Metabolic and bioprocess engineering for production of selenized yeast with increased content of seleno-methylselenocysteine

Sensory quality and metabolite dynamics in an organic selenium-enriched milk fermented by Geotrichum candidum

This study explores selenium enrichment of milk through microbial transformation, utilizing Geotrichum candidum LG-8, isolated from traditional dairy products. A combination of electronic tongue, electronic nose, HPLC-ICP-MS, and UHPLC-QTOF-MS were used to evaluate milk quality, organic selenium types, and various components. The milk retained a weakly acidic pH post-transformation, with a flavor profile rich in sour, sweet, and umami tastes. Odor analysis revealed predominant contributions from nitrogen oxides, aromatic compounds, ketones, and organic sulfides. Selenium concentrations increased with higher initial selenium levels, with 50 μg/mL yielding concentrations of MeSeCys, SeCys2, Se(IV), SeMet, and Se(VI) at 5.15, 20.80, 1.41, 75.31 and 0.44 μg/L respectively. Significant changes were observed in the milk's amino acid profile, with metabolite identification and metabolic pathways mapping(including KEGG). The findings highlight the potential of Geotrichum candidum LG-8 for generating functional organic selenium-enriched milk, offering promising nutritional benefits in dairy production.

Effect of Selenium Supplementation on Biotin and Selenobiotin Concentrations in Meyerozyma guilliermondii and Trichosporon cutaneum Cells

Numerous studies have demonstrated the efficacy of selenium compounds in preventing and treating lifestyle-related diseases such as cancer and cardiovascular disorders. The formulation of selenium-enriched supplements for humans and animals, particularly those containing selenium yeast, is highly advantageous. These products are rich in organic selenium derivatives, showing significantly higher bioavailability than inorganic forms of selenium. A particularly promising selenium analogue of sulphur-containing compounds is selenobiotin. The literature indicates that Phycomyces blakesleeanus and Escherichia coli strains can synthesise this compound. This research aimed to evaluate the effect of selenium supplementation on the biosynthesis of biotin and selenobiotin in Trichosporon cutaneum and Meyerozyma guilliermondii. The results have the potential to advance biotechnological approaches for the production of selenobiotin for various applications. A method based on affinity chromatography was used to quantify selenobiotin. The results confirmed that both yeast strains could synthesise selenobiotin in addition to biotin. In M. guilliermondii cells, selenobiotin accounted for up to 17.3% of the total biotin vitamer fraction. In comparison, in T. cutaneum cells, it accounted for up to 28.4% of the sum of biotin and its analogues. The highest levels of selenobiotin were observed in cells cultured with selenomethionine.

Metabolic engineering of Escherichia coli for seleno-methylselenocysteine production

Selenium (Se) is an essential trace element for life. Seleno-methylselenocysteine (SeMCys) can serve as a Se supplement with anticarcinogenic activity and can improve cognitive deficits. We engineered Escherichia coli for microbial production of SeMCys. The genes involved in the synthesis of SeMCys were divided into three modules-the selenocysteine (SeCys) synthesis, methyl donor synthesis and SMT modules-and expressed in plasmids with different copy numbers. The higher copy number of the SeCys synthesis module facilitated SeMCys production. The major routes for SeCys degradation were then modified. Deletion of the cysteine desulfurase gene csdA or sufS improved SeMCys production the most, and the strain that knocked out both genes doubled SeMCys production. The addition of serine in the mid-logarithmic growth phase significantly improved SeMCys synthesis. When the serine synthetic pathway was enhanced, SeMCys production increased by 12.5 %. Fed-batch culture for sodium selenite supplementation in the early stationary phase improved SeMCys production to 3.715 mg/L. This is the first report of the metabolic engineering of E. coli for the production of SeMCys and provide information on Se metabolism.

Industrial Production of Functional Foods for Human Health and Sustainability

Functional foods significantly affect social stability, human health, and food security. Plants and microorganisms are high-quality chassis for the bioactive ingredients in functional foods. Characterised by precise nutrition and the provision of both nutritive and medicinal value, functional foods serve a as key extension of functional agriculture and offer assurance of food availability for future space exploration efforts. This review summarises the main bioactive ingredients in functional foods and their functions, describes the strategies used for the nutritional fortification and industrial production of functional foods, and provides insights into the challenges and future developments in the applications of plants and microorganisms in functional foods. Our review aims to provide a theoretical basis for the development of functional foods, ensure the successful production of new products, and support the U.N. Sustainable Development Goals, including no poverty, zero hunger, and good health and well-being.

