Selenium versus sulfur: Reversibility of chemical reactions and resistance to permanent oxidation in proteins and nucleic acids

Chirality-driven strong thioredoxin reductase inhibition

Overexpression of thioredoxin reductase (TXNRD) plays crucial role in tumorigenesis. Therefore, designing TXNRD inhibitors is a promising strategy for targeted anticancer drug development. However, poor selectivity has always been a challenge, resulting in unavoidable toxicity in clinic. Herein we demonstrate a strategy to develop highly selective chiral metal complexes-based TXNRD inhibitors. By manipulating the conformation of two distinct weakly interacting groups, we optimize the compatibility between the drug and the electrophilic group within the active site of TXNRD to enhance their non-covalent interaction, thus effectively avoids the poor selectivity deriving from covalent drug interaction, on the basis of ensuring the strong inhibition. Detailed experimental and computational results demonstrate that the chiral isomeric drugs bind to the active site of TXNRD, and the interaction strength is well modulated by chirality. Especially, the meso-configuration, in which the two large sterically hindered active groups are positioned on opposite sides of the drug, exhibits the highest number of non-covalent interactions and most effective inhibition on TXNRD. Taken together, this work not only provides a novel approach for developing highly selective proteinase inhibitors, but also sheds light on possible underlying mechanisms for future application.

Exploring the Link between Oxidative Stress, Selenium Levels, and Obesity in Youth

Obesity is a worldwide increasing concern. Although in adults this is easily estimated with the body mass index, in children, who are constantly growing and whose bodies are changing, the reference points to assess weight status are age and gender, and need corroboration with complementary data, making their quantification highly difficult. The present review explores the interaction spectrum of oxidative stress, selenium status, and obesity in children and adolescents. Any factor related to oxidative stress that triggers obesity and, conversely, obesity that induces oxidative stress are part of a vicious circle, a complex chain of mechanisms that derive from each other and reinforce each other with serious health consequences. Selenium and its compounds exhibit key antioxidant activity and also have a significant role in the nutritional evaluation of obese children. The balance of selenium intake, retention, and metabolism emerges as a vital aspect of health, reflecting the complex interactions between diet, oxidative stress, and obesity. Understanding whether selenium status is a contributor to or a consequence of obesity could inform nutritional interventions and public health strategies aimed at preventing and managing obesity from an early age.

Replacing a Cysteine Ligand by Selenocysteine in a [NiFe]-Hydrogenase Unlocks Hydrogen Production Activity and Addresses the Role of Concerted Proton-Coupled Electron Transfer in Electrocatalytic Reversibility

Hydrogenases catalyze hydrogen/proton interconversion that is normally electrochemically reversible (having minimal overpotential requirement), a special property otherwise almost exclusive to platinum metals. The mechanism of [NiFe]-hydrogenases includes a long-range proton-coupled electron-transfer process involving a specific Ni-coordinated cysteine and the carboxylate of a nearby glutamate. A variant in which this cysteine has been exchanged for selenocysteine displays two distinct changes in electrocatalytic properties, as determined by protein film voltammetry. First, proton reduction, even in the presence of H2 (a strong product inhibitor), is greatly enhanced relative to H2 oxidation: this result parallels a characteristic of natural [NiFeSe]-hydrogenases which are superior H2 production catalysts. Second, an inflection (an S-shaped "twist" in the trace) appears around the formal potential, the small overpotentials introduced in each direction (oxidation and reduction) signaling a departure from electrocatalytic reversibility. Concerted proton-electron transfer offers a lower energy pathway compared to stepwise transfers. Given the much lower proton affinity of Se compared to that of S, the inflection provides compelling evidence that concerted proton-electron transfer is important in determining why [NiFe]-hydrogenases are reversible electrocatalysts.

Seleno-amino Acid Metabolism Reshapes the Tumor Microenvironment: from Cytotoxicity to Immunotherapy

Selenium (Se) is an essential trace element for biological processes. Seleno-amino acids (Se-AAs), known as the organic forms of Se, and their metabolic reprogramming have been increasingly recognized to regulate antioxidant defense, enzyme activity, and tumorigenesis. Therefore, there is emerging interest in exploring the potential application of Se-AAs in antitumor therapy. In addition to playing a vital role in inhibiting tumor growth, accumulating evidence has revealed that Se-AA metabolism could reshape the tumor microenvironment (TME) and enhance immunotherapy responses. This review presents a comprehensive overview of the current progress in multifunctional Se-AAs for antitumor treatment, with a particular emphasis on elucidating the crosstalk between Se-AA metabolism and various cell types in the TME, including tumor cells, T cells, macrophages, and natural killer cells. Furthermore, novel applications integrating Se-AAs are also discussed alongside prospects to provide new insights into this emerging field.

The Selenoproteome as a Dynamic Response Mechanism to Oxidative Stress in Hydrogenotrophic Methanogenic Communities

Methanogenesis is a critical process in the carbon cycle that is applied industrially in anaerobic digestion and biogas production. While naturally occurring in diverse environments, methanogenesis requires anaerobic and reduced conditions, although varying degrees of oxygen tolerance have been described. Microaeration is suggested as the next step to increase methane production and improve hydrolysis in digestion processes; therefore, a deeper understanding of the methanogenic response to oxygen stress is needed. To explore the drivers of oxygen tolerance in methanogenesis, two parallel enrichments were performed under the addition of H2/CO2 in an environment without reducing agents and in a redox-buffered environment by adding redox mediator 9,10-anthraquinone-2,7-disulfonate disodium. The cellular response to oxidative conditions is mapped using proteomic analysis. The resulting community showed remarkable tolerance to high-redox environments and was unperturbed in its methane production. Next to the expression of pathways to mitigate reactive oxygen species, the higher redox potential environment showed an increased presence of selenocysteine and selenium-associated pathways. By including sulfur-to-selenium mass shifts in a proteomic database search, we provide the first evidence of the dynamic and large-scale incorporation of selenocysteine as a response to oxidative stress in hydrogenotrophic methanogenesis and the presence of a dynamic selenoproteome.

