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

The human selenoproteome: recent insights into functions and regulation

Selenium (Se) is a nutritional trace mineral essential for various aspects of human health that exerts its effects mainly through its incorporation into selenoproteins as the amino acid, selenocysteine. Twenty-five selenoprotein genes have been identified in humans and several selenoproteins are broadly classified as antioxidant enzymes. As progress is made on characterizing the individual members of this protein family, however, it is becoming clear that their properties and functions are quite diverse. This review summarizes recent insights into properties of individual selenoproteins such as tissue distribution, subcellular localization, and regulation of expression. Also discussed are potential roles the different selenoproteins play in human health and disease.

Overexpression of methionine-R-sulfoxide reductases has no influence on fruit fly aging

Methionine sulfoxide reductases (Msrs) are enzymes that repair oxidized methionine residues in proteins. This function implicated Msrs in antioxidant defense and the regulation of aging. There are two known Msr types in animals: MsrA specific for the reduction of methionine-S-sulfoxide, and MsrB that catalyzes the reduction of methionine-R-sulfoxide. In a previous study, overexpression of MsrA in the nervous system of Drosophila was found to extend lifespan by 70%. Overexpression of MsrA in yeast also extended lifespan, whereas MsrB overexpression did so only under calorie restriction conditions. The effect of MsrB overexpression on lifespan has not yet been characterized in animal model systems. Here, the GAL4-UAS binary system was used to drive overexpression of cytosolic Drosophila MsrB and mitochondrial mouse MsrB2 in whole body, fatbody, and the nervous system of flies. In contrast to MsrA, MsrB overexpression had no consistent effect on the lifespan of fruit flies on either corn meal or sugar yeast diets. Physical activity, fecundity, and stress resistance were also similar in MsrB-overexpressing and control flies. Thus, MsrA and MsrB, the two proteins with similar function in antioxidant protein repair, have different effects on aging in fruit flies.

Low exchangeability of selenocysteine, the 21st amino acid, in vertebrate proteins

Selenocysteine (Sec), the 21st amino acid, is incorporated into proteins through the recoding of a termination codon, an inefficient translational process mediated by a complex molecular machinery. Sec is a rare amino acid in extant proteins, chemically similar to cysteine (Cys), found in homologous position to Cys of nonselenoprotein families. Selenoproteins account for the dependence of vertebrates on environmental selenium (Se) and have an important role in several Se-deficiency diseases. Selenoproteins are poorly characterized enzymes and reports on the functional exchangeability of Sec with Cys are limited and controversial. Whether the unique role of Sec in some selenoenzymes illustrates the broader contribution of Se to protein function is unknown (Gromer S, Johansson L, Bauer H, Arscott LD, Rauch S, Ballou DP, Williams CH Jr, Schirmer RH, Arnér ES. 2003. Active sites of thioredoxin reductases: why selenoproteins? Proc Natl Acad Sci USA. 100:12618-12623). Here, we address this question from an evolutionary perspective by the simultaneous identification of the patterns of divergence in almost half a billion years of vertebrate evolution and diversity within the human lineage for the full complement of enzymatic Sec residues in these proteomes. We complete this analysis with data for the homologous Cys residues in the same genomes. Our results indicate concerted purifying selection across Sec and Cys sites in all selenoproteomes, consistent with a unique role of Sec in protein function, low exchangeability, and an unknown degree of functional divergence with Cys homologs. The distinct biochemical properties of Sec, rather than the geographical distribution of Se, global O(2) levels or Sec metabolic cost, appear to play a major role in driving adaptive changes in vertebrate selenoproteomes. A better understanding of the selenoproteomes and neutral evolutionary patterns in other taxa will be necessary to fully assess the generality of this conclusion.

