Selenate reductase activity inEscherichia colirequires Isc iron-sulfur cluster biosynthesis genes

Selenium Metabolism and Selenoproteins in Prokaryotes: A Bioinformatics Perspective

Selenium (Se) is an important trace element that mainly occurs in the form of selenocysteine in selected proteins. In prokaryotes, Se is also required for the synthesis of selenouridine and Se-containing cofactor. A large number of selenoprotein families have been identified in diverse prokaryotic organisms, most of which are thought to be involved in various redox reactions. In the last decade or two, computational prediction of selenoprotein genes and comparative genomics of Se metabolic pathways and selenoproteomes have arisen, providing new insights into the metabolism and function of Se and their evolutionary trends in bacteria and archaea. This review aims to offer an overview of recent advances in bioinformatics analysis of Se utilization in prokaryotes. We describe current computational strategies for the identification of selenoprotein genes and generate the most comprehensive list of prokaryotic selenoproteins reported to date. Furthermore, we highlight the latest research progress in comparative genomics and metagenomics of Se utilization in prokaryotes, which demonstrates the divergent and dynamic evolutionary patterns of different Se metabolic pathways, selenoprotein families, and selenoproteomes in sequenced organisms and environmental samples. Overall, bioinformatics analyses of Se utilization, function, and evolution may contribute to a systematic understanding of how this micronutrient is used in nature.

Insights into Increasing Selenate Reductase Enzyme Activity in the Presence of Nitrogen-Doped Graphite Electrodes for Selenium Effluent Treatment

The weathering of selenium-rich rocks or anthropogenic activities such as mining or smelting can release selenium into the environment, posing a significant environmental risk. The increased monitoring and enforcement of selenium regulations have resulted in protocols to efficiently measure and treat selenium in water and effluent water. The principal aqueous forms of inorganic selenium are selenite (Se(IV)) and selenate (Se(VI)). Selenate, due to its oxy-anionic nature, high mobility, and lack of affinity to conventional adsorbents, is typically more difficult to treat and remove. Thus, it is proposed to remove selenate from water by first reducing it to selenite and then to insoluble elemental selenium, a form that has low toxicity. A naturally occurring selenate reductase enzyme from Thauera selenatis was previously shown to specifically reduce selenate to selenite. To exploit this functionality, recombinant enzyme technologies were used to produce a cell-free, enriched Thauera selenatis selenate reductase heterotrimeric enzyme complex (TsSer-αβγ). The addition of the recombinant enzyme complex to effluent water was found to successfully reduce the selenate. Interestingly, upon adding nitrogen-doped graphite electrodes to the reaction, the selenate-reducing activity significantly increased. Overall, these findings highlight a new, potentially sustainable solution to the reduction of selenate in water and effluent water.

Selenium respiration in anaerobic bacteria: Does energy generation pay off?

Selenium (Se) respiration in bacteria was revealed for the first time at the end of 1980s. Although thermodynamically-favorable, energy-dense and documented in phylogenetically-diverse bacteria, this metabolic process appears to be accompanied by a number of challenges and numerous unanswered questions. Selenium oxyanions, SeO₄^2- and SeO₃^2-, are reduced to elemental Se (Se⁰) through anaerobic respiration, the end product being solid and displaying a considerable size (up to 500 nm) at the bacterial scale. Compared to other electron acceptors used in anaerobic respiration (e.g. N, S, Fe, Mn, and As), Se is one of the few elements whose end product is solid. Furthermore, unlike other known bacterial intracellular accumulations such as volutin (inorganic polyphosphate), S⁰, glycogen or magnetite, Se⁰ has not been shown to play a nutritional or ecological role for its host. In the context of anaerobic respiration of Se oxyanions, biogenic Se⁰ appears to be a by-product, a waste that needs proper handling, and this raises the question of the evolutionary implications of this process. Why would bacteria use a respiratory substrate that is useful, in the first place, and then highly detrimental? Interestingly, in certain artificial ecosystems (e.g. upflow bioreactors) Se⁰ might help bacterial cells to increase their density and buoyancy and thus avoid biomass wash-out, ensuring survival. This review article provides an in-depth analysis of selenium respiration (model selenium respiring bacteria, thermodynamics, respiratory enzymes, and genetic determinants), complemented by an extensive discussion about the evolutionary implications and the properties of biogenic Se⁰ using published and original/unpublished results.