A Physiological Approach to Explore How Thioredoxin-Glutathione Reductase (TGR) and Peroxiredoxin (Prx) Eliminate H2O2 in Cysticerci of Taenia

Peroxiredoxins (Prxs) and glutathione peroxidases (GPxs) are the main enzymes of the thiol-dependent antioxidant systems responsible for reducing the H2O2 produced via aerobic metabolism or parasitic organisms by the host organism. These antioxidant systems maintain a proper redox state in cells. The cysticerci of Taenia crassiceps tolerate millimolar concentrations of this oxidant. To understand the role played by Prxs in this cestode, two genes for Prxs, identified in the genome of Taenia solium (TsPrx1 and TsPrx3), were cloned. The sequence of the proteins suggests that both isoforms belong to the class of typical Prxs 2-Cys. In addition, TsPrx3 harbors a mitochondrial localization signal peptide and two motifs (-GGLG- and -YP-) associated with overoxidation. Our kinetic characterization assigns them as thioredoxin peroxidases (TPxs). While TsPrx1 and TsPrx3 exhibit the same catalytic efficiency, thioredoxin-glutathione reductase from T. crassiceps (TcTGR) was five and eight times higher. Additionally, the latter demonstrated a lower affinity (>30-fold) for H2O2 in comparison with TsPrx1 and TsPrx3. The TcTGR contains a Sec residue in its C-terminal, which confers additional peroxidase activity. The aforementioned aspect implies that TsPrx1 and TsPrx3 are catalytically active at low H2O2 concentrations, and the TcTGR acts at high H2O2 concentrations. These results may explain why the T. crassiceps cysticerci can tolerate high H2O2 concentrations.

Theranostic Fluorescent Probes

The ability to gain spatiotemporal information, and in some cases achieve spatiotemporal control, in the context of drug delivery makes theranostic fluorescent probes an attractive and intensely investigated research topic. This interest is reflected in the steep rise in publications on the topic that have appeared over the past decade. Theranostic fluorescent probes, in their various incarnations, generally comprise a fluorophore linked to a masked drug, in which the drug is released as the result of certain stimuli, with both intrinsic and extrinsic stimuli being reported. This release is then signaled by the emergence of a fluorescent signal. Importantly, the use of appropriate fluorophores has enabled not only this emerging fluorescence as a spatiotemporal marker for drug delivery but also has provided modalities useful in photodynamic, photothermal, and sonodynamic therapeutic applications. In this review we highlight recent work on theranostic fluorescent probes with a particular focus on probes that are activated in tumor microenvironments. We also summarize efforts to develop probes for other applications, such as neurodegenerative diseases and antibacterials. This review celebrates the diversity of designs reported to date, from discrete small-molecule systems to nanomaterials. Our aim is to provide insights into the potential clinical impact of this still-emerging research direction.

Selenium chemistry for spatio-selective peptide and protein functionalization

The ability to construct a peptide or protein in a spatio-specific manner is of great interest for therapeutic and biochemical research. However, the various functional groups present in peptide sequences and the need to perform chemistry under mild and aqueous conditions make selective protein functionalization one of the greatest synthetic challenges. The fascinating paradox of selenium (Se) - being found in both toxic compounds and also harnessed by nature for essential biochemical processes - has inspired the recent exploration of selenium chemistry for site-selective functionalization of peptides and proteins. In this Review, we discuss such approaches, including metal-free and metal-catalysed transformations, as well as traceless chemical modifications. We report their advantages, limitations and applications, as well as future research avenues.

Essential Trace Elements in Patients with Dyslipidemia: A Meta-analysis

BACKGROUND

Lipid metabolism is a complex process that includes lipid uptake, transport, synthesis, and degradation. Trace elements are vital in maintaining normal lipid metabolism in the human body. This study explores the relationship between serum trace elements and lipid metabolism.

METHODS

In this study, we reviewed articles on the relationship between alterations in somatic levels of zinc, iron, calcium, copper, chrome, manganese, selenium, and lipid metabolism. In this systematic review and mate-analysis, databases such as PubMed, Web of Science, and China National Knowledge Infrastructure (CNKI), Wanfang was searched for articles on the relationship published between January 1, 1900, and July 12, 2022. The meta-analysis was performed using Review Manager5.3 (Cochrane Collaboration).

RESULTS

No significant association was found between serum zinc and dyslipidemia, while other serum trace elements (iron, selenium, copper, chromium, and manganese) were associated with hyperlipidemia.

CONCLUSION

The present study suggested that the human body's zinc, copper, and calcium content may be related to lipid metabolism. However, findings on lipid metabolism and Iron, Manganese have not been conclusive. In addition, the relationship between lipid metabolism disorders and selenium levels still needs to be further studied. Further research is needed on treating lipid metabolism diseases by changing trace elements.

