing the position of the substrate in the active site. A scenario for the reductase step is proposed and discussed in comparison with that of MsrA.

Studies on the reducing systems for plant and animal thioredoxin-independent methionine sulfoxide reductases B

Two distinct stereospecific methionine sulfoxide reductases (Msr), MsrA and MsrB reduce the oxidized methionine (Met), methionine sulfoxide [Met(O)], back to Met. In this report, we examined the reducing systems required for the activities of two chloroplastic MsrB enzymes (NtMsrB1 and NtMsrB2) from tobacco (Nicotiana tabacum). We found that NtMrsB1, but not NtMsrB2, could use dithiothreitol as an efficient hydrogen donor. In contrast Escherichia coli thioredoxin (Trx) could serve as a reducing agent for NtMsrB2, but not for NtMsrB1. Similar to previously reported human Trx-independent hMsrB2 and hMsrB3, NtMsrB1 could also use bovine liver thionein and selenocysteamine as reducing agents. Furthermore, the unique plant Trx-like protein CDSP32 was shown to reduce NtMsrB1, hMsrB2 and hMsrB3. All these tested Trx-independent MsrB enzymes lack an additional cysteine (resolving cysteine) that is capable of forming a disulfide bond on the enzyme during the catalytic reaction. Our results indicate that plant and animal MsrB enzymes lacking a resolving cysteine likely share a similar reaction mechanism.

Identification of MSRA gene on chromosome 8p as a candidate metastasis suppressor for human hepatitis B virus-positive hepatocellular carcinoma

Background

The prognosis of patients with hepatocellular carcinoma (HCC) still remains very dismal, which is mainly due to metastasis. In our previous studies, we found that chromosome 8p deletions might contribute to metastasis of HCC. In this study, we aimed to identify the candidate metastatic suppressor gene on chromosome 8p.

Methods

Oligo-nucleotide microarrays which included 322 genes on human chromosome 8p were constructed to analyze the difference in gene expression profiles between HCC tissues with and without metastasis. The leading differentially expressed genes were identified and selected for further analysis by real-time PCR and Western blotting. Recombinant expression plasmid vectors for each target gene were constructed and transfected into HCC cells and its in vitro effects on proliferation and invasion of HCC cells were also investigated.

Results

Sixteen leading differentially expressed genes were identified from the HCC tissues with metastasis compared with those without metastasis (p < 0.01, q < 16 %). Among of the 10 significantly down-regulated genes in HCC with metastasis, methionine sulfoxide reductase A (MSRA) had the lowest p value and false discovery rate (FDR), and was considered as a potential candidate for metastasis suppressor gene. Real-time PCR and Western blotting confirmed that the mRNA and protein expression levels of MSRA were significantly decreased in HCC with metastasis compared with those without metastasis (p < 0.001), and MSRA mRNA level in HCCLM6 cells (with high metastatic potential) was also much lower than that of other HCC cell lines. Transfection of a recombinant expression plasmid vector and overexpression of MSRA gene could obviously inhibit cell colony formation (4.33 ± 2.92 vs. 9.17 ± 3.38, p = 0.008) and invasion (7.40 ± 1.67 vs. 17.20 ± 2.59, p= 0.0001) of HCCLM6 cell line.

Conclusion

MSRA gene on chromosome 8p might possess metastasis suppressor activity in HCC.

Impairment of methionine sulfoxide reductase during UV irradiation and photoaging

During chronic UV irradiation, which is part of the skin aging process, proteins are damaged by reactive oxygen species resulting in the accumulation of oxidatively modified protein. UV irradiation generates irreversible oxidation of the side chains of certain amino acids resulting in the formation of carbonyl groups on proteins. Nevertheless, certain amino acid oxidation products such as methionine sulfoxide can be reversed back to their reduced form within proteins by specific repair enzymes, the methionine sulfoxide reductases A and B. Using quantitative confocal microscopy, the amount of methionine sulfoxide reductase A was found significantly lower in sun-exposed skin as compared to sun-protected skin. Due to the importance of the methionine sulfoxide reductase system in the maintenance of protein structure and function during aging and conditions of oxidative stress, the fate of this system was investigated after UVA irradiation of human normal keratinocytes. When keratinocytes are exposed to 15 J/cm(2) UVA, methionine sulfoxide reductase activity and content are decreased, indicating that the methionine sulfoxide reductase system is a sensitive target for UV-induced inactivation.

