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

Optogenetics with Atomic Precision─A Comprehensive Review of Optical Control of Protein Function through Genetic Code Expansion

Conditional control of protein activity is important in order to elucidate the particular functions and interactions of proteins, their regulators, and their substrates, as well as their impact on the behavior of a cell or organism. Optical control provides a perhaps optimal means of introducing spatiotemporal control over protein function as it allows for tunable, rapid, and noninvasive activation of protein activity in its native environment. One method of introducing optical control over protein activity is through the introduction of photocaged and photoswitchable noncanonical amino acids (ncAAs) through genetic code expansion in cells and animals. Genetic incorporation of photoactive ncAAs at key residues in a protein provides a tool for optical activation, or sometimes deactivation, of protein activity. Importantly, the incorporation site can typically be rationally selected based on structural, mechanistic, or computational information. In this review, we comprehensively summarize the applications of photocaged lysine, tyrosine, cysteine, serine, histidine, glutamate, and aspartate derivatives, as well as photoswitchable phenylalanine analogues. The extensive and diverse list of proteins that have been placed under optical control demonstrates the broad applicability of this methodology.

50 Years of Organoselenium Chemistry, Biochemistry and Reactivity: Mechanistic Understanding, Successful and Controversial Stories

In 1973, two major discoveries changed the face of selenium chemistry: the identification of the first mammal selenoenzyme, glutathione peroxidase 1, and the discovery of the synthetic utility of the so-called selenoxide elimination. While the chemical mechanism behind the catalytic activity of glutathione peroxidases appears to be mostly unveiled, little is known about the mechanisms of other selenoproteins and, for some of them, even the function lies in the dark. In chemistry, the capacity of organoselenides of catalyzing hydrogen peroxide activation for the practical manipulation of organic functional groups has been largely explored, and some mechanistic details have been clearly elucidated. As a paradox, despite the long-standing experience in the field, the nature of the active oxidant in various reactions still remains matter of debate. While many successes characterize these fields, the pharmacological use of organoselenides still lacks any true application, and while some organoselenides were found to be non-toxic and safe to use, to date no therapeutically approved use was granted. In this review, some fundamental and chronologically aligned topics spanning organoselenium biochemistry, chemistry and pharmacology are discussed, focusing on the current mechanistic picture describing their activity as either bioactive compounds or catalysts.

Development and Optimization of a Redox Enzyme-Based Fluorescence Biosensor for the Identification of MsrB1 Inhibitors

MsrB1 is a thiol-dependent enzyme that reduces protein methionine-R-sulfoxide and regulates inflammatory response in macrophages. Therefore, MsrB1 could be a promising therapeutic target for the control of inflammation. To identify MsrB1 inhibitors, we construct a redox protein-based fluorescence biosensor composed of MsrB1, a circularly permutated fluorescent protein, and the thioredoxin1 in a single polypeptide chain. This protein-based biosensor, named RIYsense, efficiently measures protein methionine sulfoxide reduction by ratiometric fluorescence increase. We used it for high-throughput screening of potential MsrB1 inhibitors among 6868 compounds. A total of 192 compounds were selected based on their ability to reduce relative fluorescence intensity by more than 50% compared to the control. Then, we used molecular docking simulations of the compound on MsrB1, affinity assays, and MsrB1 activity measurement to identify compounds with reliable and strong inhibitory effects. Two compounds were selected as MsrB1 inhibitors: 4-[5-(4-ethylphenyl)-3-(4-hydroxyphenyl)-3,4-dihydropyrazol-2-yl]benzenesulfonamide and 6-chloro-10-(4-ethylphenyl)pyrimido[4,5-b]quinoline-2,4-dione. They are heterocyclic, polyaromatic compounds with a substituted phenyl moiety interacting with the MsrB1 active site, as revealed by docking simulation. These compounds were found to decrease the expression of anti-inflammatory cytokines such as IL-10 and IL-1rn, leading to auricular skin swelling and increased thickness in an ear edema model, effectively mimicking the effects observed in MsrB1 knockout mice. In summary, using a novel redox protein-based fluorescence biosensor, we identified potential MsrB1 inhibitors that can regulate the inflammatory response, particularly by influencing the expression of anti-inflammatory cytokines. These compounds are promising tools for understanding MsrB1's role during inflammation and eventually controlling inflammation in therapeutic approaches.

Redox signaling and skeletal muscle adaptation during aerobic exercise

Redox regulation is a fundamental physiological phenomenon related to oxygen-dependent metabolism, and skeletal muscle is mainly regarded as a primary site for oxidative phosphorylation. Several studies have revealed the importance of reactive oxygen and nitrogen species (RONS) in the signaling process relating to muscle adaptation during exercise. To date, improving knowledge of redox signaling in modulating exercise adaptation has been the subject of comprehensive work and scientific inquiry. The primary aim of this review is to elucidate the molecular and biochemical pathways aligned to RONS as activators of skeletal muscle adaptation and to further identify the interconnecting mechanisms controlling redox balance. We also discuss the RONS-mediated pathways during the muscle adaptive process, including mitochondrial biogenesis, muscle remodeling, vascular angiogenesis, neuron regeneration, and the role of exogenous antioxidants.

Biological and Catalytic Properties of Selenoproteins

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium-carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions.

The Key Role of Chalcogenurane Intermediates in the Reduction Mechanism of Sulfoxides and Selenoxides by Thiols Explored In Silico

Sulfoxides and selenoxides oxidize thiols to disulfides while being reduced back to sulfides and selenides. While the reduction mechanism of sulfoxides to sulfides has been thoroughly explored experimentally as well as computationally, less attention has been devoted to the heavier selenoxides. In this work, we explore the reductive mechanism of dimethyl selenoxide, as an archetypal selenoxide and, for the sake of comparison, the reductive mechanism of dimethyl sulfoxide to gain insight into the role of the chalcogen on the reaction substrate. Particular attention is devoted to the key role of sulfurane and selenurane intermediates. Moreover, the capacity of these system to oxidize selenols rather than thiols, leading to the formation of selenyl sulfide bridges, is explored in silico. Notably, this analysis provides molecular insight into the role of selenocysteine in methionine sulfoxide reductase selenoenzyme. The activation strain model of chemical reactivity is employed in the studied reactions as an intuitive tool to bridge the computationally predicted effect of the chalcogen on the chalcogenoxide as well as on the chalcogenol.