Selenium-More than Just a Fortuitous Sulfur Substitute in Redox Biology

Living organisms use selenium mainly in the form of selenocysteine in the active site of oxidoreductases. Here, selenium's unique chemistry is believed to modulate the reaction mechanism and enhance the catalytic efficiency of specific enzymes in ways not achievable with a sulfur-containing cysteine. However, despite the fact that selenium/sulfur have different physicochemical properties, several selenoproteins have fully functional cysteine-containing homologues and some organisms do not use selenocysteine at all. In this review, selected selenocysteine-containing proteins will be discussed to showcase both situations: (i) selenium as an obligatory element for the protein's physiological function, and (ii) selenium presenting no clear advantage over sulfur (functional proteins with either selenium or sulfur). Selenium's physiological roles in antioxidant defence (to maintain cellular redox status/hinder oxidative stress), hormone metabolism, DNA synthesis, and repair (maintain genetic stability) will be also highlighted, as well as selenium's role in human health. Formate dehydrogenases, hydrogenases, glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases will be herein featured.

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.

Synthesis of Selenium Nanoparticles Modified by Quaternary Chitosan Covalently Bonded with Gallic Acid

Quaternary chitosan derivative with covalently bonded antioxidant (QCG) was used as media for synthesis of selenium nanoparticles (SeNPs). SeNPs were characterized using AFM, TEM, and DLS methods. The data confirmed the formation of stable nanoparticles with a positive charge (34.86-46.73 mV) and a size in the range 119.5-238.6 nm. The antibacterial and fungicidal activity of SeNPs occurred within the range of values for chitosan derivatives. In all cases, the highest activity was against C. albicans (MIC 125 µg/mL). The toxicity of the modified selenium nanoparticles to eukaryotic cells was significantly higher. Among nanoparticle samples, SeNPs that were synthesized at 55 °C demonstrated the highest toxicity against Colo357 and HaCaT cell lines. Based on these results, SeNPs loaded with doxorubicin were obtained. DOX loading efficiency was about 18%. QCG-SeNPs loaded with DOX at a concentration of 1.25 μg/mL inhibited more than 50% of hepatocarcinoma (Colo 357) cells and about 70% of keratinocytes (HaCaT).

Recent progress in the development of small-molecule fluorescent probes for detection and imaging of selenocysteine and application in thyroid disease diagnosis

Selenocysteine (SeCys) is the 21st genetically encoded amino acid present in proteins and is involved in various biological functions. Inappropriate levels of SeCys can be considered as a sign of various diseases. Therefore, small molecular fluorescent probes for the detection and imaging of SeCys in vivo in biological systems are considered to be of significant interest for understanding the physiological role of SeCys. Thus, this article mainly provides a critical evaluation of recent advances made in SeCys detection along with the biomedical applications based on small molecular fluorescent probes published in the literature during the past half a dozen years. Therefore, the article primarily deals with the rational design of fluorescent probes, wherein these were selective towards SeCys over other biologically abundant molecules, in particular the thiol-based ones. The detection has been monitored by different spectral techniques, such as fluorescence and absorption spectroscopy and in some cases even visual color changes. Further, the detection mechanism and the utility of fluorescent probes for in vitro and in vivo cell imaging applications are addressed. For clarity, the main features have been conveniently divided into four categories based on the chemical reactions of the probe, viz., in terms of the cleavage of the responsive group by the SeCys nucleophile: (i) 2,4-dinitrobene sulphonamide group, (ii) 2,4-dinitrobenesulfonate ester group, (iii) 2,4-dinitrobenzeneoxy group and (iv) miscellaneous types. Overall this article deals with the analysis of more than two dozen fluorescent probes demonstrated for selective detection of SeCys along with their applications towards disease diagnosis.

Selenoproteins and the senescence-associated epitranscriptome

Selenium is a naturally found trace element, which provides multiple benefits including antioxidant, anticancer, and antiaging, as well as boosting immunity. One unique feature of selenium is its incorporation as selenocysteine, a rare 21st amino acid, into selenoproteins. Twenty-five human selenoproteins have been discovered, and a majority of these serve as crucial antioxidant enzymes for redox homeostasis. Unlike other amino acids, incorporation of selenocysteine requires a distinctive UGA stop codon recoding mechanism. Although many studies correlating selenium, selenoproteins, aging, and senescence have been performed, it has not yet been explored if the upstream events regulating selenoprotein synthesis play a role in senescence-associated pathologies. The epitranscriptomic writer alkylation repair homolog 8 (ALKBH8) is critical for selenoprotein production, and its deficiency can significantly decrease levels of selenoproteins that are essential for reactive oxygen species (ROS) detoxification, and increase oxidative stress, one of the major drivers of cellular senescence. Here, we review the potential role of epitranscriptomic marks that govern selenocysteine utilization in regulating the senescence program.

Revealing PACMA 31 as a new chemical type TrxR inhibitor to promote cancer cell apoptosis

Thioredoxin reductase (TrxR) is a pivotal regulator of redox homeostasis, while dysregulation of redox homeostasis is a hallmark for cancer cells. Thus, there is considerable potential to inhibit the aberrantly upregulated TrxR in cancer cells to discover selective cancer therapeutic agents. Nevertheless, the structural types of TrxR inhibitors presented currently are still relatively limited. We herein report that PACMA 31, previously reported to inhibit protein disulfide isomerase (PDI), is a potent TrxR inhibitor. PACMA 31 possesses a pharmacophore scaffold that is structurally different from the announced TrxR inhibitors and exhibits effective cytotoxicity against cervical cancer cells. Our results reveal that PACMA 31 selectively inhibits TrxR over the related glutathione reductase (GR) and in the presence of reduced glutathione (GSH). Further studies with mutant enzyme and molecular docking suggest that the propynamide fragment of PACMA 31 interacts covalently with the selenocysteine residue of TrxR. Moreover, PACMA 31 effectively and selectively curbs TrxR activity in cells and further stimulates the production of reactive oxygen species (ROS) at low micromolar concentrations, which in turn triggers the accumulation of oxidized thioredoxin (Trx) and GSSG in cells. Follow-up studies demonstrate that PACMA 31 targets TrxR in cells to induce oxidative stress-mediated cancer cell apoptosis. Our results provide a new structural type of TrxR inhibitor that may serve as a useful probe for investigating the biology of TrxR-implicated pathways, and uncover a new target of PACMA 31 that contributes to it becoming a candidate for cancer treatment.