Specificity of thioredoxins and glutaredoxins as electron donors to two distinct classes of Arabidopsis plastidial methionine sulfoxide reductases B

Methionine sulfoxide reductases (MSRs) A and B reduce methionine sulfoxide (MetSO) S- and R-diastereomers, respectively, back to Met using electrons generally supplied by thioredoxin. The physiological reductants for MSRBs remain unknown in plants, which display a remarkable variety of thioredoxins (Trxs) and glutaredoxins (Grxs). Using recombinant proteins, we show that Arabidopsis plastidial MSRB1 and MSRB2, which differ regarding the number of presumed redox-active cysteines, possess specific reductants. Most simple-module Trxs, especially Trx m1 and Trx y2, are preferential and efficient electron donors towards MSRB2, while the double-module CDSP32 Trx and Grxs can reduce only MSRB1. This study identifies novel types of reductants, related to Grxs and peculiar Trxs, for MSRB proteins displaying only one redox-active cysteine.

Methionine sulfoxide reductase from the hyperthermophilic archaeon Thermococcus kodakaraensis, an enzyme designed to function at suboptimal growth temperatures

Methionine sulfoxide reductase (Msr) catalyzes the thioredoxin-dependent reduction and repair of methionine sulfoxide (MetO). Although Msr genes are not present in most hyperthermophile genomes, an Msr homolog encoding an MsrA-MsrB fusion protein (MsrAB(Tk)) was present on the genome of the hyperthermophilic archaeon Thermococcus kodakaraensis. Recombinant proteins corresponding to MsrAB(Tk) and the individual domains (MsrA(Tk) and MsrB(Tk)) were produced, purified, and biochemically examined. MsrA(Tk) and MsrB(Tk) displayed strict substrate selectivity for Met-S-O and Met-R-O, respectively. MsrAB(Tk), and in particular the MsrB domain of this protein, displayed an intriguing behavior for an enzyme from a hyperthermophile. While MsrAB(Tk) was relatively stable at temperatures up to 80 degrees C (with a half-life of approximately 30 min at 80 degrees C), a 75% decrease in activity was observed after 2.5 min at 85 degrees C, the optimal growth temperature of this archaeon. Moreover, maximal levels of MsrB activity of MsrAB(Tk) were observed at the strikingly low temperature of 30 degrees C, which also was observed for MsrB(Tk). Consistent with the low-temperature-specific biochemical properties of MsrAB(Tk), the presence of the protein was greater in T. kodakaraensis cells grown at suboptimal temperatures (60 to 70 degrees C) and could not be detected at 80 to 90 degrees C. We found that the amount of intracellular MsrAB(Tk) protein increased with exposure to higher dissolved oxygen levels, but only at suboptimal growth temperatures. While measuring background rates of the Msr enzyme reactions, we observed significant levels of MetO reduction at high temperatures without enzyme. The occurrence of nonenzymatic MetO reduction at high temperatures may explain the specific absence of Msr homologs in most hyperthermophiles. Together with the fact that the presence of Msr in T. kodakaraensis is exceptional among the hyperthermophiles, the enzyme may represent a novel strategy for this organism to deal with low-temperature environments in which the dissolved oxygen concentrations increase.

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.

NMR assignments of 1H, 13C and 15N spectra of methionine sulfoxide reductase B1 from Mus musculus

Isotopically labeled, 15N and 15N/13C forms of recombinant methionine-r-sulfoxide reductase 1 (MsrB1, SelR) from Mus musculus were produced, in which catalytic selenocysteine was replaced with cysteine. We report here the 1H, 13C and 15N NMR assignment of the reduced form of this mammalian protein.