Novel Lennard-Jones Parameters for Cysteine and Selenocysteine in the AMBER Force Field

Cysteine is a common amino acid with a thiol group that plays a pivotal role in a variety of scenarios in redox biochemistry. In contrast, selenocysteine, the 21st amino acid, is only present in 25 human proteins. Classical force-field parameters for cysteine and selenocysteine are still scarce. In this context, we present a methodology to obtain Lennard-Jones parameters for cysteine and selenocysteine in different physiologically relevant oxidation and protonation states. The new force field parameters obtained in this work are available at https://github.com/MALBECC/AMBER-parameters-database. The parameters were adjusted to reproduce water radial distribution functions obtained by density functional theory ab initio molecular dynamics. We validated the results by evaluating the impact of the choice of parameters on the structure and dynamics in classical molecular dynamics simulations of representative proteins containing catalytic cysteine/selenocysteine residues. There are significant changes in protein structure and dynamics depending on the parameters choice, specifically affecting the residues close to the catalytic sites.

Challenges and opportunities in bringing nonbiological atoms to life with synthetic metabolism

The relatively narrow spectrum of chemical elements within the microbial 'biochemical palate' limits the reach of biotechnology, because several added-value compounds can only be produced with traditional organic chemistry. Synthetic biology offers enabling tools to tackle this issue by facilitating 'biologization' of non-canonical chemical atoms. The interplay between xenobiology and synthetic metabolism multiplies routes for incorporating nonbiological atoms into engineered microbes. In this review, we survey natural assimilation routes for elements beyond the essential biology atoms [i.e., carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S)], discussing how these mechanisms could be repurposed for biotechnology. Furthermore, we propose a computational framework to identify chemical elements amenable to biologization, ranking reactions suitable to build synthetic metabolism. When combined and deployed in robust microbial hosts, these approaches will offer sustainable alternatives for smart chemical production.

Modulating antioxidant systems as a therapeutic approach to retinal degeneration

The human retina is facing a big challenge of reactive oxygen species (ROS) from endogenous and exogenous sources. Excessive ROS can cause damage to DNA, lipids, and proteins, triggering abnormal redox signaling, and ultimately lead to cell death. Thus, oxidative stress has been observed in inherited retinal diseases as a common hallmark. To counteract the detrimental effect of ROS, cells are equipped with various antioxidant defenses. In this review, we will focus on the antioxidant systems in the retina and how they can protect retina from oxidative stress. Both small antioxidants and antioxidant enzymes play a role in ROS removal. Particularly, the thioredoxin and glutaredoxin systems, as the major antioxidant systems in mammalian cells, exert functions in redox signaling regulation via modifying cysteines in proteins. In addition, the thioredoxin-like rod-derived cone viability factor (RdCVFL) and thioredoxin interacting protein (TXNIP) can modulate metabolism in photoreceptors and promote their survival. In conclusion, elevating the antioxidant capacity in retina is a promising therapy to curb the progress of inherited retinal degeneration.

The selenoprotein methionine sulfoxide reductase B1 (MSRB1)

Methionine (Met) can be oxidized to methionine sulfoxide (MetO), which exist as R- and S-diastereomers. Present in all three domains of life, methionine sulfoxide reductases (MSR) are the enzymes that reduce MetO back to Met. Most characterized among them are MSRA and MSRB, which are strictly stereospecific for the S- and R-diastereomers of MetO, respectively. While the majority of MSRs use a catalytic Cys to reduce their substrates, some employ selenocysteine. This is the case of mammalian MSRB1, which was initially discovered as selenoprotein SELR or SELX and later was found to exhibit an MSRB activity. Genomic analyses demonstrated its occurrence in most animal lineages, and biochemical and structural analyses uncovered its catalytic mechanism. The use of transgenic mice and mammalian cell culture revealed its physiological importance in the protection against oxidative stress, maintenance of neuronal cells, cognition, cancer cell proliferation, and the immune response. Coincident with the discovery of Met oxidizing MICAL enzymes, recent findings of MSRB1 regulating the innate immunity response through reversible stereospecific Met-R-oxidation of cytoskeletal actin opened up new avenues for biological importance of MSRB1 and its role in disease. In this review, we discuss the current state of research on MSRB1, compare it with other animal Msrs, and offer a perspective on further understanding of biological functions of this selenoprotein.

Oxidation and reduction of actin: Origin, impact in vitro and functional consequences in vivo

Actin is among the most abundant proteins in eukaryotic cells and assembles into dynamic filamentous networks regulated by many actin binding proteins. The actin cytoskeleton must be finely tuned, both in space and time, to fulfill key cellular functions such as cell division, cell shape changes, phagocytosis and cell migration. While actin oxidation by reactive oxygen species (ROS) at non-physiological levels are known for long to impact on actin polymerization and on the cellular actin cytoskeleton, growing evidence shows that direct and reversible oxidation/reduction of specific actin amino acids plays an important and physiological role in regulating the actin cytoskeleton. In this review, we describe which actin amino acid residues can be selectively oxidized and reduced in many different ways (e.g. disulfide bond formation, glutathionylation, carbonylation, nitration, nitrosylation and other oxidations), the cellular enzymes at the origin of these post-translational modifications, and the impact of actin redox modifications both in vitro and in vivo. We show that the regulated balance of oxidation and reduction of key actin amino acid residues contributes to the control of actin filament polymerization and disassembly at the subcellular scale and highlight how improper redox modifications of actin can lead to pathological conditions.

A critical review on sulfur reduction of aqueous selenite: Mechanisms and applications

Selenite, which is extremely toxic at high concentrations, can easily be enriched in natural aquatic environments due to human activities, which causes great harm to ecosystems. Sulfur reduction can effectively reduce soluble selenite in large quantities to nontoxic solid elemental selenium, which plays a significant role in controlling the toxicity and cycle of selenium. In view of the bright prospects of the sulfur reduction reaction of selenite, this review comprehensively summarizes the continuous development in the sulfidation of selenite. First, the geochemical characteristics of aqueous selenium in different sulfur systems involving species distribution and various phase types at Eh-pH conditions were summarized. Second, sulfur reductions of selenite with chemical sulfide in natural water environments, sulfur reductase and extracellular polymer substances containing thiol groups in sulfate-reducing bacteria have been reviewed to further understand the corresponding mechanisms, rates and influencing factors. Furthermore, applications of sulfur reduction of selenium, including removal of selenium, enrichment of selenium, synthesis of selenoproteins and prevention of leakage of selenium, were also summarized. Finally, this review identified future research needs for the sulfidation of selenite for environmental applications.

Antimicrobial Activity of Se-Nanoparticles from Bacterial Biotransformation

Selenium nanoparticles (SeNPs) are gaining importance in the food and medical fields due to their antibacterial properties. The microbial inhibition of these kinds of particles has been tested in a wide range of Gram (+) and Gram (−) pathogenic bacteria. When SeNPs are synthesized by biological methods, they are called biogenic SeNPs, which have a negative charge caused by their interaction between surface and capping layer (bioorganic material), producing their high stability. This review is focused on SeNPs synthesis by bacteria and summarizes the main factors that influence their main characteristics: shape, size and surface charge, considering the bacteria growth conditions for their synthesis. The different mechanisms of antimicrobial activity are revised, and this review describes several biosynthesis hypotheses that have been proposed due to the fact that the biological mechanism of SeNP synthesis is not fully known.