Voyage of selenium from environment to life: Beneficial or toxic?

Selenium (Se), a naturally occurring metalloid, is an essential micronutrient for life as it is incorporated as selenocysteine in proteins. Although beneficial at low doses, Se is hazardous at high concentrations and poses a serious threat to various ecosystems. Due to this contrasting 'dual' nature, Se has garnered the attention of researchers wishing to unravel its puzzling properties. In this review, we describe the impact of selenium's journey from environment to diverse biological systems, with an emphasis on its chemical advantage. We describe the uneven distribution of Se and how this affects the bioavailability of this element, which, in turn, profoundly affects the habitat of a region. Once taken up, the subsequent incorporation of Se into proteins as selenocysteine and its antioxidant functions are detailed here. The causes of improved protein function due to the incorporation of redox-active Se atom (instead of S) are examined. Subsequently, the reasons for the deleterious effects of Se, which depend on its chemical form (organo-selenium or the inorganic forms) in different organisms are elaborated. Although Se is vital for the function of many antioxidant enzymes, how the pro-oxidant nature of Se can be potentially exploited in different therapies is highlighted. Furthermore, we succinctly explain how the presence of Se in biological systems offsets the toxic effects of heavy metal mercury. Finally, the different avenues of research that are fundamental to expand our understanding of selenium biology are suggested.

Escherichia coli tRNA 2-Selenouridine Synthase (SelU): Elucidation of Substrate Specificity to Understand the Role of S-Geranyl-tRNA in the Conversion of 2-Thio- into 2-Selenouridines in Bacterial tRNA

The bacterial enzyme tRNA 2-selenouridine synthase (SelU) is responsible for the conversion of 5-substituted 2-thiouridine (R5S2U), present in the anticodon of some bacterial tRNAs, into 5-substituted 2-selenouridine (R5Se2U). We have already demonstrated using synthetic RNAs that transformation S2U→Se2U is a two-step process, in which the S2U-RNA is geranylated and the resulting geS2U-RNA is selenated. Currently, the question is how SelU recognizes its substrates and what the cellular pathway of R5S2U→R5Se2U conversion is in natural tRNA. In the study presented here, we characterized the SelU substrate requirements, identified SelU-associated tRNAs and their specific modifications in the wobble position. Finally, we explained the sequence of steps in the selenation of tRNA. The S2U position within the RNA chain, the flanking sequence of the modification, and the length of the RNA substrate, all have a key influence on the recognition by SelU. MST data on the affinity of SelU to individual RNAs confirmed the presumed process. SelU binds the R5S2U-tRNA and then catalyzes its geranylation to the R5geS2U-tRNA, which remains bound to the enzyme and is selenated in the next step of the transformation. Finally, the R5Se2U-tRNA leaves the enzyme and participates in the translation process. The enzyme does not directly catalyze the R5S2U-tRNA selenation and the R5geS2U-tRNA is the intermediate product in the linear sequence of reactions.

Selective cellular probes for mammalian thioredoxin reductase TrxR1: Rational design of RX1, a modular 1,2-thiaselenane redox probe

Quantifying the activity of key cellular redox players is crucial for understanding physiological homeostasis, and for targeting their perturbed states in pathologies including cancer and inflammatory diseases. However, cellularly-selective probes for oxidoreductase turnover are sorely lacking. We rationally developed the first probes that selectively target the mammalian selenoprotein thioredoxin reductase (TrxR), using a cyclic selenenylsulfide oriented to harness TrxR's unique selenolthiol chemistry while resisting the cellular monothiol background. Lead probe RX1 had excellent TrxR1-selective performance in cells, cross-validated by knockout, selenium starvation, knock-in, and chemical inhibitors. Its background-free fluorogenicity enabled us to perform the first quantitative high-throughput live cell screen for TrxR1 inhibitors, which indicated that tempered S_NAr electrophiles may be more selective TrxR drugs than the classical electrophiles used hitherto. The RX1 design thus sets the stage for in vivo imaging of the activity of this key oxidoreductase in health and disease, and can also drive TrxR1-inhibitor drug design.

Harnessing selenocysteine to enhance microbial cell factories for hydrogen production

Hydrogen is a clean, renewable energy source, that when combined with oxygen, produces heat and electricity with only water vapor as a biproduct. Furthermore, it has the highest energy content by weight of all known fuels. As a result, various strategies have engineered methods to produce hydrogen efficiently and in quantities that are of interest to the economy. To approach the notion of producing hydrogen from a biological perspective, we take our attention to hydrogenases which are naturally produced in microbes. These organisms have the machinery to produce hydrogen, which when cleverly engineered, could be useful in cell factories resulting in large production of hydrogen. Not all hydrogenases are efficient at hydrogen production, and those that are, tend to be oxygen sensitive. Therefore, we provide a new perspective on introducing selenocysteine, a highly reactive proteinogenic amino acid, as a strategy towards engineering hydrogenases with enhanced hydrogen production, or increased oxygen tolerance.