Regeneration mechanisms of Arabidopsis thaliana methionine sulfoxide reductases B by glutaredoxins and thioredoxins

Methionine oxidation leads to the formation of S- and R-diastereomers of methionine sulfoxide (MetSO), which are reduced back to methionine by methionine sulfoxide reductases (MSRs) A and B, respectively. MSRBs are classified in two groups depending on the conservation of one or two redox-active Cys; 2-Cys MSRBs possess a catalytic Cys-reducing MetSO and a resolving Cys, allowing regeneration by thioredoxins. The second type, 1-Cys MSRBs, possess only the catalytic Cys. The biochemical mechanisms involved in activity regeneration of 1-Cys MSRBs remain largely elusive. In the present work we used recombinant plastidial Arabidopsis thaliana MSRB1 and MSRB2 as models for 1-Cys and 2-Cys MSRBs, respectively, to delineate the Trx- and glutaredoxin-dependent reduction mechanisms. Activity assays carried out using a series of cysteine mutants and various reductants combined with measurements of free thiols under distinct oxidation conditions and mass spectrometry experiments show that the 2-Cys MSRB2 is reduced by Trx through a dithiol-disulfide exchange involving both redox-active Cys of the two partners. Regarding 1-Cys MSRB1, oxidation of the enzyme after substrate reduction leads to the formation of a stable sulfenic acid on the catalytic Cys, which is subsequently glutathionylated. The deglutathionylation of MSRB1 is achieved by both mono- and dithiol glutaredoxins and involves only their N-terminal conserved catalytic Cys. This study proposes a detailed mechanism of the regeneration of 1-Cys MSRB activity by glutaredoxins, which likely constitute physiological reductants for this type of MSR.

How an obscure archaeal gene inspired the discovery of selenocysteine biosynthesis in humans

Selenocysteine (Sec) is the 21st genetically encoded amino acid found in organisms from all three domains of life. Sec biosynthesis is unique in that it always proceeds from an aminoacyl-tRNA precursor. Even though Sec biosynthesis in bacteria was established almost two decades ago, only recently the pathway was elucidated in archaea and eukaryotes. While other aspects of Sec biology have been reviewed previously (Allmang and Krol, Biochimie 2006;88:1561-1571, Hatfield et al., Prog Nucleic Acid Res Mol Biol 2006;81:97-142, Squires and Berry, IUBMB Life 2008;60:232-235), here we review the biochemistry and evolution of Sec biosynthesis and coding and show how the knowledge of an archaeal cysteine biosynthesis pathway helped to uncover the route to Sec formation in archaea and eukaryotes.

Protein-repairing methionine sulfoxide reductases in photosynthetic organisms: gene organization, reduction mechanisms, and physiological roles

Methionine oxidation to methionine sulfoxide (MetSO) is reversed by two types of methionine sulfoxide reductases (MSRs), A and B, specific to the S- and R-diastereomers of MetSO, respectively. MSR genes are found in most organisms from bacteria to human. In the current review, we first compare the organization of the MSR gene families in photosynthetic organisms from cyanobacteria to higher plants. The analysis reveals that MSRs constitute complex families in higher plants, bryophytes, and algae compared to cyanobacteria and all non-photosynthetic organisms. We also perform a classification, based on gene number and structure, position of redox-active cysteines and predicted sub-cellular localization. The various catalytic mechanisms and potential physiological electron donors involved in the regeneration of MSR activity are then described. Data available from higher plants reveal that MSRs fulfill an essential physiological function during environmental constraints through a role in protein repair and in protection against oxidative damage. Taking into consideration the expression patterns of MSR genes in plants and the known roles of these genes in non-photosynthetic cells, other functions of MSRs are discussed during specific developmental stages and ageing in photosynthetic organisms.

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.

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.

