Escherichia coli NifS-like proteins provide selenium in the pathway for the biosynthesis of selenophosphate

Selenium metabolism and selenoproteins function in brain and encephalopathy

Selenium (Se) is an essential trace element of the utmost importance to human health. Its deficiency induces various disorders. Se species can be absorbed by organisms and metabolized to hydrogen selenide for the biosynthesis of selenoproteins, selenonucleic acids, or selenosugars. Se in mammals mainly acts as selenoproteins to exert their biological functions. The brain ranks highest in the specific hierarchy of organs to maintain the level of Se and the expression of selenoproteins under the circumstances of Se deficiency. Dyshomeostasis of Se and dysregulation of selenoproteins result in encephalopathy such as Alzheimer's disease, Parkinson's disease, depression, amyotrophic lateral sclerosis, and multiple sclerosis. This review provides a summary and discussion of Se metabolism, selenoprotein function, and their roles in modulating brain diseases based on the most currently published literature. It focuses on how Se is utilized and transported to the brain, how selenoproteins are biosynthesized and function physiologically in the brain, and how selenoproteins are involved in neurodegenerative diseases. At the end of this review, the perspectives and problems are outlined regarding Se and selenoproteins in the regulation of encephalopathy.

PRDX6 contributes to selenocysteine metabolism and ferroptosis resistance

Selenocysteine (Sec) metabolism is crucial for cellular function and ferroptosis prevention and begins with the uptake of the Sec carrier, selenoprotein P (SELENOP). Following uptake, Sec released from SELENOP is metabolized via selenocysteine lyase (SCLY), producing selenide, a substrate for selenophosphate synthetase 2 (SEPHS2), which provides the essential selenium donor, selenophosphate (H2SePO3-), for the biosynthesis of the Sec-tRNA. Here, we discovered an alternative pathway in Sec metabolism mediated by peroxiredoxin 6 (PRDX6), independent of SCLY. Mechanistically, we demonstrate that PRDX6 can readily react with selenide and interact with SEPHS2, potentially acting as a selenium delivery system. Moreover, we demonstrate the functional significance of this alternative route in human cancer cells, revealing a notable association between elevated expression of PRDX6 and human MYCN-amplified neuroblastoma subtype. Our study sheds light on a previously unrecognized aspect of Sec metabolism and its implications in ferroptosis, offering further possibilities for therapeutic exploitation.

SEPHS1: Its evolution, function and roles in development and diseases

Selenophosphate synthetase (SEPHS) was originally discovered in prokaryotes as an enzyme that catalyzes selenophosphate synthesis using inorganic selenium and ATP as substrates. However, in contrast to prokaryotes, two paralogs, SEPHS1 and SEPHS2, occur in many eukaryotes. Prokaryotic SEPHS, also known as SelD, contains either cysteine (Cys) or selenocysteine (Sec) in the catalytic domain. In eukaryotes, only SEPHS2 carries out selenophosphate synthesis and contains Sec at the active site. However, SEPHS1 contains amino acids other than Sec or Cys at the catalytic position. Phylogenetic analysis of SEPHSs reveals that the ancestral SEPHS contains both selenophosphate synthesis and another unknown activity, and that SEPHS1 lost the selenophosphate synthesis activity. The three-dimensional structure of SEPHS1 suggests that its homodimer is unable to form selenophosphate, but retains ATPase activity to produce ADP and inorganic phosphate. The most prominent function of SEPHS1 is that it is implicated in the regulation of cellular redox homeostasis. Deficiency of SEPHS1 leads to the disturbance in the expression of genes involved in redox homeostasis. Different types of reactive oxygen species (ROS) are accumulated in response to SEPHS deficiency depending on cell or tissue types. The accumulation of ROS causes pleiotropic effects such as growth retardation, apoptosis, DNA damage, and embryonic lethality. SEPHS1 deficiency in mouse embryos affects retinoic signaling and other related signaling pathways depending on the embryonal stage until the embryo dies at E11.5. Dysregulated SEPHS1 is associated with the pathogenesis of various diseases including cancer, Crohn's disease, and osteoarthritis.

Selenium status and type 2 diabetes risk

Optimal selenium (Se) status is necessary for overall health. That status can be affected by food intake pattern, age, sex, and health status. At nutritional levels of intake, Se functions metabolically as an essential constituent of some two dozen selenoproteins, most, if not all, of which have redox functions. Insufficient dietary intake of Se reduces, to varying degrees, the expression of these selenoproteins. Recent clinical and animal studies have indicated that both insufficient and excessive Se intakes may increase risk of type 2 diabetes mellitus (T2D), perhaps by way of selenoprotein actions. In this review, we discuss the current evidence linking Se status and T2D risk, and the roles of 14 selenoproteins and other proteins involved in selenoprotein biosynthesis. Understanding such results can inform the setting of safe and adequate Se intakes.

The selenophosphate synthetase family: A review

Selenophosphate synthetases use selenium and ATP to synthesize selenophosphate. This is required for biological utilization of selenium, most notably for the synthesis of the non-canonical amino acid selenocysteine (Sec). Therefore, selenophosphate synthetases underlie all functions of selenoproteins, which include redox homeostasis, protein quality control, hormone regulation, metabolism, and many others. This protein family comprises two groups, SelD/SPS2 and SPS1. The SelD/SPS2 group represent true selenophosphate synthetases, enzymes central to selenium metabolism which are present in all Sec-utilizing organisms across the tree of life. Notably, many SelD/SPS2 proteins contain Sec as catalytic residue in their N-terminal flexible selenium-binding loop, while others replace it with cysteine (Cys). The SPS1 group comprises proteins originated through gene duplications of SelD/SPS2 in metazoa in which the Sec/Cys-dependent catalysis was disrupted. SPS1 proteins do not synthesize selenophosphate and are not required for Sec synthesis. They have essential regulatory functions related to redox homeostasis and pyridoxal phosphate, which affect signaling pathways for growth and differentiation. In this review, we summarize the knowledge about the selenophosphate synthetase family acquired through decades of research, encompassing their structure, mechanism, function, and evolution.

The physiology and evolution of microbial selenium metabolism

Selenium is an essential trace element whose compounds are widely metabolized by organisms from all three domains of life. Moreover, phylogenetic evidence indicates that selenium species, along with iron, molybdenum, tungsten, and nickel, were metabolized by the last universal common ancestor of all cellular lineages, primarily for the synthesis of the 21st amino acid selenocysteine. Thus, selenium metabolism is both environmentally ubiquitous and a physiological adaptation of primordial life. Selenium metabolic reactions comprise reductive transformations both for assimilation into macromolecules and dissimilatory reduction of selenium oxyanions and elemental selenium during anaerobic respiration. This review offers a comprehensive overview of the physiology and evolution of both assimilatory and dissimilatory selenium metabolism in bacteria and archaea, highlighting mechanisms of selenium respiration. This includes a thorough discussion of our current knowledge of the physiology of selenocysteine synthesis and incorporation into proteins in bacteria obtained from structural biology. Additionally, this is the first comprehensive discussion in a review of the incorporation of selenium into the tRNA nucleoside 5-methylaminomethyl-2-selenouridine and as an inorganic cofactor in certain molybdenum hydroxylase enzymes. Throughout, conserved mechanisms and derived features of selenium metabolism in both domains are emphasized and discussed within the context of the global selenium biogeochemical cycle.

Dietary Selenium Requirement for the Prevention of Glucose Intolerance and Insulin Resistance in Middle-Aged Mice

BACKGROUND

Although dietary selenium (Se) deficiency or excess induces type 2 diabetes-like symptoms in mice, suboptimal body Se status usually causes no symptoms but may promote age-related decline in overall health.

OBJECTIVES

We sought to determine the dietary Se requirement for protection against type 2 diabetes-like symptoms in mice.