The Balance between Hydrogen Bonds, Halogen Bonds, and Chalcogen Bonds in the Crystal Structures of a Series of 1,3,4-Chalcogenadiazoles

In order to explore how specific atom-to-atom replacements change the electrostatic potentials on 1,3,4-chalcogenadiazole derivatives, and to deliberately alter the balance between intermolecular interactions, four target molecules were synthesized and characterized. DFT calculations indicated that the atom-to-atom substitution of Br with I, and S with Se enhanced the σ-hole potentials, thus increasing the structure directing ability of halogen bonds and chalcogen bonds as compared to intermolecular hydrogen bonding. The delicate balance between these intermolecular forces was further underlined by the formation of two polymorphs of 5-(4-iodophenyl)-1,3,4-thiadiazol-2-amine; Form I displayed all three interactions while Form II only showed hydrogen and chalcogen bonding. The results emphasize that the deliberate alterations of the electrostatic potential on polarizable atoms can cause specific and deliberate changes to the main synthons and subsequent assemblies in the structures of this family of compounds.

Introducing Selenocysteine into Recombinant Proteins in Escherichia coli

Selenoproteins contain the 21st amino acid, selenocysteine. Selenocysteine is the only amino acid that is synthesized on its cognate tRNA, and it is inserted at specific recoded UGA stop codons via a complex translation system. Although highly similar to cysteine, selenocysteine has unique properties, including a stronger nucleophilic ability and lower reduction potential. Efforts to site-specifically incorporate selenocysteine to create recombinant selenoproteins involve a recoded UAG stop codon and expression of the necessary selenocysteine translation machinery. This article presents a protocol for expressing and purifying selenoproteins in Escherichia coli. © 2021 Wiley Periodicals LLC. Basic Protocol: Recombinant selenoprotein production in E. coli using a rewired translation system.

Distribution of methionine sulfoxide reductases in fungi and conservation of the free-methionine-R-sulfoxide reductase in multicellular eukaryotes

Methionine, either as a free amino acid or included in proteins, can be oxidized into methionine sulfoxide (MetO), which exists as R and S diastereomers. Almost all characterized organisms possess thiol-oxidoreductases named methionine sulfoxide reductase (Msr) enzymes to reduce MetO back to Met. MsrA and MsrB reduce the S and R diastereomers of MetO, respectively, with strict stereospecificity and are found in almost all organisms. Another type of thiol-oxidoreductase, the free-methionine-R-sulfoxide reductase (fRMsr), identified so far in prokaryotes and a few unicellular eukaryotes, reduces the R MetO diastereomer of the free amino acid. Moreover, some bacteria possess molybdenum-containing enzymes that reduce MetO, either in the free or protein-bound forms. All these Msrs play important roles in the protection of organisms against oxidative stress. Fungi are heterotrophic eukaryotes that colonize all niches on Earth and play fundamental functions, in organic matter recycling, as symbionts, or as pathogens of numerous organisms. However, our knowledge on fungal Msrs is still limited. Here, we performed a survey of msr genes in almost 700 genomes across the fungal kingdom. We show that most fungi possess one gene coding for each type of methionine sulfoxide reductase: MsrA, MsrB, and fRMsr. However, several fungi living in anaerobic environments or as obligate intracellular parasites were devoid of msr genes. Sequence inspection and phylogenetic analyses allowed us to identify non-canonical sequences with potentially novel enzymatic properties. Finaly, we identified several ocurences of msr horizontal gene transfer from bacteria to fungi.

Structure and Electron-Transfer Pathway of the Human Methionine Sulfoxide Reductase MsrB3

Aims: The post-translational oxidation of methionine to methionine sulfoxide (MetSO) is a reversible process, enabling the repair of oxidative damage to proteins and the use of sulfoxidation as a regulatory switch. MetSO reductases catalyze the stereospecific reduction of MetSO. One of the mammalian MetSO reductases, MsrB3, has a signal sequence for entry into the endoplasmic reticulum (ER). In the ER, MsrB3 is expected to encounter a distinct redox environment compared with its paralogs in the cytosol, nucleus, and mitochondria. We sought to determine the location and arrangement of MsrB3 redox-active cysteines, which may couple MsrB3 activity to other redox events in the ER. Results: We determined the human MsrB3 structure by using X-ray crystallography. The structure revealed that a disulfide bond near the protein amino terminus is distant in space from the active site. Nevertheless, biochemical assays showed that these amino-terminal cysteines are oxidized by the MsrB3 active site after its reaction with MetSO. Innovation: This study reveals a mechanism to shuttle oxidizing equivalents from the primary MsrB3 active site toward the enzyme surface, where they would be available for further dithiol-disulfide exchange reactions. Conclusion: Conformational changes must occur during the MsrB3 catalytic cycle to transfer oxidizing equivalents from the active site to the amino-terminal redox-active disulfide. The accessibility of this exposed disulfide may help couple MsrB3 activity to other dithiol-disulfide redox events in the secretory pathway.

Selenium Interactions with Algae: Chemical Processes at Biological Uptake Sites, Bioaccumulation, and Intracellular Metabolism

Selenium (Se) uptake by primary producers is the most variable and important step in determining Se concentrations at higher trophic levels in aquatic food webs. We gathered data available about the Se bioaccumulation at the base of aquatic food webs and analyzed its relationship with Se concentrations in water. This important dataset was separated into lotic and lentic systems to provide a reliable model to estimate Se in primary producers from aqueous exposure. We observed that lentic systems had higher organic selenium and selenite concentrations than in lotic systems and selenate concentrations were higher in lotic environments. Selenium uptake by algae is mostly driven by Se concentrations, speciation and competition with other anions, and is as well influenced by pH. Based on Se species uptake by algae in the laboratory, we proposed an accurate mechanistic model of competition between sulfate and inorganic Se species at algal uptake sites. Intracellular Se transformations and incorporation into selenoproteins as well as the mechanisms through which Se can induce toxicity in algae has also been reviewed. We provided a new tool for risk assessment strategies to better predict accumulation in primary consumers and consequently to higher trophic levels, and we identified some research needs that could fill knowledge gaps.