Origin and evolution of the protein-repairing enzymes methionine sulphoxide reductases

The majority of extant life forms thrive in an O2-rich environment, which unavoidably induces the production of reactive oxygen species (ROS) during cellular activities. ROS readily oxidize methionine (Met) residues in proteins/peptides to form methionine sulphoxide [Met(O)] that can lead to impaired protein function. Two methionine sulphoxide reductases, MsrA and MsrB, catalyse the reduction of the S and R epimers, respectively, of Met(O) in proteins to Met. The Msr system has two known functions in protecting cells against oxidative damage. The first is to repair proteins that have lost activity due to Met oxidation and the second is to function as part of a scavenger system to remove ROS through the reversible oxidation/reduction of Met residues in proteins. Bacterial, plant and animal cells lacking MsrA are known to be more sensitive to oxidative stress. The Msr system is considered an important cellular defence mechanism to protect against oxidative stress and may be involved in ageing/senescence. MsrA is present in all known eukaryotes and eubacteria and a majority of archaea, reflecting its essential role in cellular life. MsrB is found in all eukaryotes and the majority of eubacteria and archaea but is absent in some eubacteria and archaea, which may imply a less important role of MsrB compared to MsrA. MsrA and MsrB share no sequence or structure homology, and therefore probably emerged as a result of independent evolutionary events. The fact that some archaea lack msr genes raises the question of how these archaea cope with oxidative damage to proteins and consequently of the significance of msr evolution in oxic eukaryotes dealing with oxidative stress. Our best hypothesis is that the presence of ROS-destroying enzymes such as peroxiredoxins and a lower dissolved O2 concentration in those msr-lacking organisms grown at high temperatures might account for the successful survival of these organisms under oxidative stress.

Functional diversity of cysteine residues in proteins and unique features of catalytic redox-active cysteines in thiol oxidoreductases

Thiol-dependent redox systems are involved in regulation of diverse biological processes, such as response to stress, signal transduction, and protein folding. The thiol-based redox control is provided by mechanistically similar, but structurally distinct families of enzymes known as thiol oxidoreductases. Many such enzymes have been characterized, but identities and functions of the entire sets of thiol oxidoreductases in organisms are not known. Extreme sequence and structural divergence makes identification of these proteins difficult. Thiol oxidoreductases contain a redox-active cysteine residue, or its functional analog selenocysteine, in their active sites. Here, we describe computational methods for in silico prediction of thiol oxidoreductases in nucleotide and protein sequence databases and identification of their redox-active cysteines. We discuss different functional categories of cysteine residues, describe methods for discrimination between catalytic and noncatalytic and between redox and non-redox cysteine residues and highlight unique properties of the redox-active cysteines based on evolutionary conservation, secondary and three-dimensional structures, and sporadic replacement of cysteines with catalytically superior selenocysteine residues.

An anaerobic bacterial MsrB model reveals catalytic mechanisms, advantages, and disadvantages provided by selenocysteine and cysteine in reduction of methionine-R-sulfoxide

We verified and generalized the catalytic features that selenocysteine (Sec) and cysteine (Cys) contribute to the reduction of methionine-R-sulfoxide using an anaerobic bacterial MsrB from Clostridium sp. OhILA as a model protein. The Sec-containing Clostridium MsrB form exhibited 100-fold higher activity than its Cys-containing form, revealing that Sec provided the catalytic advantage of higher activity. However, a resolving Cys was required for the thioredoxin (Trx)-dependent recycling process of the Sec-containing form. Thus, Trx could reduce the selenenylsulfide bond, but its Trx-dependent recycling process was much less efficient compared to that for the disulfide bond in the Cys-containing form, demonstrating an obvious catalytic disadvantage. These data agreed well with our previous data on mammalian MsrBs, and therefore suggested that the catalytic mechanisms, as well as the catalytic advantages and disadvantages provided by the Sec and Cys residues, are most likely conserved from anaerobic bacteria to mammals. Taken together, we propose that the use of Sec in MsrB may depend on a balance between the catalytic advantage of higher activity and the disadvantage of a less efficient regeneration process provided by this residue.