METHODS

Thirty mature (aged 4 mo) male C57BL/6J mice were fed a Se-deficient torula yeast AIN-93M diet supplemented with Na2SeO4 in graded concentrations totaling 0.01 (basal), 0.04, 0.07, 0.10, and 0.13 (control) mg Se/kg for 4 mo (n = 6) until they were middle-aged (8 mo). Droplets of whole blood were used to determine glucose tolerance and insulin sensitivity in the mice from ages 5 to 8 mo. Postmortem serum, liver, and skeletal muscle were collected to assay for selenoprotein expression and markers of glucose metabolism. Data were analyzed by 1-way ANCOVA with or without random effects for time-repeated measurements using live mice or postmortem samples, respectively.

RESULTS

Compared with control, the consumption of basal diet increased (P < 0.05) fasting serum insulin (95% CI: 52%, 182%) and leptin (95% CI: 103%, 118%) concentrations in middle-aged mice. Dietary Se insufficiency decreased (P < 0.05) 1) glucose tolerance (13-79%) and insulin sensitivity (15-65%) at ≤0.10 mg Se/kg; 2) baseline thymoma viral proto-oncogene phosphorylation on S473 (27-54%) and T308 (22-46%) at ≤0.10 and ≤0.07 mg Se/kg, respectively, in the muscle but not the liver; and 3) serum glutathione peroxidase 3 (51-83%), liver and muscle glutathione peroxidase 1 (32-84%), serum and liver selenoprotein P (28-42%), and liver and muscle selenoprotein H (39-48%) and selenoprotein W (16-73%) protein concentrations at ≤0.04, ≤0.10, ≤0.07, and ≤0.10 mg Se/kg, respectively.

CONCLUSIONS

Mice fed diets containing ≤0.10 mg Se/kg display impaired glucose tolerance and insulin sensitivity, suggesting increased susceptibility to type 2 diabetes by suboptimal Se status at levels ≤23% of nutritional needs.

Trypanosomatid selenophosphate synthetase structure, function and interaction with selenocysteine lyase

Eukaryotes from the Excavata superphylum have been used as models to study the evolution of cellular molecular processes. Strikingly, human parasites of the Trypanosomatidae family (T. brucei, T. cruzi and L. major) conserve the complex machinery responsible for selenocysteine biosynthesis and incorporation in selenoproteins (SELENOK/SelK, SELENOT/SelT and SELENOTryp/SelTryp), although these proteins do not seem to be essential for parasite viability under laboratory controlled conditions. Selenophosphate synthetase (SEPHS/SPS) plays an indispensable role in selenium metabolism, being responsible for catalyzing the formation of selenophosphate, the biological selenium donor for selenocysteine synthesis. We solved the crystal structure of the L. major selenophosphate synthetase and confirmed that its dimeric organization is functionally important throughout the domains of life. We also demonstrated its interaction with selenocysteine lyase (SCLY) and showed that it is not present in other stable assemblies involved in the selenocysteine pathway, namely the phosphoseryl-tRNA^Sec kinase (PSTK)-Sec-tRNA^Sec synthase (SEPSECS) complex and the tRNA^Sec-specific elongation factor (eEFSec) complex. Endoplasmic reticulum stress with dithiothreitol (DTT) or tunicamycin upon selenophosphate synthetase ablation in procyclic T. brucei cells led to a growth defect. On the other hand, only DTT presented a negative effect in bloodstream T. brucei expressing selenophosphate synthetase-RNAi. Furthermore, selenoprotein T (SELENOT) was dispensable for both forms of the parasite. Together, our data suggest a role for the T. brucei selenophosphate synthetase in the regulation of the parasite’s ER stress response.

Selenium is both a toxic compound and a micronutrient. As a micronutrient, it participates in the synthesis of specific proteins, selenoproteins, as the amino acid selenocysteine. The synthesis of selenocysteine is present in organisms ranging from bacteria to humans. The protist parasites of the Trypanosomatidae family, that cause major tropical diseases, conserve the complex machinery responsible for selenocysteine biosynthesis and incorporation in selenoproteins. However, this pathway has been considered dispensable for the parasitic protist cells. This has intrigued us, and lead to question that if maintained in the cell it should be under selective pressure and therefore be necessary. Also, extensive and dynamic protein-protein interactions must happen to deliver selenium-containing intermediates along the pathway in order to warrant efficient usage of biological selenium in the cell. In this study we have investigated the molecular interactions of different proteins involved in selenocysteine synthesis and its putative involvement in the endoplasmic reticulum redox homeostasis.

Biotransformation of Selenium by Lactic Acid Bacteria: Formation of Seleno-Nanoparticles and Seleno-Amino Acids

Selenium (Se) is an essential micronutrient for the majority of living organisms, and it has been identified as selenocysteine in the active site of several selenoproteins such as glutathione peroxidase, thioredoxin reductase, and deiodinases. Se deficiency in humans is associated with viral infections, thyroid dysfunction, different types of cancer, and aging. In several European countries as well as in Argentina, Se intake is below the recommended dietary Intake (RDI). Some lactic acid bacteria (LAB) can accumulate and bio-transform selenite (toxic) into Se-nanoparticles (SeNPs) and Se-amino acids (non-toxic). The microbial growth, Se metabolite distribution, and the glutathione reductase (involved in selenite reduction) activity of Se-enriched LAB were studied in this work. The ninety-six assayed strains, belonging to the genera Lactococcus, Weissella, Leuconostoc, Lactobacillus, Enterococcus, and Fructobacillus could grow in the presence of 5 ppm sodium selenite. From the total, eight strains could remove more than 80% of the added Se from the culture medium. These bacteria accumulated intracellularly between 1.2 and 2.5 ppm of the added Se, from which F. tropaeoli CRL 2034 contained the highest intracellular amount. These strains produced only the seleno-amino acid SeCys as observed by LC-ICP-MS and confirmed by LC-ESI-MS/MS. The intracellular SeCys concentrations were between 0.015 and 0.880 ppm; Lb. brevis CRL 2051 (0.873 ppm), Lb. plantarum CRL 2030 (0.867 ppm), and F. tropaeoli CRL 2034 (0.625 ppm) were the strains that showed the highest concentrations. Glutathione reductase activity values were higher when the strains were grown in the presence of Se except for the F. tropaeoli CRL 2034 strain, which showed an opposite behavior. The cellular morphology of the strains was not affected by the presence of Se in the culture medium; interestingly, all the strains were able to form spherical SeNPs as determined by transmission electron microscopy (TEM). Only two Enterococcus strains produced the volatile Se compounds dimethyl-diselenide identified by GC-MS. Our results show that Lb. brevis CRL 2051, Lb. plantarum CRL 2030, and F. tropaeoli CRL 2034 could be used for the development of nutraceuticals or as starter cultures for the bio-enrichment of fermented fruit beverages with SeCys and SeNPs.

Evaluation of effects of selenium nanoparticles on Bacillus subtilis

The present study was performed to characterize of selenium nanoparticles (Nano-Se) which were synthesized by pulsed laser ablation in liquids to obtain the aqueous selenium citrate solution. The study was conducted using bacteriological and electronic-microscopic methods. Transmission electron microscopy (TEM) and spectroscopy analyses demonstrated that nano-selenium particles obtained by the method of selenium ablation had the size of 4–8 nm. UV-Visible Spectrum colloidal solution Nano-Se exhibited absorption maxima at 210 nm. To clarify some effects of the action of Nano-Se on Bacillus subtilis, we investigated the interaction of Nano-Se with B. subtilis IMV B-7392 before and after incubation with Nano-Se, examining TEM images. It has been shown that exposure to B. subtilis IMV B-7392 in the presence of Nano-Se is accompanied by the rapid uptake of Nano-Se by bacterial culture. TEM analysis found that the electron-dense Nano-Se particles were located in the intracellular spaces of B. subtilis IMV B-7392. That does not lead to changes in cultural and morphological characteristics of B. subtilis IMV B-7392. Using TEM, it has been shown that penetration of nanoparticles in the internal compartments is accompanied with transient porosity of the cell membrane of B. subtilis IMV B-7392 without rupturing it. The effective concentration of Nano-Se 0.2 × 10–3 mg/mL was found to increase the yield of biologically active substances of B. subtilis. In order to create probiotic nano-selenium containing products, the nutrient medium of B. subtilis IMV B-7392 was enriched with Nano-Se at 0.2 × 10–3 mg/mL. It was found that particles Nano-Se are non-toxic to the culture and did not exhibit bactericidal or bacteriostatic effects. The experimentally demonstrated ability of B. subtilis to absorb selenium nanoparticles has opened up the possibility of using Nano-Se as suitable drug carriers.