Methionine sulfoxide reductase B from Corynebacterium diphtheriae catalyzes sulfoxide reduction via an intramolecular disulfide cascade

Corynebacterium diphtheriae is a human pathogen that causes diphtheria. In response to immune system-induced oxidative stress, C. diphtheriae expresses antioxidant enzymes, among which are methionine sulfoxide reductase (Msr) enzymes, which are critical for bacterial survival in the face of oxidative stress. Although some aspects of the catalytic mechanism of the Msr enzymes have been reported, several details still await full elucidation. Here, we solved the solution structure of C. diphtheriae MsrB (Cd-MsrB) and unraveled its catalytic and oxidation-protection mechanisms. Cd-MsrB catalyzes methionine sulfoxide reduction involving three redox-active cysteines. Using NMR heteronuclear single-quantum coherence spectra, kinetics, biochemical assays, and MS analyses, we show that the conserved nucleophilic residue Cys-122 is S-sulfenylated after substrate reduction, which is then resolved by a conserved cysteine, Cys-66, or by the nonconserved residue Cys-127. We noted that the overall structural changes during the disulfide cascade expose the Cys-122-Cys-66 disulfide to recycling through thioredoxin. In the presence of hydrogen peroxide, Cd-MsrB formed reversible intra- and intermolecular disulfides without losing its Cys-coordinated Zn²⁺, and only the nonconserved Cys-127 reacted with the low-molecular-weight (LMW) thiol mycothiol, protecting it from overoxidation. In summary, our structure-function analyses reveal critical details of the Cd-MsrB catalytic mechanism, including a major structural rearrangement that primes the Cys-122-Cys-66 disulfide for thioredoxin reduction and a reversible protection against excessive oxidation of the catalytic cysteines in Cd-MsrB through intra- and intermolecular disulfide formation and S-mycothiolation.

Actin reduction by MsrB2 is a key component of the cytokinetic abscission checkpoint and prevents tetraploidy

Abscission is the terminal step of cytokinesis leading to the physical separation of the daughter cells. In response to the abnormal presence of lagging chromatin between dividing cells, an evolutionarily conserved abscission/NoCut checkpoint delays abscission and prevents formation of binucleated cells by stabilizing the cytokinetic intercellular bridge (ICB). How this bridge is stably maintained for hours while the checkpoint is activated is poorly understood and has been proposed to rely on F-actin in the bridge region. Here, we show that actin polymerization is indeed essential for stabilizing the ICB when lagging chromatin is present, but not in normal dividing cells. Mechanistically, we found that a cytosolic pool of human methionine sulfoxide reductase B2 (MsrB2) is strongly recruited at the midbody in response to the presence of lagging chromatin and functions within the ICB to promote actin polymerization there. Consistently, in MsrB2-depleted cells, F-actin levels are decreased in ICBs, and dividing cells with lagging chromatin become binucleated as a consequence of unstable bridges. We further demonstrate that MsrB2 selectively reduces oxidized actin monomers and thereby counteracts MICAL1, an enzyme known to depolymerize actin filaments by direct oxidation. Finally, MsrB2 colocalizes and genetically interacts with the checkpoint components Aurora B and ANCHR, and the abscission delay upon checkpoint activation by nuclear pore defects also depends on MsrB2. Altogether, this work reveals that actin reduction by MsrB2 is a key component of the abscission checkpoint that favors F-actin polymerization and limits tetraploidy, a starting point for tumorigenesis.

Selenium and hydrogen selenide: essential micronutrient and the fourth gasotransmitter?

Selenium (Se) is an essential micronutrient required by organisms of diverse lineage. Dietary Se is converted to hydrogen selenide either enzymatically or by endogenous antioxidant proteins. This convergent biochemical step crucially underlies the subsequent biological activity of Se and argues for inclusion of hydrogen selenide as the fourth endogenous gasotransmitter alongside nitric oxide, carbon monoxide and hydrogen sulfide.Endogenously generated hydrogen selenide is incorporated into numerous 'selenoprotein' oxidoreductase enzymes, essential for maintaining redox-status homeostasis in health and disease. Direct effects of endogenous hydrogen selenide on cellular and molecular targets are currently unknown. Given exogenously, hydrogen selenide acts as a modulator of metabolism via transient inhibition of mitochondrial cytochrome C oxidase. Here we provide an overview of Se biology, its impact on several physiological systems (immune, endocrine, cardiovascular and metabolic) and its utility as a supplement in acute and critical illness states. We further explore the evidence base supporting its role as the fourth gasotransmitter and propose a strategic case towards generation of novel selenomimetic therapeutics.

Chemical Biology Approaches to Interrogate the Selenoproteome

Selenoproteins are the family of proteins that contain the amino acid selenocysteine. Many selenoproteins, including glutathione peroxidases and thioredoxin reductases, play a role in maintaining cellular redox homeostasis. There are a number of examples of homologues of selenoproteins that utilize cysteine residues, raising the question of why selenocysteines are utilized. One hypothesis is that incorporation of selenocysteine protects against irreversible overoxidation, typical of cysteine-containing homologues under high oxidative stress. Studies of selenocysteine function are hampered by challenges both in detection and in recombinant expression of selenoproteins. In fact, about half of the 25 known human selenoproteins remain uncharacterized. Historically, selenoproteins were first detected via labeling with radioactive 75Se or by use of inductively coupled plasma-mass spectrometry to monitor nonradioactive selenium. More recently, tandem mass-spectrometry techniques have been developed to detect selenocysteine-containing peptides. For example, the isotopic distribution of selenium has been used as a unique signature to identify selenium-containing peptides from unenriched proteome samples. Additionally, selenocysteine-containing proteins and peptides were selectively enriched using thiol-reactive electrophiles by exploiting the increased reactivity of selenols relative to thiols, especially under low pH conditions. Importantly, the reactivity-based enrichment of selenoproteins can differentiate between oxidized and reduced selenoproteins, providing insight into the activity state. These mass spectrometry-based selenoprotein detection approaches have enabled (1) production of selenoproteome expression atlases, (2) identification of aging-associated changes in selenoprotein expression, (3) characterization of selenocysteine reactivity across the selenoprotein family, and (4) interrogation of selenoprotein targets of small-molecule drugs. Further investigations of selenoprotein function would benefit from recombinant expression of selenoproteins. However, the endogenous mechanism of selenoprotein production makes recombinant expression challenging. Primarily, selenocysteine is biosynthesized on its own tRNA, is dependent on multiple enzymatic steps, and is highly sensitive to selenium concentrations. Furthermore, selenocysteine is encoded by the stop codon UGA, and suppression of that stop codon requires a selenocysteine insertion sequence element in the selenoprotein mRNA. In order to circumvent the low efficiency of the endogenous machinery, selenoproteins have been produced in vitro through native chemical ligation and expressed protein ligation. Attempts have also been made to engineer the endogenous machinery for increased efficiency, including recoding the selenocysteine codon, and engineering the tRNA and the selenocysteine insertion sequence element. Alternatively, genetic code expansion can be used to generate selenoproteins. This approach allows for selenoprotein production directly within its native cellular environment, while bypassing the endogenous selenocysteine incorporation machinery. Furthermore, by incorporating a caged selenocysteine by genetic code expansion, selenoprotein activity can be spatially and temporally controlled. Genetic code expansion has allowed for the expression and uncaging of human selenoproteins in E. coli and more recently in mammalian cells. Together, advances in selenoprotein detection and expression should enable a better understanding of selenoprotein function and provide insight into the necessity for selenocysteine production.