Selenized non-Saccharomyces yeasts and their potential use in fish feed

Selenium (Se) is a vital trace element, essential for growth and other biological functions in fish. Its significance lies in its role as a fundamental component of selenoproteins, which are crucial for optimal functioning of the organism. The inclusion of Se in the diets of farmed animals, including fish, has proved invaluable in mitigating the challenges arising from elemental deficiencies experienced in captivity conditions due to limitations in the content of fishmeal. Supplementing diets with Se enhances physiological responses, particularly mitigates the effects of the continuous presence of environmental stress factors. Organic Se has been shown to have higher absorption rates and a greater impact on bioavailability and overall health than inorganic forms. A characteristic feature of yeasts is their rapid proliferation and growth, marked by efficient mineral assimilation. Most of the selenized yeasts currently available in the market, and used predominantly in animal production and aquaculture, are based on Saccharomyces cerevisiae, which contains selenomethionine (Se-Met). The object of this review is to highlight the importance of selenized yeasts. In addition, it presents metabolic and productive aspects of other yeast genera that are important potential sources of organic selenium. Some yeast strains discussed produce metabolites of interest such as lipids, pigments, and amino acids, which could have applications in aquaculture and further enrich their usefulness.

Screening of Selenium/Glutathione-Enriched Candida utilis and Its Anti-inflammatory and Antioxidant Activities in Mice

This study aimed to screen a mutant of Candida utilis SE-172 with high selenite tolerance and glutathione (GSH) biosynthesis capability via 60Co γ-radiation mutagenesis to prepare selenium (Se)-enriched yeast. The maximal intracellular contents of GSH and organic Se of 22.94 mg/g and 1308.1 μg/g were obtained, respectively, under a batch culture of SE-172. The physiological mechanism underlying increased GSH and organic Se contents in Se/GSH-enriched C. utilis SE-172 was revealed based on assaying activities of γ-glutamylcysteine synthase (γ-GCS) involved in GSH biosynthesis and selenophosphate synthase (SPS) related to organic Se bioconversion, and by determining intracellular ATP and NADH contents and ATP/ADP and NADH/NAD+ ratios associated with energy supply and regeneration. Moreover, the effect of this selenized yeast on anti-inflammatory and antioxidant activities in mice with colitis was investigated. The supplementation of Se/GSH-enriched yeast decreased the dextran sodium sulfate-induced damage to colon tissues, reduced the expression of pro-inflammatory factors [interleukin (IL)-6, IL-1β, and tumor necrosis factor-α (TNF-α)] in serum, increased the antioxidant-related enzyme [superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px)] activities, and decreased the malondialdehyde content in colon. The Se/GSH-enriched C. utilis SE-172 showed potent anti-inflammatory and antioxidant activities in mice with colitis.

Enhancing Selenium Accumulation in Rhodotorula mucilaginosa Strain 6S Using a Proteomic Approach for Aquafeed Development

It is known that selenium (Se) is an essential trace element, important for the growth and other biological functions of fish. One of its most important functions is to contribute to the preservation of certain biological components, such as DNA, proteins, and lipids, providing protection against free radicals resulting from normal metabolism. The objective of this study was to evaluate and optimize selenium accumulation in the native yeast Rhodotorula mucilaginosa 6S. Sodium selenite was evaluated at different concentrations (5-10-15-20-30-40 mg/L). Similarly, the effects of different concentrations of nitrogen sources and pH on cell growth and selenium accumulation in the yeast were analyzed. Subsequently, the best cultivation conditions were scaled up to a 2 L reactor with constant aeration, and the proteome of the yeast cultured with and without sodium selenite was evaluated. The optimal conditions for biomass generation and selenium accumulation were found with ammonium chloride and pH 5.5. Incorporating sodium selenite (30 mg/L) during the exponential phase in the bioreactor after 72 h of cultivation resulted in 10 g/L of biomass, with 0.25 mg total Se/g biomass, composed of 25% proteins, 15% lipids, and 0.850 mg total carotenoids/g biomass. The analysis of the proteomes associated with yeast cultivation with and without selenium revealed a total of 1871 proteins. The results obtained showed that the dynamic changes in the proteome, in response to selenium in the experimental medium, are directly related to catalytic activity and oxidoreductase activity in the yeast. R. mucilaginosa 6S could be an alternative for the generation of selenium-rich biomass with a composition of other nutritional compounds also of interest in aquaculture, such as proteins, lipids, and pigments.