Selenocysteine β-Lyase: Biochemistry, Regulation and Physiological Role of the Selenocysteine Decomposition Enzyme

The enzyme selenocysteine β-lyase (SCLY) was first isolated in 1982 from pig livers, followed by its identification in bacteria. SCLY works as a homodimer, utilizing pyridoxal 5'-phosphate as a cofactor, and catalyzing the specific decomposition of the amino acid selenocysteine into alanine and selenide. The enzyme is thought to deliver its selenide as a substrate for selenophosphate synthetases, which will ultimately be reutilized in selenoprotein synthesis. SCLY subcellular localization is unresolved, as it has been observed both in the cytosol and in the nucleus depending on the technical approach used. The highest SCLY expression and activity in mammals is found in the liver and kidneys. Disruption of the Scly gene in mice led to obesity, hyperinsulinemia, glucose intolerance, and hepatic steatosis, with SCLY being suggested as a participant in the regulation of energy metabolism in a sex-dependent manner. With the physiological role of SCLY still not fully understood, this review attempts to discuss the available literature regarding SCLY in animals and provides avenues for possible future investigation.

Iron-sulphur cluster biogenesis via the SUF pathway

Iron-sulphur (Fe-S) clusters are versatile cofactors, which are essential for key metabolic processes in cells, such as respiration and photosynthesis, and which may have also played a crucial role in establishing life on Earth. They can be found in almost all living organisms, from unicellular prokaryotes and archaea to multicellular animals and plants, and exist in diverse forms. This review focuses on the most ancient Fe-S cluster assembly system, the sulphur utilization factor (SUF) mechanism, which is crucial in bacteria for cell survival under stress conditions such as oxidation and iron starvation, and which is also present in the chloroplasts of green microalgae and plants, where it is responsible for plastidial Fe-S protein maturation. We explain the SUF Fe-S cluster assembly process, the proteins involved, their regulation and provide evolutionary insights. We specifically focus on examples from Fe-S cluster synthesis in the model organisms Escherichia coli and Arabidopsis thaliana and discuss in an in vivo context the assembly of the [FeFe]-hydrogenase H-cluster from Chlamydomonas reinhardtii.

Methylation of selenocysteine catalysed by thiopurine S-methyltransferase

BACKGROUND

Methylation driven by thiopurine S-methylatransferase (TPMT) is crucial for deactivation of cytostatic and immunosuppressant thiopurines. Despite its remarkable integration into clinical practice, the endogenous function of TPMT is unknown.

METHODS

To address the role of TPMT in methylation of selenium compounds, we established the research on saturation transfer difference (STD) and ⁷⁷Se NMR spectroscopy, fluorescence measurements, as well as computational molecular docking simulations.

RESULTS

Using STD NMR spectroscopy and fluorescence measurements of tryptophan residues in TPMT, we determined the binding of selenocysteine (Sec) to human recombinant TPMT. By comparing binding characteristics of Sec in the absence and in the presence of methyl donor, we confirmed S-adenosylmethionine (SAM)-induced conformational changes in TPMT. Molecular docking analysis positioned Sec into the active site of TPMT with orientation relevant for methylation reaction. Se-methylselenocysteine (MeSec), produced in the enzymatic reaction, was detected by ⁷⁷Se NMR spectroscopy. A direct interaction between Sec and SAM in the active site of rTPMT and the formation of both products, MeSec and S-adenosylhomocysteine, was demonstrated using NMR spectroscopy.

CONCLUSIONS

The present study provides evidence on in vitro methylation of Sec by rTPMT in a SAM-dependant manner.

GENERAL SIGNIFICANCE

Our results suggest novel role of TPMT and demonstrate new insights into enzymatic modifications of the 21st amino acid.

Delivery of selenium to selenophosphate synthetase for selenoprotein biosynthesis

BACKGROUND

Selenophosphate, the key selenium donor for the synthesis of selenoprotein and selenium-modified tRNA, is produced by selenophosphate synthetase (SPS) from ATP, selenide, and H₂O. Although free selenide can be used as the in vitro selenium substrate for selenophosphate synthesis, the precise physiological system that donates in vivo selenium substrate to SPS has not yet been characterized completely.

SCOPE OF REVIEW

In this review, we discuss selenium metabolism with respect to the delivery of selenium to SPS in selenoprotein biosynthesis.

MAJOR CONCLUSIONS

Glutathione, selenocysteine lyase, cysteine desulfurase, and selenium-binding proteins are the candidates of selenium delivery system to SPS. The thioredoxin system is also implicated in the selenium delivery to SPS in Escherichia coli.

GENERAL SIGNIFICANCE

Selenium delivered via a protein-bound selenopersulfide intermediate emerges as a central element not only in achieving specific selenoprotein biosynthesis but also in preventing the occurrence of toxic free selenide in the cell. This article is part of a Special Issue entitled "Selenium research in biochemistry and biophysics - 200 year anniversary".

Diverse Mechanisms of Sulfur Decoration in Bacterial tRNA and Their Cellular Functions

Sulfur-containing transfer ribonucleic acids (tRNAs) are ubiquitous biomolecules found in all organisms that possess a variety of functions. For decades, their roles in processes such as translation, structural stability, and cellular protection have been elucidated and appreciated. These thionucleosides are found in all types of bacteria; however, their biosynthetic pathways are distinct among different groups of bacteria. Considering that many of the thio-tRNA biosynthetic enzymes are absent in Gram-positive bacteria, recent studies have addressed how sulfur trafficking is regulated in these prokaryotic species. Interestingly, a novel proposal has been given for interplay among thionucleosides and the biosynthesis of other thiocofactors, through participation of shared-enzyme intermediates, the functions of which are impacted by the availability of substrate as well as metabolic demand of thiocofactors. This review describes the occurrence of thio-modifications in bacterial tRNA and current methods for detection of these modifications that have enabled studies on the biosynthesis and functions of S-containing tRNA across bacteria. It provides insight into potential modes of regulation and potential evolutionary events responsible for divergence in sulfur metabolism among prokaryotes.

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.

Selenophosphate Synthetase

Selenophosphate synthetase, the selD gene product from Escherichia coli, is one of the enzymes required for the synthesis and specific insertion of selenocysteine into proteins directed by the TGA codon. Selenophosphate synthetases have been isolated from or are thought to be present in most organisms; however, the best characterized selenophosphate synthetase is from E. coli, in which both in vivo and in vitro studies have been performed. Leinfelder and coworkers showed that an E. coli mutant lacking an intact selD gene fails to incorporate Se into both the selenocysteine-containing enzyme formate dehydrogenase (FDH) and tRNA species that normally contain 2-selenouridine residues at the wobble position. Thus, this study strongly implicated selenophosphate as playing a major role in E. coli selenium metabolic pathways. The selenophosphate synthetase reaction requires some form of reduced selenium such as hydrogen selenide (HSe-) and ATP as substrates to generate a stoichiometric amount of SePO3, AMP, and orthophosphate. Studies of selenophosphate inhibition have provided further insight into the mechanism of selenophosphate synthetase. An assay by which AMP formation is measured in the absence of selenide showed that selenophosphate synthetase catalyzes hydrolysis of ATP to AMP and two orthophosphates in an uncoupled reaction. The sequencing of selenophosphate synthetase genes from various organisms reveals several conserved regions in the gene product. Recent investigations into the mechanism of selenophosphate synthetase have revealed a property of selenophosphate synthetase not previously observed. In samples of purified selenophosphate synthetase, an unusual optical absorption spectrum is seen.