Progress in the emerging role of selenoproteins in cardiovascular disease: focus on endoplasmic reticulum-resident selenoproteins

Cardiovascular diseases represent one of the most important health problems of developed countries. One of the main actors involved in the onset and development of cardiovascular diseases is the increased production of reactive oxygen species that, through lipid peroxidation, protein oxidation and DNA damage, induce oxidative stress and cell death. Basic and clinical research are ongoing to better understand the endogenous antioxidant mechanisms that counteract oxidative stress, which may allow to identify a possible therapeutic targeting/application in the field of stress-dependent cardiovascular pathologies. In this context, increasing attention is paid to the glutathione/glutathione-peroxidase and to the thioredoxin/thioredoxin-reductase systems, among the most potent endogenous antioxidative systems. These key enzymes, belonging to the selenoprotein family, have a well-established function in the regulation of the oxidative cell balance. The aim of the present review was to highlight the role of selenoproteins in cardiovascular diseases, introducing the emerging cardioprotective role of endoplasmic reticulum-resident members and in particular one of them, namely selenoprotein T or SELENOT. Accumulating evidence indicates that the dysfunction of different selenoproteins is involved in the susceptibility to oxidative stress and its associated cardiovascular alterations, such as congestive heart failure, coronary diseases, impaired cardiac structure and function. Some of them are under investigation as useful pathological biomarkers. In addition, SELENOT exhibited intriguing cardioprotective effects by reducing the cardiac ischemic damage, in terms of infarct size and performance. In conclusion, selenoproteins could represent valuable targets to treat and diagnose cardiovascular diseases secondary to oxidative stress, opening a new avenue in the field of related therapeutic strategies.

Copper-Mediated Selenazolidine Deprotection Enables One-Pot Chemical Synthesis of Challenging Proteins

While chemical protein synthesis has granted access to challenging proteins, the synthesis of longer proteins is often limited by low abundance or non-strategic placement of cysteine residues, which are essential for native chemical ligations, as well as multiple purification and isolation steps. We describe the one-pot total synthesis of human thiosulfate:glutathione sulfurtransferase (TSTD1). WT-TSTD1 was synthesized in a C-to-N synthetic approach involving multiple NCL reactions, Cu^II -mediated deprotection of selenazolidine (Sez), and chemoselective deselenization. The seleno-analog Se-TSTD1, in which the active site Cys is replaced with selenocysteine, was also synthesized with a kinetically controlled ligation with an N-to-C synthetic approach. The catalytic activity of the two proteins indicated that Se-TSTD1 possessed only four-fold lower activity than WT-TSTD1, thus suggesting that selenoproteins can have physiologically comparable sulfutransferase activity to their cysteine counterparts.

The thioredoxin system and cancer therapy: a review

Thioredoxin (Trx), thioredoxin reductase (TrxR), and NADPH are key members of the Trx system that is involved in redox regulation and antioxidant defense. In recent years, several researchers have provided information about the roles of the Trx system in cancer development and progression. These reports indicated that many tumor cells express high levels of Trx and TrxR, which can be responsible for drug resistance in tumorigenesis. Inhibition of the Trx system may thus contribute to cancer therapy and improving chemotherapeutic agents. There are now a number of effective natural and synthetic inhibitors with chemotherapy applications possessing antitumor activity ranging from oxidative stress induction to apoptosis. In this article, we first described the features and functions of the Trx system and then reviewed briefly its correlations with cancer. Finally, we summarized the present knowledge about the Trx/TrxR inhibitors as anticancer drugs.

Using chemical synthesis to optimise antimicrobial peptides in the fight against antimicrobial resistance

The emergence of multidrug-resistant bacteria has necessitated the urgent need for novel antibacterial agents. Antimicrobial peptides (AMPs), the host-defence molecules of most living organisms, have shown great promise as potential antibiotic candidates due to their multiple mechanisms of action which result in very low or negligible induction of resistance. However, the development of AMPs for clinical use has been limited by their potential toxicity to animal cells, low metabolic stability and high manufacturing cost. Extensive efforts have therefore been directed towards the development of enhanced variants of natural AMPs to overcome these aforementioned limitations. In this review, we present our efforts focused on development of efficient strategies to prepare several recently discovered AMPs including antitubercular peptides. The design and synthesis of more potent and stable AMP analogues with synthetic modifications made to the natural peptides containing glycosylated residues or disulfide bridges are described.

Plant selenium toxicity: Proteome in the crosshairs

The metalloid element, selenium (Se) is in many ways special and perhaps because of this its research in human and plant systems is of great interest. Despite its non-essentiality, higher plants take it up and metabolize it via sulfur pathways, but higher amounts of Se cause toxic symptoms in plants. However, the molecular mechanisms of selenium phytotoxicity have been only partly revealed; the data obtained so far point out that Se toxicity targets the plant proteome. Besides seleno- and oxyproteins, nitroproteins are also formed due to Se stress. In order to minimize proteomic damages induced by Se, certain plants are able to redirect selenocysteine away from protein synthesis thus preventing Se-protein formation. Additionally, the damaged or malformed selenoproteins, oxyproteins and nitroproteins may be removed by proteasomes. Based on the literature this review sets Se toxicity mechanisms into a new concept and it draws attention to the importance of Se-induced protein-level changes.

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

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

The thioredoxin system as a target for mercury compounds

BACKGROUND

Mercury interaction with selenium in vivo has been recognized for >50 years. Several researchers attempted to use selenium to mitigate the detrimental effects of mercurial compounds but the results were controversial. Selenium pools in living organisms are quite low and the high affinity of mercury to bind selenols pointed out selenoproteins as possible targets of toxicity. Such was the case of the selenoenzyme thioredoxin reductase (TrxR) which is an integrant part of the thioredoxin system. Given the important role of this redox system for cellular functioning and the high affinity of mercury for TrxR's active site, this interaction can be key to understand the mechanism by which Hg causes cell death.

SCOPE OF THE REVIEW

This review discusses the current state of knowledge concerning the interaction between mercury compounds and the thioredoxin system, its implications for the development of toxicity and the effects of selenium co-exposure.

MAJOR CONCLUSIONS

The mechanism of toxicity of mercurials is a complex chain of events starting with inhibition of the selenoenzyme, TrxR. Selenium supplementation protects TrxR from the toxicity of inorganic forms of mercury (i.e., Hg(II)) to a certain extent, but not from methylmercury. When TrxR is inhibited, thioredoxin is reduced by alternative mechanisms involving glutathione and glutaredoxin and only when this pathway is hampered does cell death occur.