Prolonged selenium-deficient diet in MsrA knockout mice causes enhanced oxidative modification to proteins and affects the levels of antioxidant enzymes in a tissue-specific manner

The methionine sulfoxide reductase (Msr) system (comprised of MsrA and MsrB) is responsible for reducing methionine sulfoxide (MetO) to methionine. One major form of MsrB is a selenoprotein. Following prolonged selenium deficient diet (SD), through F2 generation, the MsrA -/- mice exhibited higher protein-MetO and carbonyl levels relative to their wild-type (WT) control in most organs. More specifically, the SD diet caused alteration in the expression and/or activities of certain antioxidants as follows: lowering the specific activity of MsrB in the MsrA -/- cerebellum in comparison to WT mice; lowering the activities of glutathione peroxidase (Gpx) and thioredoxin reductase (Trr) especially in brains of MsrA -/- mice; elevation of the cellular levels of selenoprotein P (SelP) in most tissues of the MsrA -/- relative to WT. Unexpectedly, the expression and activity of glucose-6-phosphate dehydrogenase (G6PD) were mainly elevated in lungs and hearts of MsrA -/- mice. Moreover, the body weight of the MsrA -/- mice lagged behind the WT mice body weight up to 120 days of the SD diet. In summary, it is suggested that the lack of the MsrA gene in conjunction with prolonged SD diet causes decreased antioxidant capability and enhanced protein oxidation.

Methionine sulfoxide reduction and the aging process

Aging has been described for multicellular and asymmetrically dividing organisms, but the mechanisms are poorly understood. Oxidation of proteins is considered to be one of the major factors that leads to aging. Oxidative damage to proteins results in the oxidation of certain amino acid residues, among which oxidation of sulfur-containing amino acids, methionine and cysteine, is notable because of the susceptibility of these residues to damage, and occurrence of repair mechanisms. Methionine sulfoxide reductases, MsrA and MsrB, are thioredoxin-dependent oxidoreductases that reduce oxidized forms of methionine, methionine sulfoxides, in a stereospecific manner. These enzymes are present in all cell types and have shown to be regulating life spans in mammals, insects, and yeast. Here, their roles in modulating yeast life span are discussed.

Free methionine-(R)-sulfoxide reductase from Escherichia coli reveals a new GAF domain function

The reduction of methionine sulfoxide (MetO) is mediated by methionine sulfoxide reductases (Msr). The MsrA and MsrB families can reduce free MetO and MetO within a peptide or protein context. This process is stereospecific with the S- and R-forms of MetO repaired by MsrA and MsrB, respectively. Cell extracts from an MsrA(-)B(-) knockout of Escherichia coli have several remaining Msr activities. This study has identified an enzyme specific for the free form of Met-(R)-O, fRMsr, through proteomic analysis. The recombinant enzyme exhibits the same substrate specificity and is as active as MsrA family members. E. coli fRMsr is, however, 100- to 1,000-fold more active than non-selenocysteine-containing MsrB enzymes for free Met-(R)-O. The crystal structure of E. coli fRMsr was previously determined, but no known function was assigned. Thus, the function of this protein has now been determined. The structural similarity of the E. coli and yeast proteins suggests that most fRMsrs use three cysteine residues for catalysis and the formation of a disulfide bond to enclose a small active site cavity. This latter feature is most likely a key determinant of substrate specificity. Moreover, E. coli fRMsr is the first GAF domain family member to show enzymatic activity. Other GAF domain proteins substitute the Cys residues and others to specifically bind cyclic nucleotides, chromophores, and many other ligands for signal potentiation. Therefore, Met-(R)-O may represent a signaling molecule in response to oxidative stress and nutrients via the TOR pathway in some organisms.