Selenocysteine Lyase

Selenocysteine is a naturally occurring analog of cysteine in which the sulfur atom of the latter is replaced with selenium. This seleno-amino acid occurs as a specific component of various selenoproteins and selenium-dependent enzymes. Incorporation of selenocysteine into these proteins occurs cotranslationally as directed by the UGA codon. For this process, a special tRNA having an anticodon complimentary to UGA, tRNASec, is utilized. In Escherichia coli and related bacteria, this tRNA first is amino acylated with serine, and the seryl-tRNASec is converted to selenocysteyl-tRNASec. The specific incorporation of selenocysteine into proteins directed by the UGA codon depends on the synthesis of selenocysteyl-tRNASec. Included in the selenium delivery protein category are rhodaneses that mobilize selenium from inorganic sources and NIFS-like proteins that liberate elemental selenium from selenocysteine. The NIFS protein from Azotobacter vinelandii was found to serve as an efficient catalyst in vitro for delivery of selenium from free selenocysteine to Escherichia coli selenophosphate synthetase for selenophosphate formation. The widespread distribution of selenocysteine lyase in numerous bacterial species was reported and the bacterial enzymes, like the pig liver enzyme, required pyridoxal phosphate as cofactor. Three NIFS-like genes were isolated from E. coli by Esaki and coworkers and the expressed gene products were isolated and characterized. One of these NIFS-like proteins also exhibited a high preference for selenocysteine over cysteine. M. vannielii, an anaerobic methane-producing organism, that grows in a mineral medium containing formate as sole organic carbon source, synthesizes several specific selenoenzymes required for growth and energy production under these conditions.

Formation of a Ternary Complex for Selenocysteine Biosynthesis in Bacteria

The synthesis of selenocysteine-containing proteins (selenoproteins) involves the interaction of selenocysteine synthase (SelA), tRNA (tRNA(Sec)), selenophosphate synthetase (SelD, SPS), a specific elongation factor (SelB), and a specific mRNA sequence known as selenocysteine insertion sequence (SECIS). Because selenium compounds are highly toxic in the cellular environment, the association of selenium with proteins throughout its metabolism is essential for cell survival. In this study, we demonstrate the interaction of SPS with the SelA-tRNA(Sec) complex, resulting in a 1.3-MDa ternary complex of 27.0 ± 0.5 nm in diameter and 4.02 ± 0.05 nm in height. To assemble the ternary complex, SPS undergoes a conformational change. We demonstrated that the glycine-rich N-terminal region of SPS is crucial for the SelA-tRNA(Sec)-SPS interaction and selenoprotein biosynthesis, as revealed by functional complementation experiments. Taken together, our results provide new insights into selenoprotein biosynthesis, demonstrating for the first time the formation of the functional ternary SelA-tRNA(Sec)-SPS complex. We propose that this complex is necessary for proper selenocysteine synthesis and may be involved in avoiding the cellular toxicity of selenium compounds.

Importance of selenium and selenoprotein for brain function: From antioxidant protection to neuronal signalling

Multiple biological functions of selenium manifest themselves mainly via 25 selenoproteins that have selenocysteine at their active centre. Selenium is vital for the brain and seems to participate in the pathology of disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and epilepsy. Since selenium was shown to be involved in diverse functions of the central nervous system, such as motor performance, coordination, memory and cognition, a possible role of selenium and selenoproteins in brain signalling pathways may be assumed. The aim of the present review is to analyse possible relations between selenium and neurotransmission. Selenoproteins seem to be of special importance in the development and functioning of GABAergic (GABA, γ-aminobutyric acid) parvalbumin positive interneurons of the cerebral cortex and hippocampus. Dopamine pathway might be also selenium dependent as selenium shows neuroprotection in the nigrostriatal pathway and also exerts toxicity towards dopaminergic neurons under higher concentrations. Recent findings also point to acetylcholine neurotransmission involvement. The role of selenium and selenoproteins in neurotransmission might not only be limited to their antioxidant properties but also to inflammation, influencing protein phosphorylation and ion channels, alteration of calcium homeostasis and brain cholesterol metabolism. Moreover, a direct signalling function was proposed for selenoprotein P through interaction with post-synaptic apoliprotein E receptors 2 (ApoER2).

Evolution of selenophosphate synthetases: emergence and relocation of function through independent duplications and recurrent subfunctionalization

Selenoproteins are proteins that incorporate selenocysteine (Sec), a nonstandard amino acid encoded by UGA, normally a stop codon. Sec synthesis requires the enzyme Selenophosphate synthetase (SPS or SelD), conserved in all prokaryotic and eukaryotic genomes encoding selenoproteins. Here, we study the evolutionary history of SPS genes, providing a map of selenoprotein function spanning the whole tree of life. SPS is itself a selenoprotein in many species, although functionally equivalent homologs that replace the Sec site with cysteine (Cys) are common. Many metazoans, however, possess SPS genes with substitutions other than Sec or Cys (collectively referred to as SPS1). Using complementation assays in fly mutants, we show that these genes share a common function, which appears to be distinct from the synthesis of selenophosphate carried out by the Sec- and Cys- SPS genes (termed SPS2), and unrelated to Sec synthesis. We show here that SPS1 genes originated through a number of independent gene duplications from an ancestral metazoan selenoprotein SPS2 gene that most likely already carried the SPS1 function. Thus, in SPS genes, parallel duplications and subsequent convergent subfunctionalization have resulted in the segregation to different loci of functions initially carried by a single gene. This evolutionary history constitutes a remarkable example of emergence and evolution of gene function, which we have been able to trace thanks to the singular features of SPS genes, wherein the amino acid at a single site determines unequivocally protein function and is intertwined to the evolutionary fate of the entire selenoproteome.

Selenoproteins: molecular pathways and physiological roles

Selenium is an essential micronutrient with important functions in human health and relevance to several pathophysiological conditions. The biological effects of selenium are largely mediated by selenium-containing proteins (selenoproteins) that are present in all three domains of life. Although selenoproteins represent diverse molecular pathways and biological functions, all these proteins contain at least one selenocysteine (Sec), a selenium-containing amino acid, and most serve oxidoreductase functions. Sec is cotranslationally inserted into nascent polypeptide chains in response to the UGA codon, whose normal function is to terminate translation. To decode UGA as Sec, organisms evolved the Sec insertion machinery that allows incorporation of this amino acid at specific UGA codons in a process requiring a cis-acting Sec insertion sequence (SECIS) element. Although the basic mechanisms of Sec synthesis and insertion into proteins in both prokaryotes and eukaryotes have been studied in great detail, the identity and functions of many selenoproteins remain largely unknown. In the last decade, there has been significant progress in characterizing selenoproteins and selenoproteomes and understanding their physiological functions. We discuss current knowledge about how these unique proteins perform their functions at the molecular level and highlight new insights into the roles that selenoproteins play in human health.

The genome of Spironucleus salmonicida highlights a fish pathogen adapted to fluctuating environments

Spironucleus salmonicida causes systemic infections in salmonid fish. It belongs to the group diplomonads, binucleated heterotrophic flagellates adapted to micro-aerobic environments. Recently we identified energy-producing hydrogenosomes in S. salmonicida. Here we present a genome analysis of the fish parasite with a focus on the comparison to the more studied diplomonad Giardia intestinalis. We annotated 8067 protein coding genes in the ∼12.9 Mbp S. salmonicida genome. Unlike G. intestinalis, promoter-like motifs were found upstream of genes which are correlated with gene expression, suggesting a more elaborate transcriptional regulation. S. salmonicida can utilise more carbohydrates as energy sources, has an extended amino acid and sulfur metabolism, and more enzymes involved in scavenging of reactive oxygen species compared to G. intestinalis. Both genomes have large families of cysteine-rich membrane proteins. A cluster analysis indicated large divergence of these families in the two diplomonads. Nevertheless, one of S. salmonicida cysteine-rich proteins was localised to the plasma membrane similar to G. intestinalis variant-surface proteins. We identified S. salmonicida homologs to cyst wall proteins and showed that one of these is functional when expressed in Giardia. This suggests that the fish parasite is transmitted as a cyst between hosts. The extended metabolic repertoire and more extensive gene regulation compared to G. intestinalis suggest that the fish parasite is more adapted to cope with environmental fluctuations. Our genome analyses indicate that S. salmonicida is a well-adapted pathogen that can colonize different sites in the host.