GENERAL SIGNIFICANCE

Understanding the molecular mechanism of mercury toxicity and the mechanisms of enzymatic compensation allows the design of mitigation strategies and, since TxrR and Trx exist in the plasma, puts forward the possibility for future use of changes in activity/expression of these enzymes as biomarkers of mercury toxicity, thus refining the risk assessment process.

Selenium-Related Transcriptional Regulation of Gene Expression

The selenium content of the body is known to control the expression levels of numerous genes, both so-called selenoproteins and non-selenoproteins. Selenium is a trace element essential to human health, and its deficiency is related to, for instance, cardiovascular and myodegenerative diseases, infertility and osteochondropathy called Kashin–Beck disease. It is incorporated as selenocysteine to the selenoproteins, which protect against reactive oxygen and nitrogen species. They also participate in the activation of the thyroid hormone, and play a role in immune system functioning. The synthesis and incorporation of selenocysteine occurs via a special mechanism, which differs from the one used for standard amino acids. The codon for selenocysteine is a regular in-frame stop codon, which can be passed by a specific complex machinery participating in translation elongation and termination. This includes a presence of selenocysteine insertion sequence (SECIS) in the 3′-untranslated part of the selenoprotein mRNAs. Nonsense-mediated decay is involved in the regulation of the selenoprotein mRNA levels, but other mechanisms are also possible. Recent transcriptional analyses of messenger RNAs, microRNAs and long non-coding RNAs combined with proteomic data of samples from Keshan and Kashin–Beck disease patients have identified new possible cellular pathways related to transcriptional regulation by selenium.

Selenium-Dependent Antioxidant Enzymes: Actions and Properties of Selenoproteins

Unlike other essential trace elements that interact with proteins in the form of cofactors, selenium (Se) becomes co-translationally incorporated into the polypeptide chain as part of 21st naturally occurring amino acid, selenocysteine (Sec), encoded by the UGA codon. Any protein that includes Sec in its polypeptide chain is defined as selenoprotein. Members of the selenoproteins family exert various functions and their synthesis depends on specific cofactors and on dietary Se. The Se intake in productive animals such as chickens affect nutrient utilization, production performances, antioxidative status and responses of the immune system. Although several functions of selenoproteins are unknown, many disorders are related to alterations in selenoprotein expression or activity. Selenium insufficiency and polymorphisms or mutations in selenoproteins' genes and synthesis cofactors are involved in the pathophysiology of many diseases, including cardiovascular disorders, immune dysfunctions, cancer, muscle and bone disorders, endocrine functions and neurological disorders. Finally, heavy metal poisoning decreases mRNA levels of selenoproteins and increases mRNA levels of inflammatory factors, underlying the antagonistic effect of Se. This review is an update on Se dependent antioxidant enzymes, presenting the current state of the art and is focusing on results obtained mainly in chicken.

Methionine sulfoxide reductase B3 requires resolving cysteine residues for full activity and can act as a stereospecific methionine oxidase

The oxidation of methionine residues in proteins occurs during oxidative stress and can lead to an alteration in protein function. The enzyme methionine sulfoxide reductase (Msr) reverses this modification. Here, we characterise the mammalian enzyme Msr B3. There are two splice variants of this enzyme that differ only in their N-terminal signal sequence, which directs the protein to either the endoplasmic reticulum (ER) or mitochondria. We demonstrate here that the enzyme can complement a bacterial strain, which is dependent on methionine sulfoxide reduction for growth, that the purified recombinant protein is enzymatically active showing stereospecificity towards R-methionine sulfoxide, and identify the active site and two resolving cysteine residues. The enzyme is efficiently recycled by thioredoxin only in the presence of both resolving cysteine residues. These results show that for this isoform of Msrs, the reduction cycle most likely proceeds through a three-step process. This involves an initial sulfenylation of the active site thiol followed by the formation of an intrachain disulfide with a resolving thiol group and completed by the reduction of this disulfide by a thioredoxin-like protein to regenerate the active site thiol. Interestingly, the enzyme can also act as an oxidase catalysing the stereospecific formation of R-methionine sulfoxide. This result has important implications for the role of this enzyme in the reversible modification of ER and mitochondrial proteins.

Functional characterisation of the methionine sulfoxide reductase repertoire in Trypanosoma brucei

To combat the deleterious effects that oxidation of the sulfur atom in methionine to sulfoxide may bring, aerobic cells express repair pathways involving methionine sulfoxide reductases (MSRs) to reverse the above reaction. Here, we show that Trypanosoma brucei, the causative agent of African trypanosomiasis, expresses two distinct trypanothione-dependent MSRs that can be distinguished from each other based on sequence, sub-cellular localisation and substrate preference. One enzyme found in the parasite's cytosol, shows homology to the MSRA family of repair proteins and preferentially metabolises the S epimer of methionine sulfoxide. The second, which contains sequence motifs present in MSRBs, is restricted to the mitochondrion and can only catalyse reduction of the R form of peptide-bound methionine sulfoxide. The importance of these proteins to the parasite was demonstrated using functional genomic-based approaches to produce cells with reduced or elevated expression levels of MSRA, which exhibited altered susceptibility to exogenous H₂O₂. These findings identify new reparative pathways that function to fix oxidatively damaged methionine within this medically important parasite.

Selenoprotein MsrB1 deficiency exacerbates acetaminophen-induced hepatotoxicity via increased oxidative damage

Acetaminophen (APAP) overdose induces acute liver damage and failure via reactive oxygen species production and glutathione (GSH) depletion. Methionine sulfoxide reductase B1 (MsrB1) is an antioxidant selenoenzyme that specifically catalyzes the reduction of methionine R-sulfoxide residues. In this study, we used MsrB1 gene-knockout mice and primary hepatocytes to investigate the effect of MsrB1 on APAP-induced hepatotoxicity. Analyses of histological alterations and serum indicators of liver damage showed that MsrB1^-/- mice were more susceptible to APAP-induced acute liver injury than wild-type (MsrB1^+/+) mice. Consistent with the in vivo results, primary MsrB1^-/- hepatocytes displayed higher susceptibility to APAP-induced cytotoxicity than MsrB1^+/+ cells. MsrB1 deficiency increased hepatic oxidative stress after APAP challenge such as hydrogen peroxide production, lipid peroxidation, and protein oxidation levels. Additionally, basal and APAP-induced ratios of reduced-to-oxidized GSH (GSH/GSSG) were significantly lower in MsrB1^-/- than in MsrB1^+/+ livers. Nrf2 nuclear accumulation and heme oxygenase-1 expression levels after APAP challenge were lower in MsrB1^-/- than in MsrB1^+/+ livers, suggesting that MsrB1 deficiency attenuates the APAP-induced activation of Nrf2. Collectively, the results of this study suggest that selenoprotein MsrB1 plays a protective role against APAP-induced hepatotoxicity via its antioxidative function.