Analysis of Bioavailability and Induction of Glutathione Peroxidase by Dietary Nanoelemental, Organic and Inorganic Selenium

Dietary organic selenium (Se) is commonly utilized to increase formation of selenoproteins, including the major antioxidant protein, glutathione peroxidase (GPx). Inorganic Se salts, such as sodium selenite, are also incorporated into selenoproteins, and there is evidence that nanoelemental Se added to the diet may also be effective. We conducted two trials, the first investigated inorganic Se (selenite), organic Se (L-selenomethionine) and nanoelemental Se, in conventional mice. Their bioavailability and effectiveness to increase GPx activity were examined. The second trial focused on determining the mechanism by which dietary Se is incorporated into tissue, utilising both conventional and germ-free (GF) mice. Mice were fed a diet with minimal Se, 0.018 parts per million (ppm), and diets with Se supplementation, to achieve 0.07, 0.15, 0.3 and 1.7 ppm Se, for 5 weeks (first trial). Mass spectrometry, Western blotting and enzymatic assays were used to investigate bioavailability, protein levels and GPx activity in fresh frozen tissue (liver, ileum, plasma, muscle and feces) from the Se fed animals. Inorganic, organic and nanoelemental Se were all effectively incorporated into tissues. The high Se diet (1.7 ppm) resulted in the highest Se levels in all tissues and plasma, independent of the Se source. Interestingly, despite being ~11 to ~25 times less concentrated than the high Se, the lower Se diets (0.07; 0.15) resulted in comparably high Se levels in liver, ileum and plasma for all Se sources. GPx protein levels and enzyme activity were significantly increased by each diet, relative to control. We hypothesised that bacteria may be a vector for the conversion of nanoelemental Se, perhaps in exchange for S in sulphate metabolising bacteria. We therefore investigated Se incorporation from low sulphate diets and in GF mice. All forms of selenium were bioavailable and similarly significantly increased the antioxidant capability of GPx in the intestine and liver of GF mice and mice with sulphate free diets. Se from nanoelemental Se resulted in similar tissue levels to inorganic and organic sources in germ free mice. Thus, endogenous mechanisms, not dependent on bacteria, reduce nanoelemental Se to the metabolite selenide that is then converted to selenophosphate, synthesised to selenocysteine, and incorporated into selenoproteins. In particular, the similar efficacy of nanoelemental Se in comparison to organic Se in both trials is important in the view of the currently limited cheap sources of Se.

Developments in the study and applications of bacterial transformations of selenium species

Microbial bio-transformations of the essential trace element selenium are now recognized to occur among a wide variety of microorganisms. These transformations are used to convert this element into its assimilated form of selenocysteine, which is at the active center of a number of key enzymes, and to produce selenium nanoparticles, quantum dots, metal selenides, and methylated selenium species that are indispensable for biotechnological and bioremediation applications. The focus of this review is to present the state-of-the-art of all aspects of the investigations into the bacterial transformations of selenium species, and to consider the characterization and biotechnological uses of these transformations and their products.

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.

Bacillus safensis JG-B5T affects the fate of selenium by extracellular production of colloidally less stable selenium nanoparticles

Understanding the impact of microorganisms on the mobility of selenium (Se) is important for predicting the fate of toxic Se in the environment and improving wastewater treatment technologies. The bacteria strain Bacillus safensis JG-B5T, isolated from soil in a uranium mining waste pile, can influence the Se speciation in the environment and engineered systems. However, the mechanism and conditions of this process remain unknown. This study found that the B. safensis JG-B5T is an obligate aerobic microorganism with an ability to reduce 70% of 2.5 mM selenite to produce red spherical biogenic elemental selenium nanoparticles (BioSeNPs). Only extracellular production of BioSeNPs was observed using transmission electron microscopy. The two-chamber reactor experiments, genome analysis and corona proteins identified on BioSeNPs suggested that the selenite reduction process was primarily mediated through membrane-associated proteins, like succinate dehydrogenase. Extracellular presence and low colloidal stability of BioSeNPs as indicated by ζ-potential measurements, render B. safensis JG-B5T an attractive candidate in wastewater treatment as it provides easy way of recovering Se while maintaining low Se discharge. As this microorganism decreases Se mobility, it will affect Se bioavailability in the environment and decreases its toxicity.