Biochemistry of selenium-derivatized naturally occurring and unnatural nucleic acids

Selenium (Se) can provide unique biochemical and biological functions, and properties to macromolecules, including protein and RNA. Although Se has not yet been found in DNA, identification of the presence of Se in natural tRNAs has led to discovery of the naturally occurring 2-selenouridine and 5-[(methylamino)methyl]-2-selenouridine (mnm(5)se(2)U). The Se-atoms at C(2) of the modified uridines are introduced by 2-selenouridine synthase via displacement of the S-atoms in the corresponding 2-thiouridine nucleotides of the tRNAs, and selenophosphate is used as the Se donor. The research indicated that mnm(5)se(2)U is located at the first or wobble position of the anticodons in several bacterial tRNAs, including tRNA(Lys), tRNA(Glu), and tRNA(Gln). The 2-seleno functionality on this modified nucleotide probably improves the translation accuracy and/or efficiency. These observations in vivo suggest that the presence of Se can provide natural RNAs with useful properties to better function and survival. To further investigate the biochemical and structural properties of Se-derivatized nucleic acids (SeNA), we have pioneered chemical and enzymatic synthesis of Se-derivatized nucleic acids, and introduced Se into both RNA and DNA at a variety of positions by atom-specific replacement of oxygen. This review outlines the recent advancements in chemical and biochemical syntheses, and studies of SeNAs, and their potential applications in structural and functional investigation of nucleic acids and their protein complexes.

Evolutionary dynamics of eukaryotic selenoproteomes: large selenoproteomes may associate with aquatic life and small with terrestrial life

In silico and metabolic labeling studies of the selenoproteomes of several eukaryotes revealed distinct selenoprotein patterns as well as an ancient origin of selenoproteins and massive, independent losses in land plants, fungi, nematodes, insects and some protists, suggesting that the environment plays an important role in selenoproteome evolution.

Background

Selenocysteine (Sec) is a selenium-containing amino acid that is co-translationally inserted into nascent polypeptides by recoding UGA codons. Selenoproteins occur in both eukaryotes and prokaryotes, but the selenoprotein content of organisms (selenoproteome) is highly variable and some organisms do not utilize Sec at all.

Results

We analyzed the selenoproteomes of several model eukaryotes and detected 26 and 29 selenoprotein genes in the green algae Ostreococcus tauri and Ostreococcus lucimarinus, respectively, five in the social amoebae Dictyostelium discoideum, three in the fly Drosophila pseudoobscura, and 16 in the diatom Thalassiosira pseudonana, including several new selenoproteins. Distinct selenoprotein patterns were verified by metabolic labeling of O. tauri and D. discoideum with ⁷⁵Se. More than half of the selenoprotein families were shared by unicellular eukaryotes and mammals, consistent with their ancient origin. Further analyses identified massive, independent selenoprotein losses in land plants, fungi, nematodes, insects and some protists. Comparative analyses of selenoprotein-rich and -deficient organisms revealed that aquatic organisms generally have large selenoproteomes, whereas several groups of terrestrial organisms reduced their selenoproteomes through loss of selenoprotein genes and replacement of Sec with cysteine.

Conclusion

Our data suggest many selenoproteins originated at the base of the eukaryotic domain and show that the environment plays an important role in selenoproteome evolution. In particular, aquatic organisms apparently retained and sometimes expanded their selenoproteomes, whereas the selenoproteomes of some terrestrial organisms were reduced or completely lost. These findings suggest a hypothesis that, with the exception of vertebrates, aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element due to an unknown environmental factor.