Studies of model organisms are very powerful. However, to appreciate the enormous diversity of genetic and cell biological processes we need to extend the number of available model organisms. For example, there are very few model organisms for diverse microbial eukaryotes, a group of organisms which indeed represents the vast majority of the eukaryotic diversity. To this end, we have developed a system to do genetic modification on the Atlantic salmon pathogen Spironucleus salmonicida. Using this system we could show that the organism is capable of producing hydrogen within specialised compartments. Here we present the genome sequence of S. salmonicida together with a thorough annotation. We compare the results with the closest available model organism, the human intestinal parasite Giardia intestinalis. The fish parasite has a more elaborate system for regulation of gene expression, as well as a larger metabolic capacity. This indicates that S. salmonicida is a well-adapted pathogen that can deal with fluctuating environments, an important trait to be able to establish systemic infections in the host. The development of S. salmonicida into a model system will benefit the studies of fish infections, as well as cell biological processes.

Absence of selenoprotein P but not selenocysteine lyase results in severe neurological dysfunction

Dietary selenium restriction in mammals causes bodily selenium to be preferentially retained in the brain relative to other organs. Almost all the known selenoproteins are found in brain, where expression is facilitated by selenocysteine (Sec)-laden selenoprotein P. The brain also expresses selenocysteine lyase (Scly), an enzyme that putatively salvages Sec and recycles the selenium for selenoprotein translation. We compared mice with a genetic deletion of Scly to selenoprotein P (Sepp1) knockout mice for similarity of neurological impairments and whether dietary selenium modulates these parameters. We report that Scly knockout mice do not display neurological dysfunction comparable to Sepp1 knockout mice. Feeding a low-selenium diet to Scly knockout mice revealed a mild spatial learning deficit without disrupting motor coordination. Additionally, we report that the neurological phenotype caused by the absence of Sepp1 is exacerbated in male vs. female mice. These findings indicate that Sec recycling via Scly becomes limiting under selenium deficiency and suggest the presence of a complementary mechanism for processing Sec. Our studies illuminate the interaction between Sepp1 and Scly in the distribution and turnover of body and brain selenium and emphasize the consideration of sex differences when studying selenium and selenoproteins in vertebrate biology.

Biochemical discrimination between selenium and sulfur 1: a single residue provides selenium specificity to human selenocysteine lyase

Selenium and sulfur are two closely related basic elements utilized in nature for a vast array of biochemical reactions. While toxic at higher concentrations, selenium is an essential trace element incorporated into selenoproteins as selenocysteine (Sec), the selenium analogue of cysteine (Cys). Sec lyases (SCLs) and Cys desulfurases (CDs) catalyze the removal of selenium or sulfur from Sec or Cys and generally act on both substrates. In contrast, human SCL (hSCL) is specific for Sec although the only difference between Sec and Cys is the identity of a single atom. The chemical basis of this selenium-over-sulfur discrimination is not understood. Here we describe the X-ray crystal structure of hSCL and identify Asp146 as the key residue that provides the Sec specificity. A D146K variant resulted in loss of Sec specificity and appearance of CD activity. A dynamic active site segment also provides the structural prerequisites for direct product delivery of selenide produced by Sec cleavage, thus avoiding release of reactive selenide species into the cell. We thus here define a molecular determinant for enzymatic specificity discrimination between a single selenium versus sulfur atom, elements with very similar chemical properties. Our findings thus provide molecular insights into a key level of control in human selenium and selenoprotein turnover and metabolism.

Structural insights into the catalytic mechanism of Escherichia coli selenophosphate synthetase

Selenophosphate synthetase (SPS) catalyzes the synthesis of selenophosphate, the selenium donor for the biosynthesis of selenocysteine and 2-selenouridine residues in seleno-tRNA. Selenocysteine, known as the 21st amino acid, is then incorporated into proteins during translation to form selenoproteins which serve a variety of cellular processes. SPS activity is dependent on both Mg(2+) and K(+) and uses ATP, selenide, and water to catalyze the formation of AMP, orthophosphate, and selenophosphate. In this reaction, the gamma phosphate of ATP is transferred to the selenide to form selenophosphate, while ADP is hydrolyzed to form orthophosphate and AMP. Most of what is known about the function of SPS has derived from studies investigating Escherichia coli SPS (EcSPS) as a model system. Here we report the crystal structure of the C17S mutant of SPS from E. coli (EcSPS(C17S)) in apo form (without ATP bound). EcSPS(C17S) crystallizes as a homodimer, which was further characterized by analytical ultracentrifugation experiments. The glycine-rich N-terminal region (residues 1 through 47) was found in the open conformation and was mostly ordered in both structures, with a magnesium cofactor bound at the active site of each monomer involving conserved aspartate residues. Mutating these conserved residues (D51, D68, D91, and D227) along with N87, also found at the active site, to alanine completely abolished AMP production in our activity assays, highlighting their essential role for catalysis in EcSPS. Based on the structural and biochemical analysis of EcSPS reported here and using information obtained from similar studies done with SPS orthologs from Aquifex aeolicus and humans, we propose a catalytic mechanism for EcSPS-mediated selenophosphate synthesis.

Proteomic characterization of a selenium-metabolizing probiotic Lactobacillus reuteri Lb26 BM for nutraceutical applications

Selenium (Se), Se-cysteines and selenoproteins have received growing interest in the nutritional field as redox-balance modulating agents. The aim of this study was to establish the Se-concentrating and Se-metabolizing capabilities of the probiotic Lactobacillus reuteri Lb26 BM, for nutraceutical applications. A comparative proteomic approach was employed to study the bacteria grown in a control condition (MRS modified medium) and in a stimulated condition (4.38 mg/L of sodium selenite). The total protein extract was separated into two pI ranges: 4-7 and 6-11; the 25 identified proteins were divided into five functional classes: (i) Se metabolism; (ii) energy metabolism; (iii) stress/adhesion; (iv) cell shape and transport; (v) proteins involved in other functions. All the experimental results indicate that L. reuteri Lb26 BM is able to metabolize Se(IV), incorporating it into selenoproteins, through the action of a selenocysteine lyase, thus enhancing organic Se bioavailability. This involves endo-ergonic reactions balanced by an increase of substrate-level phosphorylation, chiefly through lactic fermentation. Nevertheless, when L. reuteri was grown on Se a certain degree of stress was observed, and this has to be taken into account for future applicative purposes. The proteomic approach has proven to be a powerful tool for the metabolic characterization of potential Se-concentrating probiotics.

Selenium controls transcription of paralogous formate dehydrogenase genes in the termite gut acetogen, Treponema primitia

The termite gut spirochete, Treponema primitia, is a CO(2)-reductive acetogen that is phylogenetically distinct from other distantly related and more extensively studied acetogens such as Moorella thermoacetica. Research on T. primitia has revealed details about the role of spirochetes in CO(2)-reductive acetogenesis, a process important to the mutualism occurring between termites and their gut microbial communities. Here, a locus of the T. primitia genome containing Wood-Ljungdahl pathway genes for CO(2)-reductive acetogenesis was sequenced. This locus contained methyl-branch genes of the pathway (i.e. for the reduction of CO(2) to the level of methyl-tetrahydrofolate) including paralogous genes for cysteine and selenocysteine (Sec) variants of formate dehydrogenase (FDH) and genes for Sec incorporation. The FDH variants affiliated phylogenetically with hydrogenase-linked FDH enzymes, suggesting that T. primitia FDH enzymes utilize electrons derived directly from molecular H(2). Sub-nanomolar concentrations of selenium decreased transcript levels of the cysteine variant FDH gene. Selenium concentration did not markedly influence the level of mRNA upstream of the Sec-codon in the Sec variant FDH; however, the level of transcript extending downstream of the Sec-codon increased incrementally with increasing selenium concentrations. The features and regulation of these FDH genes are an indication that T. primitia may experience dynamic selenium availability in its H(2)-rich gut environment.