Co-metabolism of Na+/K+ ion regulated physiological enhancement on selenium-accumulation in Saccharomyces yeasts

BACKGROUND

Selenium is an important nutritional supplement that mainly exists naturally in soil as inorganic selenium. Saccharomyces cerevisiae cells are excellent medium for converting inorganic selenium in nature into organic selenium.

RESULTS

Under the co-stimulation of sodium selenite (Na2SeO3) and potassium selenite (K2SeO3), the activity of selenophosphate synthetase (SPS) was improved up to about five folds more than conventional Na2SeO3 group with the total selenite salts content of 30 mg/L. Transcriptome analysis first revealed that due to the sharing pathway between sodium ion (Na+) and potassium ion (K+), the K+ largely regulates the metabolisms of amino acid and glutathione under the accumulation of selenite salt. Furthermore, K+ could improve the tolerance performance and selenium-biotransformation yields of Saccharomyces cerevisiae cells under Na2SeO3 salt stimulation.

CONCLUSION

The important role of K+ in regulating the intracellular selenium accumulation especially in terms of amino acid metabolism and glutathione, suggested a new direction for the development of selenium-enrichment supplements with Saccharomyces cerevisiae cell factory. © 2024 Society of Chemical Industry.

Optimized methyl donor and reduced precursor degradation pathway for seleno-methylselenocysteine production in Bacillus subtilis

BACKGROUND

Seleno-methylselenocysteine (SeMCys) is an effective component of selenium supplementation with anti-carcinogenic potential that can ameliorate neuropathology and cognitive deficits. In a previous study, a SeMCys producing strain of Bacillus subtilis GBACB was generated by releasing feedback inhibition by overexpression of cysteine-insensitive serine O-acetyltransferase, enhancing the synthesis of S-adenosylmethionine as methyl donor by overexpression of S-adenosylmethionine synthetase, and expressing heterologous selenocysteine methyltransferase. In this study, we aimed to improve GBACB SeMCys production by synthesizing methylmethionine as a donor to methylate selenocysteine and by inhibiting the precursor degradation pathway.

RESULTS

First, the performance of three methionine S-methyltransferases that provide methylmethionine as a methyl donor for SeMCys production was determined. Integration of the NmMmt gene into GBACB improved SeMCys production from 20.7 to 687.4 μg/L. Next, the major routes for the degradation of selenocysteine, which is the precursor of SeMCys, were revealed by comparing selenocysteine hyper-accumulating and non-producing strains at the transcriptional level. The iscSB knockout strain doubled SeMCys production. Moreover, deleting sdaA, which is responsible for the degradation of serine as a precursor of selenocysteine, enhanced SeMCys production to 4120.3 μg/L. Finally, the culture conditions in the flasks were optimized. The strain was tolerant to higher selenite content in the liquid medium and the titer of SeMCys reached 7.5 mg/L.

CONCLUSIONS

The significance of methylmethionine as a methyl donor for SeMCys production in B. subtilis is reported, and enhanced precursor supply facilitates SeMCys synthesis. The results represent the highest SeMCys production to date and provide insight into Se metabolism.

Effects of Selenium Enrichment on Dough Fermentation Characteristics of Baker's Yeast

In this research, the effect of selenium (Se) enrichment on dough fermentation characteristics of yeast and the possible mechanisms was investigated. Then, the Se-enriched yeast was used as starter to make Se-enriched bread, and the difference between Se-enriched bread and common bread was investigated. It was found Se enrichment increased CO₂ production and sugar consumption rate of Saccharomyces cerevisiae (S. cerevisiae) in dough fermentation, and had positive impacts on final volume and rheological index of dough. The mechanism is possibly related to higher activity and protein expression of hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK), citrate synthase (CS), isocitrate dehydrogenase (ICD), and α-ketoglutarate dehydrogenase (α-KGDHC) in Se-enriched yeast. Moreover, Se-enriched bread (Se content: 11.29 μg/g) prepared by using Se-enriched yeast as starter exhibited higher overall acceptability on sensory, cell density in stomatal morphology, and better elasticity and cohesiveness on texture properties than common bread, which may be due to effect of higher CO₂ production on dough quality. These results indicate Se-enriched yeast could be used as both Se-supplements and starter in baked-foods making.