Amblyomma maculatum SECIS binding protein 2 and putative selenoprotein P are indispensable for pathogen replication and tick fecundity

Selenium, a vital trace element, is incorporated into selenoproteins to produce selenocysteine. Our previous studies have revealed an adaptive co-evolutionary process that has enabled the spotted fever-causing tick-borne pathogen Rickettsia parkeri to survive by manipulating an antioxidant defense system associated with selenium, which includes a full set of selenoproteins and other antioxidants in ticks. Here, we conducted a systemic investigation of SECIS binding protein 2 (SBP2) and putative selenoprotein P (SELENOP) by transcript silencing in adult female Gulf-coast ticks (Amblyomma maculatum). Knockdown of the SBP2 and SELENOP genes depleted the respective transcript levels of these tick selenogenes, and caused differential regulation of other antioxidants. Importantly, the selenium level in the immature and mature tick stages increased significantly after a blood meal, but the selenium level decreased in ticks after the SBP2 and SELENOP knockdowns. Moreover, the SBP2 knockdown significantly impaired both transovarial transmission of R. parkeri to tick eggs and egg hatching. Overall, our data offer new insight into the relationship between the SBP2 selenoprotein synthesis gene and the putative tick SELENOP gene. It also augments our understanding of selenoprotein synthesis, selenium maintenance and utilization, and bacterial colonization of a tick vector.

Astragaloside IV rescues MPP+-induced mitochondrial dysfunction through upregulation of methionine sulfoxide reductase A

Methionine sulfoxide reductase (Msr) repairs oxidatively damaged proteins through acting as an antioxidant. Oxidative stress has been postulated to cause the mitochondrial dysfunction that is associated with aging and certain diseases, including Parkinson's disease (PD). The present study investigated the protective effects of astragaloside IV (AS-IV) on 1-methyl-4-phenylpyridinium (MPP⁺)-induced mitochondrial dysfunction through MsrA in PC12 cells. This revealed that oxidative stress reduced the expression of MsrA following MPP⁺ treatment. AS-IV was demonstrated to protect PC12 cells from MPP⁺-induced oxidative damage through upregulating MsrA. MsrA expression was dependent on the Sirt1-FOXO3a signaling pathway. In addition, knockdown of MsrA reduced the protective effects of AS-IV, indicating that the antioxidant effects of AS-UV occurred through MsrA. These results suggest that AS-IV exerts antioxidant effects and regulates mitochondrial function. Thus, AS-IV may serve as an effective therapeutic agent for aging and PD.

Au Nanoclusters and Photosensitizer Dual Loaded Spatiotemporal Controllable Liposomal Nanocomposites Enhance Tumor Photodynamic Therapy Effect by Inhibiting Thioredoxin Reductase

Photodynamic therapy (PDT) is a minimally invasive therapeutic procedure of tumors with high selectivity and low side effect. However, it is usually not efficient in long-lasting tumor control. One of the main reasons is tumor cells develop some protective mechanisms that help them to deal with oxidative stress in the environment. The thioredoxin system in cancer is an important antioxidant defense system. Au nanoclusters could effectively inhibit thioredoxin reductase (TrxR) in tumor cell cytoplasm. Herein, Au nanoclusters and photosensitizer Chlorine 6 (Ce6) are co-loaded in spatiotemporal controllable liposomal nanocomposites. pH responsive molecule inserted in lipid bilayer greatly contributes to the instability of the lipid membrane in lysosomal at low pH environment. Then the payloads can rapidly release into cytoplasm. Au nanoclusters effectively inhibit TrxR in cytoplasm and enhance the photodynamic-induced intracellular reactive oxygen-free radical concentration, improving the effect of PDT. Breast cancer is chosen as a tumor model and the Au nanoclusters and photosensitizer co-loaded liposomal nanocomposites are studied to improve the effect of PDT both in vitro and in vivo, and its corresponding mechanism is investigated. This study develops a new application of gold nanoclusters and provides a new train of thoughts for enhancing the effect of PDT.

Redox Pioneer: Professor Vadim N. Gladyshev

Professor Vadim N. Gladyshev is recognized here as a Redox Pioneer, because he has published an article on antioxidant/redox biology that has been cited more than 1000 times and 29 articles that have been cited more than 100 times. Gladyshev is world renowned for his characterization of the human selenoproteome encoded by 25 genes, identification of the majority of known selenoprotein genes in the three domains of life, and discoveries related to thiol oxidoreductases and mechanisms of redox control. Gladyshev's first faculty position was in the Department of Biochemistry, the University of Nebraska. There, he was a Charles Bessey Professor and Director of the Redox Biology Center. He then moved to the Department of Medicine at Brigham and Women's Hospital, Harvard Medical School, where he is Professor of Medicine and Director of the Center for Redox Medicine. His discoveries in redox biology relate to selenoenzymes, such as methionine sulfoxide reductases and thioredoxin reductases, and various thiol oxidoreductases. He is responsible for the genome-wide identification of catalytic redox-active cysteines and for advancing our understanding of the general use of cysteines by proteins. In addition, Gladyshev has characterized hydrogen peroxide metabolism and signaling and regulation of protein function by methionine-R-sulfoxidation. He has also made important contributions in the areas of aging and lifespan control and pioneered applications of comparative genomics in redox biology, selenium biology, and aging. Gladyshev's discoveries have had a profound impact on redox biology and the role of redox control in health and disease. He is a true Redox Pioneer. Antioxid. Redox Signal. 25, 1-9.

Why Nature Chose Selenium

The authors were asked by the Editors of ACS Chemical Biology to write an article titled "Why Nature Chose Selenium" for the occasion of the upcoming bicentennial of the discovery of selenium by the Swedish chemist Jöns Jacob Berzelius in 1817 and styled after the famous work of Frank Westheimer on the biological chemistry of phosphate [Westheimer, F. H. (1987) Why Nature Chose Phosphates, Science 235, 1173-1178]. This work gives a history of the important discoveries of the biological processes that selenium participates in, and a point-by-point comparison of the chemistry of selenium with the atom it replaces in biology, sulfur. This analysis shows that redox chemistry is the largest chemical difference between the two chalcogens. This difference is very large for both one-electron and two-electron redox reactions. Much of this difference is due to the inability of selenium to form π bonds of all types. The outer valence electrons of selenium are also more loosely held than those of sulfur. As a result, selenium is a better nucleophile and will react with reactive oxygen species faster than sulfur, but the resulting lack of π-bond character in the Se-O bond means that the Se-oxide can be much more readily reduced in comparison to S-oxides. The combination of these properties means that replacement of sulfur with selenium in nature results in a selenium-containing biomolecule that resists permanent oxidation. Multiple examples of this gain of function behavior from the literature are discussed.