Integration of a Diselenide Unit Generates Fluorogenic Camptothecin Prodrugs with Improved Cytotoxicity to Cancer Cells

A diselenide/disulfide unit was introduced into camptothecin (CPT), and two selenoprodrugs (e.g., CPT-Se3 and CPT-Se4) were identified to show improved potency in killing cancer cells and inhibiting tumor growth in vivo. Interestingly, the intrinsic fluorescence of CPT was severely quenched by the diselenide bond. Both the selenoprodrugs were activated by glutathione with a nearly complete recovery of CPT's fluorescence. The activation of prodrugs was accompanied by the production of selenol intermediates, which catalyzed the constant conversion of glutathione and oxygen to oxidized glutathione and superoxides. The diselenide unit is widely employed in constructing thiol-responsive materials. However, the selenol intermediates were largely ignored in the activation process prior to this study. Our work verified that integration of the diselenide unit may further enhance the parent drug's efficacy. Also, the discovery of the fluorescence quenching property of the diselenide/disulfide bond further shed light on constructing novel theranostic agents.

Onopordopicrin from the new genus Shangwua as a novel thioredoxin reductase inhibitor to induce oxidative stress-mediated tumor cell apoptosis

Isolation and identification of natural products from plants is an essential approach for discovering drug candidates. Herein we report the characterization of three sesquiterpene lactones from a new genus Shangwua, e.g. onopordopicrin (ONP), C2, and C3, and evaluation of their pharmacological functions in interfering cellular redox signaling. Compared to C2 and C3, ONP shows the most potency in killing cancer cells. Further experiments demonstrate that ONP robustly inhibits thioredoxin reductase (TrxR), which leads to perturbation of cellular redox homeostasis with the favor of oxidative stress. Knockdown of the TrxR sensitizes cells to the ONP treatment while overexpression of the enzyme reduces the potency of ONP, underpinning the correlation of TrxR inhibition to the cytotoxicity of ONP. The discovery of ONP expands the library of the natural TrxR inhibitors, and the disclosure of the action mechanism of ONP provides a foundation for the further development of ONP as an anticancer agent.

Intein-based Design Expands Diversity of Selenocysteine Reporters

The presence of selenocysteine in a protein confers many unique properties that make the production of recombinant selenoproteins desirable. Targeted incorporation of Sec into a protein of choice is possible by exploiting elongation factor Tu-dependent reassignment of UAG codons, a strategy that has been continuously improved by a variety of means. Improving selenoprotein yield by directed evolution requires selection and screening markers that are titratable, have a high dynamic range, enable high-throughput screening, and can discriminate against nonspecific UAG decoding. Current screening techniques are limited to a handful of reporters where a cysteine (Cys) or Sec residue normally affords activity. Unfortunately, these existing Cys/Sec-dependent reporters lack the dynamic range of more ubiquitous reporters or suffer from other limitations. Here we present a versatile strategy to adapt established reporters for specific Sec incorporation. Inteins are intervening polypeptides that splice themselves from the precursor protein in an autocatalytic splicing reaction. Using an intein that relies exclusively on Sec for splicing, we show that this intein cassette can be placed in-frame within selection and screening markers, affording reporter activity only upon successful intein splicing. Furthermore, because functional splicing can only occur when a catalytic Sec is present, the amount of synthesized reporter directly measures UAG-directed Sec incorporation. Importantly, we show that results obtained with intein-containing reporters are comparable to the Sec incorporation levels determined by mass spectrometry of isolated recombinant selenoproteins. This result validates the use of these intein-containing reporters to screen for evolved components of a translation system yielding increased selenoprotein amounts.

Organoselenium ligands for heterogeneous and nanocatalytic systems: development and applications

Organoselenium ligands have attracted great attention among researchers during the past two decades. Various homogeneous, heterogeneous and nanocatalytic systems have been designed using such ligands. Although reports on selenium ligated homogeneous catalysts are quite high in number, significant work has also been done on the development of heterogeneous and nanocatalytic systems using organoselenium ligands. A review article, focusing on the utility of organoselenium compounds in the development of catalytic systems, was published in 2012 (A. Kumar, G. K. Rao, F. Saleem and A. K. Singh, Dalton Trans., 2012, 41, 11949). Moreover, it mainly covered the homogeneous catalysts. There are no review articles in the literature on heterogeneous and nanocatalytic systems designed using organoselenium compounds and their applications. Hence, this perspective aims to cover the developments pertaining to the synthetic aspects of such catalytic systems (using organoselenium compounds) and their applications in catalysis of a variety of chemical transformations. Salient features and advantages of organoselenium compounds have also been highlighted to justify the rationale behind their use in catalyst development. Their performance in various chemical transformations [viz. Suzuki-Miyaura coupling, Heck coupling, Sonogashira coupling, O-arylation of phenol, transfer hydrogenation of aldehydes and ketones, aldehyde-alkyne-amine (A3) coupling, hydration of nitriles, conversion of aldehydes to amides, cross-dehydrogenative coupling (CDC), photodegradation of substrates (formic acid, methylene blue), reduction of nitrophenols, electrolysis (hydrogen evolution reaction and oxygen reduction reactions), organocatalysis and dye sensitized solar cells] and relevant aspects of catalytic processes (such as recyclability, substrate scope and green aspects) have been critically analyzed. Future perspectives have also been discussed.

Selective, Modular Probes for Thioredoxins Enabled by Rational Tuning of a Unique Disulfide Structure Motif

Specialized cellular networks of oxidoreductases coordinate the dithiol/disulfide-exchange reactions that control metabolism, protein regulation, and redox homeostasis. For probes to be selective for redox enzymes and effector proteins (nM to μM concentrations), they must also be able to resist non-specific triggering by the ca. 50 mM background of non-catalytic cellular monothiols. However, no such selective reduction-sensing systems have yet been established. Here, we used rational structural design to independently vary thermodynamic and kinetic aspects of disulfide stability, creating a series of unusual disulfide reduction trigger units designed for stability to monothiols. We integrated the motifs into modular series of fluorogenic probes that release and activate an arbitrary chemical cargo upon reduction, and compared their performance to that of the literature-known disulfides. The probes were comprehensively screened for biological stability and selectivity against a range of redox effector proteins and enzymes. This design process delivered the first disulfide probes with excellent stability to monothiols yet high selectivity for the key redox-active protein effector, thioredoxin. We anticipate that further applications of these novel disulfide triggers will deliver unique probes targeting cellular thioredoxins. We also anticipate that further tuning following this design paradigm will enable redox probes for other important dithiol-manifold redox proteins, that will be useful in revealing the hitherto hidden dynamics of endogenous cellular redox systems.