Identification of proteins undergoing expression level modifications in WI-38 SV40 fibroblasts overexpressing methionine sulfoxide reductase A

Methionine sulfoxide reductase A overexpressing WI-38 SV40 human fibroblasts have been previously shown to exhibit higher resistance to oxidative stress by decreasing intracellular reactive oxygen species content and oxidative damage to proteins [C.R. Picot, I. Petropoulos, M. Perichon, M. Moreau, C. Nizard, B. Friguet, Overexpression of MsrA protects WI-38 SV40 human fibroblasts against H(2)O(2)-mediated oxidative stress, Free Radic Biol Med 39 (2005) 1332-1341]. In order to get further insight into the molecular mechanisms underlying this resistance to oxidative stress, proteins that are differentially expressed in methionine sulfoxide reductase A overexpressing cells were identified by 2D gel and Western blot quantitative analyses. Five proteins were shown to be differentially expressed and were identified by mass spectrometry, some of them were related to either cellular protection against oxidative stress, apoptosis or premature ageing.

Ecdysone induction of MsrA protects against oxidative stress in Drosophila

The methionine sulfoxide reductases MsrA and MsrB reduce Met(O) to Met in epimer-specific fashion. In Drosophila, the major ecdysone induced protein is MsrA, which is regulated by the EcR-USP complex. We tested Kc cells for induction of MsrA, MsrB, EcR, and CAT by ecdysone and found that MsrA and the EcR were induced by ecdysone, but MsrB and CAT were not. When we tested for resistance to 20mM H2O2 toxicity, viability of Kc cells was reduced 3-fold. Pretreatment with 0.2 microM ecdysone for 48 h prior to exposure to H2O2, increased viability to 77% of controls. The EcR-deficient L57-3-11 knockout line was not responsive to ecdysone, and H2O2 resistance of both control and ecdysone-treated L57-3-11 cells was similar to that of the ecdysone-untreated Kc cells. These results show that hormonal regulation of MsrA is implicated in conferring protection against oxidative stress in the Drosophila model.

Specific activity of methionine sulfoxide reductase in CD-1 mice is significantly affected by dietary selenium but not zinc

Reactive oxygen species-mediated oxidation of methionine residues in protein results in a racemic mixture of R and S forms of methionine sulfoxide (MetO). MetO is reduced back to methionine by the methionine sulfoxide reductases MsrA and MsrB. MsrA is specific toward the S form and MsrB is specific toward the R form of MetO. MsrB is a selenoprotein reported to contain zinc (Zn). To determine the effects of dietary selenium (Se) and Zn on Msr activity, CD-1 mice (N=16/group) were fed, in a 2 x 2 design, diets containing 0 or 0.2 microg Se/g and 3 or 15 microg Zn/g. As an oxidative stress, half of the mice received L-buthionine sulfoximine (BSO; ip; 2 mmol/kg, three times per week for the last 3 wk); the others received saline. After 9.5 wk, Msr (the combined specific activities of MsrA and MsrB) was measured in the brain, kidney, and liver. Se deficiency decreased (p<0.0001) Msr in all three tissues, but Zn had no direct effect. BSO treatment was expected to result in increased Msr activity; this was not seen. Additionally, we found that the ratio of MetO to methionine in liver protein was increased (indicative of oxidative damage) by Se deficiency. The results show that Se deficiency increases oxidation of methionyl residues in protein, that Se status affects Msr (most likely through effects on the selenoprotein MsrB), and that marginal Zn deficiency has little effect on Msr in liver and kidney. Finally, the results show that the oxidative effects of limited BSO treatment did not upregulate Msr activity.

Multivariate analysis of elements in Chinese brake fern as determined using neutron activation analysis

Pytoremediaton of arsenic (As) contamination using Chinese brake fern (Pteris vittata L.), an As hyperaccumulator has proven potential because of its cost-effectiveness and environmental harmonies. Aiming to investigate the elemental correlation in Chinese brake fern, 20 elements (As, Br, Ca, Ce, Co, Cr, Eu, Fe, Hf, La, Na, Nd, K, Rb, Se, Sm, Sr, Th, Yb and Zn) were measured in the fronds and roots of the fern by neutron activation analysis. The ferns were sampled from two sites with high geogenic As levels: Zimudang (ZMD) and Lanmuchang (LMC) in Guizhou Province, China. Multivariate statistic analysis was performed to explore the interrelationship between these elements, especially between As and other elements. As was found to be positively related to K, Na, La, and Sm in both the roots and the fronds, suggesting that these four elements might operate as synergies to As during uptake and transportation processes. Se was positively related to most of the other cations measured, except in the fronds of the fern at ZMD, where Br replaced Se as positively related to the other cations. The difference of As and Se in correlation with other cationic elements suggested that the two anionic elements play different roles in elemental uptake processes. Our findings of elemental correlation highlight the importance of the anion- cation balance in Chinese brake fern.