Binding of ATP and its derivatives to selenophosphate synthetase from Escherichia coli

Mechanistically similar selenophosphate synthetases (SPS) have been isolated from different organisms. SPS from Escherichia coli is an ATP-dependent enzyme with a C-terminal glycine-rich Walker sequence that has been assumed to take part in the first step of ATP binding. Three C-terminally truncated mutants of SPS, containing the N-terminal 238 (SPS(238)), 262 (SPS(262)), and 332 (SPS(332)) amino acids of the 348-amino-acid protein, have been extracted from cell pellets, and two of these (SPS(262) and SPS(332)) have been purified to homogeneity. SPS(238) has been obtained in a highly purified form. Binding of the fluorescent ATP-derivative TNP-ATP and Mn-ATP to the proteins was examined for all truncated mutants of SPS and a catalytically inactive C17S mutant. It has been shown that TNP-ATP can be used as a structural probe for ATP-binding sites of SPS. We observed two TNP-ATP binding sites per molecule of enzyme for wild-type SPS and SPS(332) mutant and one TNP-ATP binding site for SPS(238) mutant. The stoichiometry of Mn-ATP-binding was 2 mol of ATP per mol of protein determined with [(14)C]ATP by HPLC gel-filtration column chromatography under saturating conditions. The binding stoichiometries for SPS(332), SPS(262), and SPS(238) were 2, 1.6, and 1, respectively. The C17S mutant exhibits about one third of wild type SPS TNP-ATP-binding ability and converts 12% of ATP in the ATPase reaction to ADP in the absence of selenide. The C-terminus contributes two thirds to the TNP-ATP binding; SPS(238) likely has one ATP-binding site removed by truncation.

Functions and cellular localization of cysteine desulfurase and selenocysteine lyase in Trypanosoma brucei

Nfs-like proteins have cysteine desulfurase (CysD) activity, which removes sulfur (S) from cysteine, and provides S for iron-sulfur cluster assembly and the thiolation of tRNAs. These proteins also have selenocysteine lyase activity in vitro, and cleave selenocysteine into alanine and elemental selenium (Se). It was shown previously that the Nfs-like protein called Nfs from the parasitic protist Trypanosoma brucei is a genuine CysD. A second Nfs-like protein is encoded in the nuclear genome of T. brucei. We called this protein selenocysteine lyase (SCL) because phylogenetic analysis reveals that it is monophyletic with known eukaryotic selenocysteine lyases. The Nfs protein is located in the mitochondrion, whereas the SCL protein seems to be present in the nucleus and cytoplasm. Unexpectedly, downregulation of either Nfs or SCL protein leads to a dramatic decrease in both CysD and selenocysteine lyase activities concurrently in the mitochondrion and the cytosolic fractions. Because loss of Nfs causes a growth phenotype but loss of SCL does not, we propose that Nfs can fully complement SCL, whereas SCL can only partially replace Nfs under our growth conditions.

Inhibition of selenium metabolism in the oral pathogen Treponema denticola

In this report we provide evidence that the antimicrobial action of stannous salts and a gold drug, auranofin, against Treponema denticola is mediated through inhibition of the metabolism of selenium for synthesis of selenoproteins.

Crystal structures of catalytic intermediates of human selenophosphate synthetase 1

Selenophosphate synthetase catalyzes the synthesis of the highly active selenium donor molecule selenophosphate, a key intermediate in selenium metabolism. We have determined the high-resolution crystal structure of human selenophosphate synthetase 1 (hSPS1). An unexpected reaction intermediate, with a tightly bound phosphate and ADP at the active site has been captured in the structure. An enzymatic assay revealed that hSPS1 possesses low ADP hydrolysis activity in the presence of phosphate. Our structural and enzymatic results suggest that consuming the second high-energy phosphoester bond of ATP could protect the labile product selenophosphate during catalytic reaction. We solved another hSPS1 structure with potassium ions at the active sites. Comparing the two structures, we were able to define the monovalent cation-binding site of the enzyme. The detailed mechanism of the ADP hydrolysis step and the exact function of the monovalent cation for hSPS1 catalytic reaction are proposed.

Identification of proteins interacting with selenocysteine lyase

Yeast two-hybrid screening of mouse cDNA libraries was performed to identify proteins interacting with selenocysteine lyase (SCL), which decomposes L-selenocysteine. Several proteins related to spermatogenesis, protein synthesis, and cell viability/apoptosis were identified as potential interactors. Major urinary proteins 1 and 2 interacted with SCL and inhibited its activity. Coimmunoprecipitation revealed interactions between SCL and each of two selenophosphate synthetase isozymes.

Structure of selenophosphate synthetase essential for selenium incorporation into proteins and RNAs

Selenophosphate synthetase (SPS) catalyzes the activation of selenide with adenosine 5'-triphosphate (ATP) to generate selenophosphate, the essential reactive selenium donor for the formation of selenocysteine (Sec) and 2-selenouridine residues in proteins and RNAs, respectively. Many SPS are themselves Sec-containing proteins, in which Sec replaces Cys in the catalytically essential position (Sec/Cys). We solved the crystal structures of Aquifex aeolicus SPS and its complex with adenosine 5'-(alpha,beta-methylene) triphosphate (AMPCPP). The ATP-binding site is formed at the subunit interface of the homodimer. Four Asp residues coordinate four metal ions to bind the phosphate groups of AMPCPP. In the free SPS structure, the two loop regions in the ATP-binding site are not ordered, and no enzyme-associated metal is observed. This suggests that ATP binding, metal binding, and the formation of their binding sites are interdependent. To identify the amino-acid residues that contribute to SPS activity, we prepared six mutants of SPS and examined their selenide-dependent ATP consumption. Mutational analyses revealed that Sec/Cys13 and Lys16 are essential. In SPS.AMPCPP, the N-terminal loop, including the two residues, assumes different conformations ("open" and "closed") between the two subunits. The AMPCPP gamma-phosphate group is solvent-accessible, suggesting that a putative nucleophile could attack the ATP gamma-phosphate group to generate selenophosphate and adenosine 5'-diphosphate (ADP). Selenide attached to Sec/Cys13 as -Se-Se(-)/-S-Se(-) could serve as the nucleophile in the "closed" conformation. A water molecule, fixed close to the beta-phosphate group, could function as the nucleophile in subsequent ADP hydrolysis to orthophosphate and adenosine 5'-monophosphate.

Functional analysis of Arabidopsis genes involved in mitochondrial iron-sulfur cluster assembly

Machinery for the assembly of the iron-sulfur ([Fe-S]) clusters that function as cofactors in a wide variety of proteins has been identified in microbes, insects, and animals. Homologs of the genes involved in [Fe-S] cluster biogenesis have recently been found in plants, as well, and point to the existence of two distinct systems in these organisms, one located in plastids and one in mitochondria. Here we present the first biochemical confirmation of the activity of two components of the mitochondrial machinery in Arabidopsis, AtNFS1 and AtISU1. Analysis of the expression patterns of the corresponding genes, as well as AtISU2 and AtISU3, and the phenotypes of plants in which these genes are up or down-regulated are consistent with a role for the mitochondrial [Fe-S] assembly system in the maturation of proteins required for normal plant development.