Nickel as a virulence factor in the Class I bacterial carcinogen, Helicobacter pylori

Helicobacter pylori is a human bacterial pathogen that causes peptic ulcers and has been designated a Class I carcinogen by the International Agency for Research on Cancer (IARC). Its ability to survive in the acid environment of the stomach, to colonize the stomach mucosa, and to cause cancer, are linked to two enzymes that require nickel-urease and hydrogenase. Thus, nickel is an important virulence factor and the proteins involved in nickel trafficking are potential antibiotic targets. This review summarizes the nickel biochemistry of H. pylori with a focus on the roles of nickel in virulence, nickel homeostasis, maturation of urease and hydrogenase, and the unique nickel trafficking that occurs between the hydrogenase maturation pathway and urease nickel incorporation that is mediated by the metallochaperone HypA and its partner, HypB.

Did Amino Acid Side Chain Reactivity Dictate the Composition and Timing of Aminoacyl-tRNA Synthetase Evolution?

The twenty amino acids in the standard genetic code were fixed prior to the last universal common ancestor (LUCA). Factors that guided this selection included establishment of pathways for their metabolic synthesis and the concomitant fixation of substrate specificities in the emerging aminoacyl-tRNA synthetases (aaRSs). In this conceptual paper, we propose that the chemical reactivity of some amino acid side chains (e.g., lysine, cysteine, homocysteine, ornithine, homoserine, and selenocysteine) delayed or prohibited the emergence of the corresponding aaRSs and helped define the amino acids in the standard genetic code. We also consider the possibility that amino acid chemistry delayed the emergence of the glutaminyl- and asparaginyl-tRNA synthetases, neither of which are ubiquitous in extant organisms. We argue that fundamental chemical principles played critical roles in fixation of some aspects of the genetic code pre- and post-LUCA.

Redox reactions of organoselenium compounds: Implication in their biological activity

Antioxidant activity of organoselenium compounds belonging to different classes i.e. functionalized aliphatic, aromatic and cyclic selenoethers, are compared on the basis of their ability to scavenge reactive oxygen species like hydroxyl and peroxyl radicals and to exhibit glutathione peroxidase (GPx) like catalytic activity. The comparative analysis has revealed that the antioxidant activity of the organoselenium compounds show direct correlation with the energy of the highest occupied molecular orbital (HOMO) and neighboring group participation that stabilizes the reaction intermediate. Finally, structural features responsible for improving the rate of reaction of organoselenium compounds with free radical/molecular oxidants have been discussed on the basis of the compounds screened at our institute.

Can Selenoenzymes Resist Electrophilic Modification? Evidence from Thioredoxin Reductase and a Mutant Containing α-Methylselenocysteine

Selenocysteine (Sec) is the 21st proteogenic amino acid in the genetic code. Incorporation of Sec into proteins is a complex and bioenergetically costly process that evokes the following question: "Why did nature choose selenium?" An answer that has emerged over the past decade is that Sec confers resistance to irreversible oxidative inactivation by reactive oxygen species. Here, we explore the question of whether this concept can be broadened to include resistance to reactive electrophilic species (RES) because oxygen and related compounds are merely a subset of RES. To test this hypothesis, we inactivated mammalian thioredoxin reductase (Sec-TrxR), a mutant containing α-methylselenocysteine [(αMe)Sec-TrxR], and a cysteine ortholog TrxR (Cys-TrxR) with various electrophiles, including acrolein, 4-hydroxynonenal, and curcumin. Our results show that the acrolein-inactivated Sec-TrxR and the (αMe)Sec-TrxR mutant could regain 25% and 30% activity, respectively, when incubated with 2 mM H₂O₂ and 5 mM imidazole. In contrast, Cys-TrxR did not regain activity under the same conditions. We posit that Sec enzymes can undergo a repair process via β-syn selenoxide elimination that ejects the electrophile, leaving the enzyme in the oxidized selenosulfide state. (αMe)Sec-TrxR was created by incorporating the non-natural amino acid (αMe)Sec into TrxR by semisynthesis and allowed for rigorous testing of our hypothesis. This Sec derivative enables higher resistance to both oxidative and electrophilic inactivation because it lacks a backbone C_α-H, which prevents loss of selenium through the formation of dehydroalanine. This is the first time this unique amino acid has been incorporated into an enzyme and is an example of state-of-the-art protein engineering.