Selenoproteinless animals: selenophosphate synthetase SPS1 functions in a pathway unrelated to selenocysteine biosynthesis

Proteins containing the 21st amino acid, selenocysteine (Sec), have been described in all three domains of life, but the composition of selenoproteomes in organisms varies significantly. Here, we report that aquatic arthropods possess many selenoproteins also detected in other animals and unicellular eukaryotes, and that most of these proteins were either lost or replaced with cysteine-containing homologs in insects. As a result of this selective selenoproteome reduction, fruit flies and mosquitoes have three known selenoproteins, and the honeybee, Apis mellifera, a single detected candidate selenoprotein. Moreover, we identified the red flour beetle, Tribolium castaneum, and the silkworm, Bombyx mori, as the first animals that lack any Sec-containing proteins. These insects also lost the Sec biosynthesis and insertion machinery, but selenophosphate synthetase 1 (SPS1), an enzyme previously implicated in Sec biosynthesis, is present in all insects, including T. castaneum and B. mori. These data indicate that SPS1 functions in a pathway unrelated to selenoprotein synthesis. Since SPS1 evolved from a protein that utilizes selenium for Sec biosynthesis, an attractive possibility is that SPS1 may define a new pathway of selenium utilization in animals.

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.

SelenoDB 1.0 : a database of selenoprotein genes, proteins and SECIS elements

Selenoproteins are a diverse group of proteins usually misidentified and misannotated in sequence databases. The presence of an in-frame UGA (stop) codon in the coding sequence of selenoprotein genes precludes their identification and correct annotation. The in-frame UGA codons are recoded to cotranslationally incorporate selenocysteine, a rare selenium-containing amino acid. The development of ad hoc experimental and, more recently, computational approaches have allowed the efficient identification and characterization of the selenoproteomes of a growing number of species. Today, dozens of selenoprotein families have been described and more are being discovered in recently sequenced species, but the correct genomic annotation is not available for the majority of these genes. SelenoDB is a long-term project that aims to provide, through the collaborative effort of experimental and computational researchers, automatic and manually curated annotations of selenoprotein genes, proteins and SECIS elements. Version 1.0 of the database includes an initial set of eukaryotic genomic annotations, with special emphasis on the human selenoproteome, for immediate inspection by selenium researchers or incorporation into more general databases. SelenoDB is freely available at http://www.selenodb.org.

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.

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.

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.

Important roles of multiple Sp1 binding sites and epigenetic modifications in the regulation of the methionine sulfoxide reductase B1 (MsrB1) promoter

Background

Methionine sulfoxide reductases (Msrs) are enzymes that catalyze the reduction of oxidized methionine residues. Most organisms that were genetically modified to lack the MsrA gene have shown shortening of their life span. Methionine sulfoxide reductases B (MsrB) proteins codified by three separate genes, named MsrB1, MsrB2, and MsrB3, are included in the Msrs system. To date, the mechanisms responsible for the transcriptional regulation of MsrB genes have not been reported. The aim of this study was to investigate the regulation of MsrB1 selenoprotein levels through transcriptional regulation of the MsrB1 gene in MDA-MB231 and MCF-7 breast carcinoma cell lines.

Results

A MsrB1 gene promoter is located 169 base pairs upstream from the transcription start site. It contains three Sp1 binding sites which are sufficient for maximal promoter activity in transient transfection experiments.

High levels of MsrB1 transcript, protein and promoter activity were detected in low metastatic MCF7 human breast cancer cells. On the contrary, very low levels of both MsrB1 transcript and promoter activity were detected in the highly metastatic counterpart MDA-MB231 cells.

A pivotal role for Sp1 in the constitutive expression of the MsrB1 gene was demonstrated through transient expression of mutant MsrB1 promoter-reporter gene constructs and chromatin immunoprecipitation experiments.

Since Sp1 is ubiquitously expressed, these sites, while necessary, are not sufficient to explain the patterns of gene expression of MsrB1 in various human breast cancer cells. MDA-MB231 cells can be induced to express MsrB1 by treatment with 5-Aza-2'-deoxycytidine, a demethylating agent. Therefore, the MsrB1 promoter is controlled by epigenetic modifications.