Elevated levels of brain-pathologies associated with neurodegenerative diseases in the methionine sulfoxide reductase A knockout mouse

One of the posttranslational modifications to proteins is methionine oxidation, which is readily reversible by the methionine sulfoxide reductase (Msr) system. Thus, accumulation of faulty proteins due to a compromised Msr system may lead to the development of aging-associated diseases like neurodegenerative diseases. In particular, it was interesting to monitor the consequential effects of methionine oxidation in relation to markers that are associated with Alzheimer's disease as methionine oxidation was implied to play a role in beta-amyloid toxicity. In this study, a knockout mouse strain of the methionine sulfoxide reductase A gene (MsrA ( -/- )) caused an enhanced neurodegeneration in brain hippocampus relative to its wild-type control mouse brain. Additionally, a loss of astrocytes integrity, elevated levels of beta-amyloid deposition, and tau phosphorylation were dominant in various regions of the MsrA ( -/- ) hippocampus but not in the wild-type. Also, a comparison between cultured brain slices of the hippocampal region of both mouse strains showed more sensitivity of the MsrA ( -/- ) cultured cells to H(2)O(2) treatment. It is suggested that a deficiency in MsrA activity fosters oxidative-stress that is manifested by the accumulation of faulty proteins (via methionine oxidation), deposition of aggregated proteins, and premature brain cell death.

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.

High-affinity and cooperative binding of oxidized calmodulin by methionine sulfoxide reductase

Methionines can play an important role in modulating protein-protein interactions associated with intracellular signaling, and their reversible oxidation to form methionine sulfoxides [Met(O)] in calmodulin (CaM) and other signaling proteins has been suggested to couple cellular redox changes to protein functional changes through the action of methionine sulfoxide reductases (Msr). Prior measurements indicate the full recovery of target protein activation upon the stereospecific reduction of oxidized CaM by MsrA, where the formation of the S-stereoisomer of Met(O) selectively inhibits the CaM-dependent activation of the Ca-ATPase. However, the physiological substrates of MsrA remain unclear, as neither the binding specificities nor affinities of protein targets have been measured. To assess the specificity of binding and its possible importance in the maintenance of CaM function, we have measured the kinetics of repair and the binding affinity between oxidized CaM and MsrA. Reduction of Met(O) in fully oxidized CaM by MsrA is sensitive to the protein fold, as repair of the intact protein is incomplete, with >6 Met(O) remaining in each CaM following MsrA reduction. In contrast, following proteolytic digestion, MsrA is able to fully reduce one-half of the oxidized methionines, indicating that surface-accessible Met(O) within folded proteins need not be substrates for MsrA repair. Mutation of the active site (i.e., C72S) in MsrA permitted equilibrium-binding measurements using both ensemble and single-molecule fluorescence correlation spectroscopy measurements. We observe cooperative binding of two MsrA to each CaMox with an apparent affinity (K = 70 +/- 10 nM) that is 3 orders of magnitude greater than the Michaelis constant (KM = 68 +/- 4 microM). The high-affinity and cooperative interaction between MsrA and CaMox suggests an important regulatory role of MsrA in the binding and reduction of Met(O) in functionally sensitive proteins, such that multiple MsrA proteins are recruited to simultaneously bind and reduce Met(O) in highly oxidized proteins. Given the suggested role of Met(O) in modulating reversible binding interactions between proteins associated with cellular signaling, these results indicate an ability of MsrA to selectively reduce Met(O) within highly surface-accessible sequences to maintain cellular function as part of an adaptive response to oxidative stress.

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.