CdS quantum dots as fluorescence probes for the sensitive and selective detection of highly reactive HSe− ions in aqueous solution

Water-soluble cadmium sulfide (CdS) quantum dots (QDs) capped by mercaptoacetic acid were synthesized by aqueous-phase arrested precipitation, and characterized by transmission electron microscopy, spectrofluorometry, and UV-Vis spectrophotometry. The prepared luminescent water-soluble CdS QDs were evaluated as fluorescence probes for the detection of highly reactive hydrogen selenide ions (HSe(-) ions). The quenching of the fluorescence emission of CdS QDs with the addition of HSe(-) ions is due to the elimination of the S(2-) vacancies which are luminescence centers. Quantitative analysis based on chemical interaction between HSe(-) ions and the surface of CdS QDs is very simple, easy to develop, and has demonstrated very high sensitivity and selectivity features. The effect of foreign ions (common anions and biologically relevant cations) on the fluorescence of the CdS QDs was examined to evaluate the selectivity. Only Cu(2+) and S(2-) ions exhibit significant effects on the fluorescence of CdS QDs. With the developed method, we are able to determine the concentration of HSe(-) ions in the range from 0.10 to 4.80 micromol L(-1), and the limit of detection is 0.087 micromol L(-1). The proposed method was successfully applied to monitor the obtained HSe(-) ions from the reaction of glutathione with selenite. To the best of our knowledge, this is the first report on fluorescence analysis of HSe(-) ions in aqueous solution.

Escherichia coli Biotin Synthase Produces Selenobiotin. Further Evidence of the Involvement of the [2Fe-2S]2+ Cluster in the Sulfur Insertion Step

Biotin synthase, a member of the "radical SAM" family, catalyzes the final step of the biotin biosynthetic pathway, namely, the insertion of a sulfur atom into dethiobiotin. The as-isolated enzyme contains a [2Fe-2S](2+) cluster, but the active enzyme requires an additional [4Fe-4S](2+) cluster, which is formed in the presence of Fe(NH(4))(2)(SO(4))(2) and Na(2)S in the in vitro assay. The role of the [4Fe-4S](2+) cluster is to mediate the electron transfer to SAM, while the [2Fe-2S](2+) cluster is involved in the sulfur insertion step. To investigate the selenium version of the reaction, we have depleted the enzyme of its iron and sulfur and reconstituted the resulting apoprotein with FeCl(3) and Na(2)Se to yield a [2Fe-2Se](2+) cluster. This enzyme was assayed in vitro with Na(2)Se in place of Na(2)S to enable the formation of a [4Fe-4Se](2+) cluster. Selenobiotin was produced, but the activity was lower than that of the as-isolated [2Fe-2S](2+) enzyme in the presence of Na(2)S. The [2Fe-2Se](2+) enzyme was additionally assayed with Na(2)S, to reconstitute a [4Fe-4S](2+) cluster, in case the latter was more efficient than a [4Fe-4Se](2+) cluster for the electron transfer. Indeed, the activity was improved, but in that case, a mixture of biotin and selenobiotin was produced. This was unexpected if one considers the [2Fe-2S](2+) center as the sulfur source (either as the ultimate donor or via another intermediate), unless some exchange of the chalcogenide has taken place in the cluster. This latter point was seen in the resonance Raman spectrum of the reacted enzyme which clearly indicated the presence of both the [2Fe-2Se](2+) and [2Fe-2S](2+) clusters. No exchange was observed in the absence of reaction. These observations bring supplementary proof that the [2Fe-2S](2+) cluster is implicated in the sulfur insertion step.

Selenocysteine in proteins-properties and biotechnological use

Selenocysteine (Sec), the 21st amino acid, exists naturally in all kingdoms of life as the defining entity of selenoproteins. Sec is a cysteine (Cys) residue analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group in Cys. The selenium atom gives Sec quite different properties from Cys. The most obvious difference is the lower pK(a) of Sec, and Sec is also a stronger nucleophile than Cys. Proteins naturally containing Sec are often enzymes, employing the reactivity of the Sec residue during the catalytic cycle and therefore Sec is normally essential for their catalytic efficiencies. Other unique features of Sec, not shared by any of the other 20 common amino acids, derive from the atomic weight and chemical properties of selenium and the particular occurrence and properties of its stable and radioactive isotopes. Sec is, moreover, incorporated into proteins by an expansion of the genetic code as the translation of selenoproteins involves the decoding of a UGA codon, otherwise being a termination codon. In this review, we will describe the different unique properties of Sec and we will discuss the prerequisites for selenoprotein production as well as the possible use of Sec introduction into proteins for biotechnological applications. These include residue-specific radiolabeling with gamma or positron emitters, the use of Sec as a reactive handle for electophilic probes introducing fluorescence or other peptide conjugates, as the basis for affinity purification of recombinant proteins, the trapping of folding intermediates, improved phasing in X-ray crystallography, introduction of 77Se for NMR spectroscopy, or, finally, the analysis or tailoring of enzymatic reactions involving thiol or oxidoreductase (redox) selenolate chemistry.

STRUCTURE, FUNCTION, AND FORMATION OF BIOLOGICAL IRON-SULFUR CLUSTERS

Iron-sulfur [Fe-S] clusters are ubiquitous and evolutionary ancient prosthetic groups that are required to sustain fundamental life processes. Owing to their remarkable structural plasticity and versatile chemical/electronic features [Fe-S] clusters participate in electron transfer, substrate binding/activation, iron/sulfur storage, regulation of gene expression, and enzyme activity. Formation of intracellular [Fe-S] clusters does not occur spontaneously but requires a complex biosynthetic machinery. Three different types of [Fe-S] cluster biosynthetic systems have been discovered, and all of them are mechanistically unified by the requirement for a cysteine desulfurase and the participation of an [Fe-S] cluster scaffolding protein. Important mechanistic questions related to [Fe-S] cluster biosynthesis involve the molecular details of how [Fe-S] clusters are assembled on scaffold proteins, how [Fe-S] clusters are transferred from scaffolds to target proteins, how various accessory proteins participate in [Fe-S] protein maturation, and how the biosynthetic process is regulated.

Characterization of potential selenium-binding proteins in the selenophosphate synthetase system

Selenophosphate, an activated form of selenium that can serve as a selenium donor, is generated by the selD gene product, selenophosphate synthetase (SPS). Selenophosphate is required by several bacteria and by mammals for the specific synthesis of Secys-tRNA, the precursor of selenocysteine in selenoenzymes. Although free selenide can be used in vitro for synthesis of selenophosphate, the physiological system that donates selenium to SPS is incompletely characterized. To detect potential selenium-delivery proteins, two known sulfurtransferases and glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12) were analyzed for ability to bind and transfer selenium. Rhodanese (EC 2.8.1.1) was shown to bind selenium tightly, with only part of the selenium being available as substrate for SPS in the presence of added reductant. 3-Mercaptopyruvate sulfurtransferase (3-MST; EC 2.8.1.2) and GAPDH also bound selenium supplied as selenodiglutathione formed from SeO3(2-) and glutathione. Selenium bound to 3-MST and GAPDH was released more readily than that from rhodanese and also was more available as a substrate for SPS. Although rhodanese retained tightly bound selenium under aerobic conditions, the protein gradually became insoluble, whereas GAPDH containing bound selenium was stable at neutral pH for a long period. These results indicate that 3-MST and GAPDH have more suitable potentials as a physiological selenium-delivery protein than rhodanese. In the presence of a selenium-binding protein, a low level of selenodiglutathione formed from SeO3(2-) and glutathione could effectively replace the high concentrations of selenide routinely used as substrate in the SPS in vitro assays.