Conclusion

The results of this study provide the first insights into the transcriptional regulation of the human MsrB1 gene, including the discovery that the Sp1 transcription factor may play a central role in its expression. We also demonstrated that the MsrB1 promoter activity appears to be controlled by epigenetic modifications such as methylation.

High-throughput identification of catalytic redox-active cysteine residues

Cysteine (Cys) residues often play critical roles in proteins; however, identification of their specific functions has been limited to case-by-case experimental approaches. We developed a procedure for high-throughput identification of catalytic redox-active Cys in proteins by searching for sporadic selenocysteine-Cys pairs in sequence databases. This method is independent of protein family, structure, and taxon. We used it to selectively detect the majority of known proteins with redox-active Cys and to make additional predictions, one of which was verified. Rapid accumulation of sequence information from genomic and metagenomic projects should allow detection of many additional oxidoreductase families as well as identification of redox-active Cys in these proteins.

Catalases are NAD(P)H-dependent tellurite reductases

Reactive oxygen species damage intracellular targets and are implicated in cancer, genetic disease, mutagenesis, and aging. Catalases are among the key enzymatic defenses against one of the most physiologically abundant reactive oxygen species, hydrogen peroxide. The well-studied, heme-dependent catalases accelerate the rate of the dismutation of peroxide to molecular oxygen and water with near kinetic perfection. Many catalases also bind the cofactors NADPH and NADH tenaciously, but, surprisingly, NAD(P)H is not required for their dismutase activity. Although NAD(P)H protects bovine catalase against oxidative damage by its peroxide substrate, the catalytic role of the nicotinamide cofactor in the function of this enzyme has remained a biochemical mystery to date. Anions formed by heavy metal oxides are among the most highly reactive, natural oxidizing agents. Here, we show that a natural isolate of Staphylococcus epidermidis resistant to tellurite detoxifies this anion thanks to a novel activity of its catalase, and that a subset of both bacterial and mammalian catalases carry out the NAD(P)H-dependent reduction of soluble tellurite ion (TeO₃²⁻) to the less toxic, insoluble metal, tellurium (Te°), in vitro. An Escherichia coli mutant defective in the KatG catalase/peroxidase is sensitive to tellurite, and expression of the S. epidermidis catalase gene in a heterologous E. coli host confers increased resistance to tellurite as well as to hydrogen peroxide in vivo, arguing that S. epidermidis catalase provides a physiological line of defense against both of these strong oxidizing agents. Kinetic studies reveal that bovine catalase reduces tellurite with a low Michaelis-Menten constant, a result suggesting that tellurite is among the natural substrates of this enzyme. The reduction of tellurite by bovine catalase occurs at the expense of producing the highly reactive superoxide radical.

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.

The thioredoxin system in cancer

Thioredoxin (Trx), NADPH and thioredoxin reductase (TrxR) comprise a thioredoxin system which exists in nearly all living cells. It functions in thiol-dependent thiol-disulfide exchange reactions crucial to control of the reduced intracellular redox environment, cellular growth, defense against oxidative stress or control of apoptosis and has multi-facetted roles in mammalian cells including implications in cancer. Eg reduced Trx activates DNA binding of transcription factors and is involved in antioxidant defense through repair of oxidatively damaged proteins or as an electron donor to peroxiredoxins. The Trx system functions in synthesis of deoxyribonucleotides for DNA synthesis, both replication and repair, by ribonucleotide reductase. Trx and truncated Trx (Trx80) act in modulation of immune cell function. TrxR isoforms in the cytosol and the mitochondria are essential selenoenzymes with a selenocysteine in the active site. These enzymes display a remarkably broad substrate specificity but are also targets for existing chemotherapeutic drugs. Mammalian TrxR enzymes are linked to selenium metabolism as a result of being selenoproteins, but can also directly reduce low molecular selenium compounds like selenite and have been implicated in the chemoprevention effects of selenium against cancer. Numerous scientific reports describe higher expression of Trx and TrxR in some, but not all tumors. Some data suggest that high Trx could be linked to resistance to chemotherapies while others suggest that high Trx and TrxR may induce apoptosis and reduce the mitotic index of certain tumors linked to the p53 dependent cell death. Recent data suggest that TrxR is essential for the carcinogenic process and invasive phenotype of cancer. Both Trx and TrxR have been regarded as interesting targets for chemotherapy.