Different Oxidation Pathways of 2-Selenouracil and 2-Thiouracil, Natural Components of Transfer RNA

Sulfur- and selenium-modified uridines present in the wobble position of transfer RNAs (tRNAs) play an important role in the precise reading of genetic information and tuning of protein biosynthesis in all three domains of life. Both sulfur and selenium chalcogens functionally operate as key elements of biological molecules involved in the protection of cells against oxidative damage. In this work, 2-thiouracil (S2Ura) and 2-selenouracil (Se2Ura) were treated with hydrogen peroxide at 1:0.5, 1:1, and 1:10 molar ratios and at selected pH values ranging from 5 to 8. It was found that Se2Ura was more prone to oxidation than its sulfur analog, and if reacted with H₂O₂ at a 1:1 or lower molar ratio, it predominantly produced diselenide Ura-Se-Se-Ura, which spontaneously transformed to a previously unknown Se-containing two-ring compound. Its deselenation furnished the major reaction product, a structure not related to any known biological species. Under the same conditions, only a small amount of S2Ura was oxidized to form Ura-SO₂H and uracil (Ura). In contrast, 10-fold excess hydrogen peroxide converted Se2Ura and S2Ura into corresponding Ura-SeO_nH and Ura-SO_nH intermediates, which decomposed with the release of selenium and sulfur oxide(s) to yield Ura as either a predominant or exclusive product, respectively. Our results confirmed significantly different oxidation pathways of 2-selenouracil and 2-thiouracil.

The roles of chalcogenides in O2 protection of H2ase active sites

At some point, all HER (Hydrogen Evolution Reaction) catalysts, important in sustainable H2O splitting technology, will encounter O2 and O2-damage. The [NiFeSe]-H2ases and some of the [NiFeS]-H2ases, biocatalysts for reversible H2 production from protons and electrons, are exemplars of oxygen tolerant HER catalysts in nature. In the hydrogenase active sites oxygen damage may be extensive (irreversible) as it is for the [FeFe]-H2ase or moderate (reversible) for the [NiFe]-H2ases. The affinity of oxygen for sulfur, in [NiFeS]-H2ase, and selenium, in [NiFeSe]-H2ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. In contrast, the [FeFe]-H2ase active site has less possibilities for chalcogen-oxygen uptake and a greater chance for O2-attack on iron. Exposure to O2 typically leads to irreversible damage. Despite the evidence of S/Se-oxygenation in the active sites of hydrogenases, there are limited reported synthetic models. This perspective will give an overview of the studies of O2 reactions with the hydrogenases and biomimetics with focus on our recent studies that compare sulfur and selenium containing synthetic analogues of the [NiFe]-H2ase active sites.

Sanguinarine as a new chemical entity of thioredoxin reductase inhibitor to elicit oxidative stress and promote tumor cell apoptosis

The alteration of redox homeostasis is a hallmark of cancer cells. As a critical player in regulating cellular redox signaling, thioredoxin reductase (TrxR) enzymes are increasingly recognized as attractive targets for anticancer drug development. We reported herein the natural product sanguinarine (SAN) as a potent inhibitor of TrxR with a new chemical scaffold. Inhibition of TrxR leads to accumulation of the oxidized thioredoxin, elicits oxidative stress, and finally promotes apoptosis of cancer cells. Further synthesis of different model compounds of SAN demonstrated that the phenanthridinium unit is responsible for the TrxR inhibition. The core structure of SAN, e.g., the phenanthridinium moiety, is different from those of known TrxR inhibitors, and thus SAN is a new chemical entity of TrxR inhibitors and may serve a lead for further development. In addition, as the phenanthridinium scaffold is widely present in natural products, the disclosure of TrxR inhibition by such unit sheds light in understanding the pharmacological actions of these molecules.

Actinobacterial Degradation of 2-Hydroxyisobutyric Acid Proceeds via Acetone and Formyl-CoA by Employing a Thiamine-Dependent Lyase Reaction

The tertiary branched short-chain 2-hydroxyisobutyric acid (2-HIBA) has been associated with several metabolic diseases and lysine 2-hydroxyisobutyrylation seems to be a common eukaryotic as well as prokaryotic post-translational modification in proteins. In contrast, the underlying 2-HIBA metabolism has thus far only been detected in a few microorganisms, such as the betaproteobacterium Aquincola tertiaricarbonis L108 and the Bacillus group bacterium Kyrpidia tusciae DSM 2912. In these strains, 2-HIBA can be specifically activated to the corresponding CoA thioester by the 2-HIBA-CoA ligase (HCL) and is then isomerized to 3-hydroxybutyryl-CoA in a reversible and B₁₂-dependent mutase reaction. Here, we demonstrate that the actinobacterial strain Actinomycetospora chiangmaiensis DSM 45062 degrades 2-HIBA and also its precursor 2-methylpropane-1,2-diol via acetone and formic acid by employing a thiamine pyrophosphate-dependent lyase. The corresponding gene is located directly upstream of hcl, which has previously been found only in operonic association with the 2-hydroxyisobutyryl-CoA mutase genes in other bacteria. Heterologous expression of the lyase gene from DSM 45062 in E. coli established a 2-hydroxyisobutyryl-CoA lyase activity in the latter. In line with this, analysis of the DSM 45062 proteome reveals a strong induction of the lyase-HCL gene cluster on 2-HIBA. Acetone is likely degraded via hydroxylation to acetol catalyzed by a MimABCD-related binuclear iron monooxygenase and formic acid appears to be oxidized to CO₂ by selenium-dependent dehydrogenases. The presence of the lyase-HCL gene cluster in isoprene-degrading Rhodococcus strains and Pseudonocardia associated with tropical leafcutter ant species points to a role in degradation of biogenic short-chain ketones and highly branched organic compounds.