Intracellular repair of oxidation-damaged α-synuclein fails to target C-terminal modification sites

Cellular oxidative stress serves as a common denominator in many neurodegenerative disorders, including Parkinson's disease. Here we use in-cell NMR spectroscopy to study the fate of the oxidation-damaged Parkinson's disease protein alpha-synuclein (α-Syn) in non-neuronal and neuronal mammalian cells. Specifically, we deliver methionine-oxidized, isotope-enriched α-Syn into cultured cells and follow intracellular protein repair by endogenous enzymes at atomic resolution. We show that N-terminal α-Syn methionines Met1 and Met5 are processed in a stepwise manner, with Met5 being exclusively repaired before Met1. By contrast, C-terminal methionines Met116 and Met127 remain oxidized and are not targeted by cellular enzymes. In turn, persisting oxidative damage in the C-terminus of α-Syn diminishes phosphorylation of Tyr125 by Fyn kinase, which ablates the necessary priming event for Ser129 modification by CK1. These results establish that oxidative stress can lead to the accumulation of chemically and functionally altered α-Syn in cells.

α-synuclein is a protein linked to the occurrence of Parkinson's disease. Here, the authors use time-resolved in-cell NMR spectroscopy to study the repair of methionine-oxidized α-synuclein by endogenous cellular enzymes.

Regulation of protein function by reversible methionine oxidation and the role of selenoprotein MsrB1

SIGNIFICANCE

Protein structure and function can be regulated via post-translational modifications by numerous enzymatic and nonenzymatic mechanisms. Regulation involving oxidation of sulfur-containing residues emerged as a key mechanism of redox control. Unraveling the participants and principles of such regulation is necessary for understanding the biological significance of redox control of cellular processes.

RECENT ADVANCES

Reversible oxidation of methionine residues by monooxygenases of the Mical family and subsequent reduction of methionine sulfoxides by a selenocysteine-containing methionine sulfoxide reductase B1 (MsrB1) was found to control the assembly and disassembly of actin in mammals, and the Mical/MsrB pair similarly regulates actin in fruit flies. This finding has opened up new avenues for understanding the use of stereospecific methionine oxidation in regulating cellular processes and the roles of MsrB1 and Micals in regulation of actin dynamics.

CRITICAL ISSUES

So far, Micals have been the only known partners of MsrB1, and actin is the only target. It is important to identify additional substrates of Micals and characterize other Mical-like enzymes.

FUTURE DIRECTIONS

Oxidation of methionine, reviewed here, is an emerging but not well-established mechanism. Studies suggest that methionine oxidation is a form of oxidative damage of proteins, a modification that alters protein structure or function, a tool in redox signaling, and a mechanism that controls protein function. Understanding the functional impact of reversible oxidation of methionine will require identification of targets, substrates, and regulators of Micals and Msrs. Linking the biological processes, in which these proteins participate, might also lead to insights into disease conditions, which involve regulation of actin by Micals and Msrs.

Genetic code flexibility in microorganisms: novel mechanisms and impact on physiology

The genetic code, initially thought to be universal and immutable, is now known to contain many variations, including biased codon usage, codon reassignment, ambiguous decoding and recoding. As a result of recent advances in the areas of genome sequencing, biochemistry, bioinformatics and structural biology, our understanding of genetic code flexibility has advanced substantially in the past decade. In this Review, we highlight the prevalence, evolution and mechanistic basis of genetic code variations in microorganisms, and we discuss how this flexibility of the genetic code affects microbial physiology.

The discovery of methionine sulfoxide reductase enzymes: An historical account and future perspectives

L-Methionine (L-Met) is the only sulphur-containing proteinogenic amino acid together with cysteine. Its importance is highlighted by it being the initiator amino acid for protein synthesis in all known living organisms. L-Met, free or inserted into proteins, is sensitive to oxidation of its sulfide moiety, with formation of L-Met sulfoxide. The sulfoxide could not be inserted into proteins, and the oxidation of L-Met in proteins often leads to the loss of biological activity of the affected molecule. Key discoveries revealed the existence, in rats, of a metabolic pathway for the reduction of free L-Met sulfoxide and, later, in Escherichia coli, of the enzymatic reduction of L-Met sulfoxide inserted in proteins. Upon oxidation, the sulphur atom becomes a new stereogenic center, and two stable diastereoisomers of L-Met sulfoxide exist. A fundamental discovery revealed the existence of two unrelated families of enzymes, MsrA and MsrB, whose members display opposite stereospecificity of reduction for the two sulfoxides. The importance of Msrs is additionally emphasized by the discovery that one of the only 25 selenoproteins expressed in humans is a Msr. The milestones on the road that led to the discovery and characterization of this group of antioxidant enzymes are recounted in this review.

Corynebacterium glutamicum methionine sulfoxide reductase A uses both mycoredoxin and thioredoxin for regeneration and oxidative stress resistance

Oxidation of methionine leads to the formation of the S and R diastereomers of methionine sulfoxide (MetO), which can be reversed by the actions of two structurally unrelated classes of methionine sulfoxide reductase (Msr), MsrA and MsrB, respectively. Although MsrAs have long been demonstrated in numerous bacteria, their physiological and biochemical functions remain largely unknown in Actinomycetes. Here, we report that a Corynebacterium glutamicum methionine sulfoxide reductase A (CgMsrA) that belongs to the 3-Cys family of MsrAs plays important roles in oxidative stress resistance. Deletion of the msrA gene in C. glutamicum resulted in decrease of cell viability, increase of ROS production, and increase of protein carbonylation levels under various stress conditions. The physiological roles of CgMsrA in resistance to oxidative stresses were corroborated by its induced expression under various stresses, regulated directly by the stress-responsive extracytoplasmic-function (ECF) sigma factor SigH. Activity assays performed with various regeneration pathways showed that CgMsrA can reduce MetO via both the thioredoxin/thioredoxin reductase (Trx/TrxR) and mycoredoxin 1/mycothione reductase/mycothiol (Mrx1/Mtr/MSH) pathways. Site-directed mutagenesis confirmed that Cys56 is the peroxidatic cysteine that is oxidized to sulfenic acid, while Cys204 and Cys213 are the resolving Cys residues that form an intramolecular disulfide bond. Mrx1 reduces the sulfenic acid intermediate via the formation of an S-mycothiolated MsrA intermediate (MsrA-SSM) which is then recycled by mycoredoxin and the second molecule of mycothiol, similarly to the glutathione/glutaredoxin/glutathione reductase (GSH/Grx/GR) system. However, Trx reduces the Cys204-Cys213 disulfide bond in CgMsrA produced during MetO reduction via the formation of a transient intermolecular disulfide bond between Trx and CgMsrA. While both the Trx/TrxR and Mrx1/Mtr/MSH pathways are operative in reducing CgMsrA under stress conditions in vivo, the Trx/TrxR pathway alone is sufficient to reduce CgMsrA under normal conditions. Based on these results, a catalytic model for the reduction of CgMsrA by Mrx1 and Trx is proposed.