Dynamic evolution of selenocysteine utilization in bacteria: a balance between selenoprotein loss and evolution of selenocysteine from redox active cysteine residues

Comparative genomics and evolutionary analyses to examine the dynamics of selenocysteine utilization in bacteria reveal a dynamic balance between selenoprotein origin and loss.

Background

Selenocysteine (Sec) is co-translationally inserted into protein in response to UGA codons. It occurs in oxidoreductase active sites and often is catalytically superior to cysteine (Cys). However, Sec is used very selectively in proteins and organisms. The wide distribution of Sec and its restricted use have not been explained.

Results

We conducted comparative genomics and phylogenetic analyses to examine dynamics of Sec decoding in bacteria at both selenium utilization trait and selenoproteome levels. These searches revealed that 21.5% of sequenced bacteria utilize Sec, their selenoproteomes have 1 to 31 selenoproteins, and selenoprotein-rich organisms are mostly Deltaproteobacteria or Firmicutes/Clostridia. Evolutionary histories of selenoproteins suggest that Cys-to-Sec replacement is a general trend for most selenoproteins. In contrast, only a small number of Sec-to-Cys replacements were detected, and these were mostly restricted to formate dehydrogenase and selenophosphate synthetase families. In addition, specific selenoprotein gene losses were observed in many sister genomes. Thus, the Sec/Cys replacements were mostly unidirectional, and increased utilization of Sec by existing protein families was counterbalanced by loss of selenoprotein genes or entire selenoproteomes. Lateral transfers of the Sec trait were an additional factor, and we describe the first example of selenoprotein gene transfer between archaea and bacteria. Finally, oxygen requirement and optimal growth temperature were identified as environmental factors that correlate with changes in Sec utilization.

Conclusion

Our data reveal a dynamic balance between selenoprotein origin and loss, and may account for the discrepancy between catalytic advantages provided by Sec and the observed low number of selenoprotein families and Sec-utilizing organisms.

Thionein can serve as a reducing agent for the methionine sulfoxide reductases

It has been generally accepted, primarily from studies on methionine sulfoxide reductase (Msr) A, that the biological reducing agent for the members of the Msr family is reduced thioredoxin (Trx), although high levels of DTT can be used as the reductant in vitro. Preliminary experiments using both human recombinant MsrB2 (hMsrB2) and MsrB3 (hMsrB3) showed that although DTT can function in vitro as the reducing agent, Trx works very poorly, prompting a more careful comparison of the ability of DTT and Trx to function as reducing agents with the various members of the Msr family. Escherichia coli MsrA and MsrB and bovine MsrA efficiently use either Trx or DTT as reducing agents. In contrast, hMsrB2 and hMsrB3 show <10% of the activity with Trx as compared with DTT, raising the possibility that, in animal cells, Trx may not be the direct hydrogen donor or that there may be a Trx-independent reducing system required for MsrB2 and MsrB3 activity. A heat-stable protein has been detected in bovine liver that, in the presence of EDTA, can support the Msr reaction in the absence of either Trx or DTT. This protein has been identified as a zinc-containing metallothionein (Zn-MT). The results indicate that thionein (T), which is formed when the zinc is removed from Zn-MT, can function as a reducing system for the Msr proteins because of its high content of cysteine residues and that Trx can reduce oxidized T.

Selenoproteins--tracing the role of a trace element in protein function

Selenium is an important component of several enzymes, replacing sulfur in cysteine residues. Its discovery and significance are described in this primer.