Catalysis of Peroxide Reduction by Fast Reacting Protein Thiols

Life on Earth evolved in the presence of hydrogen peroxide, and other peroxides also emerged before and with the rise of aerobic metabolism. They were considered only as toxic byproducts for many years. Nowadays, peroxides are also regarded as metabolic products that play essential physiological cellular roles. Organisms have developed efficient mechanisms to metabolize peroxides, mostly based on two kinds of redox chemistry, catalases/peroxidases that depend on the heme prosthetic group to afford peroxide reduction and thiol-based peroxidases that support their redox activities on specialized fast reacting cysteine/selenocysteine (Cys/Sec) residues. Among the last group, glutathione peroxidases (GPxs) and peroxiredoxins (Prxs) are the most widespread and abundant families, and they are the leitmotif of this review. After presenting the properties and roles of different peroxides in biology, we discuss the chemical mechanisms of peroxide reduction by low molecular weight thiols, Prxs, GPxs, and other thiol-based peroxidases. Special attention is paid to the catalytic properties of Prxs and also to the importance and comparative outlook of the properties of Sec and its role in GPxs. To finish, we describe and discuss the current views on the activities of thiol-based peroxidases in peroxide-mediated redox signaling processes.

Processive Recoding and Metazoan Evolution of Selenoprotein P: Up to 132 UGAs in Molluscs

Selenoproteins typically contain a single selenocysteine, the 21st amino acid, encoded by a context-redefined UGA. However, human selenoprotein P (SelenoP) has a redox-functioning selenocysteine in its N-terminal domain and nine selenium transporter-functioning selenocysteines in its C-terminal domain. Here we show that diverse SelenoP genes are present across metazoa with highly variable numbers of Sec-UGAs, ranging from a single UGA in certain insects, to 9 in common spider, and up to 132 in bivalve molluscs. SelenoP genes were shaped by a dynamic evolutionary process linked to selenium usage. Gene evolution featured modular expansions of an ancestral multi-Sec domain, which led to particularly Sec-rich SelenoP proteins in many aquatic organisms. We focused on molluscs, and chose Pacific oyster Magallana gigas as experimental model. We show that oyster SelenoP mRNA with 46 UGAs is translated full-length in vivo. Ribosome profiling indicates that selenocysteine specification occurs with ∼5% efficiency at UGA1 and approaches 100% efficiency at distal 3' UGAs. We report genetic elements relevant to its expression, including a leader open reading frame and an RNA structure overlapping the initiation codon that modulates ribosome progression in a selenium-dependent manner. Unlike their mammalian counterparts, the two SECIS elements in oyster SelenoP (3'UTR recoding elements) do not show functional differentiation in vitro. Oysters can increase their tissue selenium level up to 50-fold upon supplementation, which also results in extensive changes in selenoprotein expression.

Selenium cytotoxicity in Tetrahymena thermophila: New clues about its biological effects and cellular resistance mechanisms

Selenium is an essential micronutrient but at high concentrations can produce severe cytotoxicity and genomic damage. We have evaluated the cytotoxicity, ultrastructural and mitochondrial alterations of the two main selenium inorganic species; selenite and selenate, in the eukaryotic microorganism Tetrahymena thermophila. In this ciliate, selenite is more toxic than selenate. Their LC₅₀ values were calculated as 27.65 μM for Se(IV) and 56.88 mM for Se(VI). Significant levels of peroxides/hydroperoxides are induced under low-moderate selenite or selenate concentrations. Se(VI) exposures induce an immediate mitochondrial membrane depolarization. Selenium treated cells show an intense vacuolization and some of them present numerous discrete and small electrondense particles, probably selenium deposits. Mitochondrial fusion, an intense swelling in peripheral mitochondria and mitophagy are detected in selenium treated cells, especially in those exposed to Se (IV). qRT-PCR analysis of diverse genes, encoding relevant antioxidant enzymes or other proteins, like metallothioneins, involved in an environmental general stress response, have shown that they may be crucial against Se(IV) and/or Se (VI) cytotoxicity.

Oxidation of organic diselenides and ditellurides by H2O2 for bioinspired catalyst design

The reactivity of diselenides and ditellurides of general formula (RX)2 (X = Se, Te; R = H, CH3, Ph) toward hydrogen peroxide was studied through a computational approach based on accurate Density Functional Theory (DFT) calculations. The aliphatic and aromatic dichalcogenides have been chosen in light of their activity in glutathione peroxidase (GPx)-like catalytic cycles and their promising features as efficient antioxidant compounds. The reaction products, the energetics and the mechanistic details of these oxidations are discussed. Analogous disulfides are included in our analysis for completeness. We find that the barrier for oxidation of dichalcogenides decreases from disulfides to diselenides to ditellurides. On the other hand, variation of the substituents at the chalcogen nucleus has relatively little effect on the reactivity.

Characterization of the [NiFeSe] hydrogenase from Desulfovibrio vulgaris Hildenborough

The [NiFeSe] hydrogenases are a subgroup of the well-characterized family of [NiFe] hydrogenases, in which a selenocysteine is a ligand to the nickel atom in the binuclear NiFe active site instead of cysteine. These enzymes display very interesting catalytic properties for biological hydrogen production and bioelectrochemical applications: high H₂ production activity, bias for H₂ evolution, low H₂ inhibition, and some degree of O₂ tolerance. Here we describe the methodologies employed to study the [NiFeSe] hydrogenase isolated from the sulfate-reducing bacteria D. vulgaris Hildenborough and the creation of a homologous expression system for production of variant forms of the enzyme.

Selenium as a pleiotropic agent for medical discovery and drug delivery

Selenium as a biologically active element lends much support to health maintenance and disease prevention. It is now presenting pleiotropic effects on therapy and drug delivery. In this study, a profiling on the physiological functions, therapeutic significances, clinical/preclinical performances, and biomedical and drug delivery applications of selenium in different modalities was carried out. Major interests focused on selenium-based nanomedicines in confronting various diseases pertaining to selenium or not, especially in antitumor and antidiabetes. Furthermore, the article exclusively discusses selenium nanoparticles featured by ameliorative functions with emphasis on their applications in medical practice and drug delivery. The state-of-the-art in medical discovery as well as research and development on selenium and nano-selenium is discussed in this review.