Maintenance of proteins and aging: the role of oxidized protein repair

According to the free radical theory of aging proposed by Denham Harman (Journal of Gerontology 1956, 11, pp. 298-300), the continuous oxidative damage to cellular components over an organism's life span is a causal factor of the aging process. The age-related build-up of oxidized protein is therefore resulting from increased protein oxidative damage and/or decreased elimination of oxidized proteins. In this mini-review, we will address the fate, during aging, of the protein maintenance systems that are involved in the degradation of irreversibly oxidized proteins and in the repair of reversible protein oxidative damage with a special focus on the methionine sulfoxide reductases system. Since these protein degradation and repair systems have been found to be impaired with age, it is proposed that not only failure of redox homeostasis but, as importantly, failure of protein maintenance are critical factors in the aging process.

Molecular evolution of peptide methionine sulfoxide reductases (MsrA and MsrB): on the early development of a mechanism that protects against oxidative damage

Methionine sulfoxide reductases, enzymes that reverse the oxidation of methionine residues, have been described in a wide range of species. The reduction of the diastereoisomers of oxidized methionine is catalyzed by two different monomeric methionine sulfoxide reductases (MsrA and MsrB) and is best understood as an evolutionary response to high levels of oxygen either in the Earth's atmosphere or possibly in more localized environments. Phylogenetic analyses of these proteins suggest that their distribution is the outcome of a complex history including many paralogy and lateral gene transfer events.

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.

Ablation of the mammalian methionine sulfoxide reductase A affects the expression level of cysteine deoxygenase

Methionine sulfoxide reductases (Msrs) are able to reduce methionine sulfoxide to methionine both in proteins and free amino acids. By their action it is possible to regulate the function of specific proteins and the cellular antioxidant defense against oxidative damage. Similarly, cysteine deoxygenase (CDO) may be involved in the regulation of protein function and antioxidant defense mechanisms by its ability to oxidized cysteine residues. The two enzymes' involvement in sulfur amino-acids metabolism seems to be connected. Lack of methionine sulfoxide reductase A (MsrA) in liver of MsrA-/- led to a significant drop in the cellular level of thiol groups and lowered the CDO level of expression. Moreover, following selenium deficient diet (applied to decrease the expression levels of selenoproteins like MsrB), the latter effect was maintained while the basal levels of thiol decreased in both mouse strains. We suggest that both enzymes are working in coordination to balance cellular antioxidant defense.

Protein-carbonyl accumulation in the non-replicative senescence of the methionine sulfoxide reductase A (msrA) knockout yeast strain

The major enzyme of the methionine sulfoxide reductase (Msr) system is MsrA. Senescing msrA knockout mother yeast cells accumulated significant amounts of protein-carbonyl both at 5 generation-old (young) and 21 generation-old (old) cultures, while the control mother cells showed significant levels of protein-carbonyl mainly in the old culture. The Msr activities of both yeast strains declined with age and exposure of cells to H(2)O(2) caused an accumulation of protein-carbonyl especially in the msrA knockout strain. It is suggested that a compromised MsrA activity may serve as a marker for non-replicative aging.

Genes of the antioxidant system of the honey bee: annotation and phylogeny

Antioxidant enzymes perform a variety of vital functions including the reduction of life-shortening oxidative damage. We used the honey bee genome sequence to identify the major components of the honey bee antioxidant system. A comparative analysis of honey bee with Drosophila melanogaster and Anopheles gambiae shows that although the basic components of the antioxidant system are conserved, there are important species differences in the number of paralogs. These include the duplication of thioredoxin reductase and the expansion of the thioredoxin family in fly; lack of expansion of the Theta, Delta and Omega GST classes in bee and no expansion of the Sigma class in dipteran species. The differential expansion of antioxidant gene families among honey bees and dipteran species might reflect the marked differences in life history and ecological niches between social and solitary species.