Selenophosphate synthetase genes from lung adenocarcinoma cells: Sps1 for recycling L-selenocysteine and Sps2 for selenite assimilation

A labile selenium donor compound monoselenophosphate is synthesized from selenide and ATP by selenophosphate synthetase (SPS). In the present study, Sps1 and Sps2 were cloned from a cDNA library prepared from human lung adenocarcinoma cells (NCIH441). The human lung Sps1 has been cloned as an ORF of 1,179 bp, identical in sequence to that of the recently revised human liver Sps1. The in-frame TGA codon of the lung Sps2 was genetically altered to TGT (Cys) to obtain the Sps2Cys gene. Expression of the recombinant plasmids containing Sps1 or Sps2Cys was highly toxic to Escherichia coli host cells grown aerobically. Accordingly, the human lung Sps homologs were characterized by an in vivo complementation assay using a selD mutant strain. An added selenium source and a low salt concentration (0.1-0.25% NaCl) in the medium were required for reproducible and sensitive in vivo complementation. Sps2Cys effectively complemented the selD mutant, and the resulting formate dehydrogenase H activity was as high as that of WT E. coli MC4100. In contrast, only a weak complementation of the selD mutant by the Sps1 gene was observed when cells were grown in selenite media. Better complementation with added l-selenocysteine suggested involvement of a selenocysteine lyase for mobilization of selenium. Based on this apparent substrate specificity of the Sps1 and Sps2 gene products we suggest that the Sps1-encoded enzyme depends on a selenium salvage system that recycles l-selenocysteine, whereas the Sps2 enzyme can function with a selenite assimilation system.

Domain organization of Salmonella typhimurium formylglycinamide ribonucleotide amidotransferase revealed by X-ray crystallography

Formylglycinamide ribonucleotide amidotransferase (FGAR-AT) catalyzes the ATP-dependent conversion of formylglycinamide ribonucleotide (FGAR) and glutamine to formylglycinamidine ribonucleotide (FGAM), ADP, P(i), and glutamate in the fourth step of the purine biosynthetic pathway. In eukaryotes and Gram-negative bacteria, FGAR-AT is encoded by the purL gene as a multidomain protein with a molecular mass of about 140 kDa. In Gram-positive bacteria and archaebacteria FGAR-AT is a complex of three proteins: PurS, PurL, and PurQ. We have determined the structure of FGAR-AT (PurL) from Salmonella typhimurium at 1.9 A resolution using X-ray crystallography. PurL is the last remaining enzyme in the purine biosynthetic pathway to have its structure determined. The structure reveals four domains: an N-terminal domain structurally homologous to a PurS dimer, a linker region, an FGAM synthetase domain homologous to an aminoimidazole ribonucleotide synthetase (PurM) dimer, and a triad glutaminase domain. The domains are intricately linked by interdomain interactions and peptide connectors. The fold common to PurM and the central region of PurL represents a superfamily for which HypE, SelD, and ThiL are predicted to be members. A structural ADP molecule was found bound to a site related to the putative active site by pseudo-2-fold symmetry and two sulfate ions were found at the putative active site. These observations and the structural similarities between PurM and StPurL were used to model the substrates FGAR and ATP in the StPurL active site. A glutamylthioester intermediate was found in the glutaminase domain at Cys1135. The N-terminal (PurS-like) domain is hypothesized to form the putative channel through which ammonia passes from the glutaminase domain to the FGAM synthetase domain.

A suf operon requirement for Fe-S cluster assembly during iron starvation in Escherichia coli

The suf and isc operons of Escherichia coli have been implicated in Fe-S cluster assembly. However, it has been unclear why E. coli has two systems for Fe-S cluster biosynthesis. We have examined the regulatory characteristics and mutant phenotypes of both operons to discern if the two operons have redundant functions or if their cellular roles are divergent. Both operons are similarly induced by hydrogen peroxide and the iron chelator 2,2'-dipyridyl, although by different mechanisms. Regulation of the isc operon is mediated by IscR, whereas the suf operon requires OxyR and IHF for the response to oxidative stress and Fur for induction by iron starvation. Simultaneous deletion of iscS and most suf genes is synthetically lethal. However, although the suf and isc operons have overlapping functions, they act as distinct complexes because the SufS desulphurase alone cannot substitute for the IscS enzyme. In addition, suf deletion mutants are more sensitive to iron starvation than isc mutants, and the activity of the Fe-S enzyme gluconate dehydratase is diminished in the suf mutant during iron starvation. These findings are consistent with the model that the isc operon encodes the housekeeping Fe-S cluster assembly system in E. coli, whereas the suf operon is specifically adapted to synthesize Fe-S clusters when iron or sulphur metabolism is disrupted by iron starvation or oxidative stress.

Enhanced selenium tolerance and accumulation in transgenic Arabidopsis expressing a mouse selenocysteine lyase

Selenium (Se) toxicity is thought to be due to nonspecific incorporation of selenocysteine (Se-Cys) into proteins, replacing Cys. In an attempt to direct Se flow away from incorporation into proteins, a mouse (Mus musculus) Se-Cys lyase (SL) was expressed in the cytosol or chloroplasts of Arabidopsis. This enzyme specifically catalyzes the decomposition of Se-Cys into elemental Se and alanine. The resulting SL transgenics were shown to express the mouse enzyme in the expected intracellular location, and to have SL activities up to 2-fold (cytosolic lines) or 6-fold (chloroplastic lines) higher than wild-type plants. Se incorporation into proteins was reduced 2-fold in both types of SL transgenics, indicating that the approach successfully redirected Se flow in the plant. Both the cytosolic and chloroplastic SL plants showed enhanced shoot Se concentrations, up to 1.5-fold compared with wild type. The cytosolic SL plants showed enhanced tolerance to Se, presumably because of their reduced protein Se levels. Surprisingly, the chloroplastic SL transgenics were less tolerant to Se, indicating that (over) production of elemental Se in the chloroplast is toxic. Expression of SL in the cytosol may be a useful approach for the creation of plants with enhanced Se phytoremediation capacity.

Snapshots of the cystine lyase C-DES during catalysis. Studies in solution and in the crystalline state

The cystine lyase (C-DES) of Synechocystis is a pyridoxal-5'-phosphate-dependent enzyme distantly related to the family of NifS-like proteins. The crystal structure of an N-terminal modified variant has recently been determined. Herein, the reactivity of this enzyme variant was investigated spectroscopically in solution and in the crystalline state to follow the course of the reaction and to determine the catalytic mechanism on a molecular level. Using the stopped-flow technique, the reaction with the preferred substrate cystine was found to follow biphasic kinetics leading to the formation of absorbing species at 338 and 470 nm, attributed to the external aldimine and the alpha-aminoacrylate; the reaction with cysteine also exhibited biphasic behavior but only the external aldimine accumulated. The same reaction intermediates were formed in crystals as seen by polarized absorption microspectrophotometry, thus indicating that C-DES is catalytically competent in the crystalline state. The three-dimensional structure of the catalytically inactive mutant C-DES(K223A) in the presence of cystine showed the formation of an external aldimine species, in which two alternate conformations of the substrate were observed. The combined results allow a catalytic mechanism to be proposed involving interactions between cystine and the active site residues Arg-360, Arg-369, and Trp-251*; these residues reorient during the beta-elimination reaction, leading to the formation of a hydrophobic pocket that stabilizes the enolimine tautomer of the aminoacrylate and the cysteine persulfide product.

Selenosugars are key and urinary metabolites for selenium excretion within the required to low-toxic range

Essential micronutrient selenium is excreted into the urine andor expired after being transformed to methylated metabolites. Monomethylated selenium is excreted into the urine in response to a supply within the required to low-toxic range, whereas tri- and dimethylated selenium increase with excessive supply at a toxic dose. Here we show that the major urinary selenium metabolite within the required to low-toxic range is a selenosugar. The structure of 1beta-methylseleno-N-acetyl-d-galactosamine was deduced from the spectroscopic data and confirmed by chemical synthesis. This metabolite was also detected in the liver, and an additional metabolite increased with inhibition of methylation. The latter metabolite was again a selenosugar conjugated with glutathione instead of a methyl group and was assumed to be a precursor for methylation to the former metabolite. A metabolic pathway for the urinary excretion of selenium, i.e., from the glutathione-S-conjugated selenosugar to the methylated one, was proposed. Urinary monomethylated (selenosugar) and trimethylated selenium can be used as specific indices that increase within the required to low-toxic range and with a distinct toxic dose, respectively.