Methionine sulfoxide reductase in Helicobacter pylori: interaction with methionine-rich proteins and stress-induced expression

The reductive repair of oxidized methionine residues performed by methionine sulfoxide reductase is important for the gastric pathogen Helicobacter pylori to maintain persistent stomach colonization. Methionine-containing proteins that are targeted for repair by Msr were identified from whole-cell extracts (after cells were exposed to O(2) stress) by using a coimmunoprecipitation approach. Proteins identified as Msr-interacting included catalase, GroEL, thioredoxin-1 (Trx1), and site-specific recombinase; with one exception (Trx1, the reductant for Msr) all these proteins have approximately twofold higher methionine (Met) content than other proteins. These Met-rich proteins were purified and were shown to individually form a cross-linked adduct with Msr. Catalase-specific activity in an msr strain was one-half that of the parent strain; this difference was only observed under oxidative stress conditions, and the activity was restored to nearly wild-type levels by adding Msr plus dithiothreitol to msr strain extracts. In agreement with the cross-linking study, pure Msr used Trx1 but not Trx2 as a reductant. Comparative structure modeling classified the H. pylori Msr in class II within the MsrB family, like the Neisseria enzymes. Pure H. pylori enzyme reduced only the R isomer of methyl p-tolyl-sulfoxide with an apparent K(m) of 4.1 mM for the substrate. Stress conditions (peroxide, peroxynitrite, and iron starvation) all caused approximately 3- to 3.5-fold transcriptional up-regulation of msr. Neither the O(2) level during growth nor the use of background regulatory mutants had a significant effect on msr transcription. Late log and stationary phase cultures had the highest Msr protein levels and specific activity.

Glutathione peroxidase 3 of Saccharomyces cerevisiae regulates the activity of methionine sulfoxide reductase in a redox state-dependent way

Glutathione peroxidase (Gpx) is one of the most important anti-oxidant enzymes in yeast. Gpx3 is a ubiquitously expressed isoform that modulates the activities of redox-sensitive thiol proteins, particularly those involved in signal transduction pathways and protein translocation. In order to search for the interaction partners of Gpx3, we carried out immunoprecipitation/2-dimensional gel electrophoresis (IP-2DE), MALDI-TOF mass spectrometry, and a pull down assay. We found that Mxr1, a peptide methionine sulfoxide reductase, interacts with Gpx3. By reducing methionine sulfoxide to methionine, Mxr1 reverses the inactivation of proteins caused by the oxidation of critical methionine residues. Gpx3 can interact with Mxr1 through the formation of an intermolecular disulfide bond. When oxidative stress is induced by H(2)O(2), this interaction is compromised and the free Mxr1 then repairs the oxidized proteins. Our findings imply that this interaction links redox sensing machinery of Gpx3 to protein repair activity of Mxr1. Based on these results, we propose that Gpx3 functions as a redox-dependent exquisite regulator of the protein repair activity of Mxr1.

Plant methionine sulfoxide reductase A and B multigenic families

Methionine oxidation to methionine sulfoxide (MetSo), which results in modification of activity and conformation for many proteins, is reversed by an enzyme present in most organisms and termed as methionine sulfoxide reductase (MSR). On the basis of substrate stereospecificity, two types of MSR, A and B, that do not share any sequence similarity, have been identified. In the present review, we first compare the multigenic MSR families in the three plant species for which the genome is fully sequenced: Arabidopsis thaliana, Oryza sativa, and Populus trichocarpa. The MSR gene content is larger in A. thaliana (five MSRAs and nine MSRBs) compared to P. trichocarpa (five MSRAs and four MSRBs) and O. sativa (four MSRAs and three MSRBs). A complete classification based on gene structure, sequence identity, position of conserved reactive cysteines and predicted subcellular localization is proposed. On the basis of in silico and experimental data originating mainly from Arabidopsis, we report that some MSR genes display organ-specific expression patterns and that those encoding plastidic MSRs are highly expressed in photosynthetic organs. We also show that the expression of numerous MSR genes is enhanced by environmental conditions known to generate oxidative stress. Thioredoxins (TRXs) constitute very likely physiological electron donors to plant MSR proteins for the catalysis of MetSO reduction, but the specificity between the numerous TRXs and methionine sulfoxide reductases (MSRs) present in plants remains to be investigated. The essential role of plant MSRs in protection against oxidative damage has been recently demonstrated on transgenic Arabidopsis plants modified in the content of cytosolic or plastidic MSRA.