Enhanced selenocysteine biosynthesis for seleno-methylselenocysteine production in Bacillus subtilis

Seleno-methylselenocysteine (SeMCys) is an effective component for selenium supplementation with anti-carcinogenic potential and can ameliorate neuropathology and cognitive deficits. In this study, we aimed to engineer Bacillus subtilis 168 for the microbial production of SeMCys. First, the accumulation of intracellular selenocysteine (SeCys) as the precursor of SeMCys was enhanced through overexpression of serine O-acetyltransferase, which was desensitized against feedback inhibition by cysteine. Next, the S-adenosylmethionine (SAM) synthetic pathway was optimized to improve methyl donor availability through expression of S-adenosylmethionine synthetase. Further, SeMCys was successfully produced through expression of the selenocysteine methyltransferase in SeCys and SAM-producing strain. The increased expression level of selenocysteine methyltransferase benefited the SeMCys production. Finally, all the heterologous genes were integrated into the genome of B. subtilis, and the strain produced SeMCys at a titer of 18.4 μg/L in fed-batch culture. This is the first report on the metabolic engineering of B. subtilis for microbial production of SeMCys and provides a good starting point for future pathway engineering to achieve the industrial-grade production of SeMCys. KEY POINTS: • Expression of the feedback-insensitive serine O-acetyltransferase provided B. subtilis the ability of accumulating SeCys. • SAM production was enhanced through expressing S-adenosylmethionine synthetase in B. subtilis. • Expression of selenocysteine methyltransferase in SeCys and SAM-accumulating strain facilitated SeMCys production.

Increased SIRT1 Concentration Following Four Years of Selenium and Q10 Intervention Associated with Reduced Cardiovascular Mortality at 10-Year Follow-Up-Sub-Study of a Previous Prospective Double-Blind Placebo-Controlled Randomized Clinical Trial

Background: Selenium and coenzyme Q₁₀ (SeQ₁₀) possess antioxidant and anti-inflammatory properties, potentially mediated via Sirtuin1 (SIRT1). We aimed to investigate the influence of a SeQ₁₀ intervention on SIRT1 concentration, with potential interactions with microRNAs. Methods: In this sub-study of a prospective double-blind placebo-controlled clinical trial, healthy subjects (mean age 76 years) were randomized to receive an active treatment (n = 165, combined 200 µg/day of Se and 200 mg/day of Q₁₀) or a placebo (n = 161). SIRT1 concentration and microRNAs were measured with ELISA and PCR, respectively. Results: After four years, SIRT1 concentration was increased in the active treatment group, with mean (SD) ng/mL of 469 (436) vs. 252 (162), p < 0.001, and decreased in the placebo group, 190 (186) vs. 269 (172), p = 0.002, and the differences between the groups were significant (p = 0.006, adjusted). Those who suffered CV death during a 10-year follow-up (n = 25 and n = 52 in the active treatment and placebo groups, respectively) had significantly lower baseline SIRT1 concentrations compared to the survivors (p < 0.001). MiR-130a-3p was significantly downregulated during the intervention and correlated inversely with SIRT1 at baseline (r = -0.466, p = 0.007). Conclusion: The increased SIRT1 concentration after the SeQ₁₀ intervention associated with reduced CV mortality, partly mediated via miR-1303a-3p, suggests that SIRT1 is an additional mediator of the intervention, preventing vascular ageing.

Accumulation and Enrichment of Trace Elements by Yeast Cells and Their Applications: A Critical Review

Maintaining the homeostasis balance of trace elements is crucial for the health of organisms. Human health is threatened by diseases caused by a lack of trace elements. Saccharomyces cerevisiae has a wide and close relationship with human daily life and industrial applications. It can not only be used as fermentation products and single-cell proteins, but also as a trace elements supplement that is widely used in food, feed, and medicine. Trace-element-enriched yeast, viz., chromium-, iron-, zinc-, and selenium-enriched yeast, as an impactful microelements supplement, is more efficient, more environmentally friendly, and safer than its inorganic and organic counterparts. Over the last few decades, genetic engineering has been developing large-scaled genetic re-design and reconstruction in yeast. It is hoped that engineered yeast will include a higher concentration of trace elements. In this review, we compare the common supplement forms of several key trace elements. The mechanisms of detoxification and transport of trace elements in yeast are also reviewed thoroughly. Moreover, genes involved in the transport and detoxification of trace elements are summarized. A feasible way of metabolic engineering transformation of S. cerevisiae to produce trace-element-enriched yeast is examined. In addition, the economy, safety, and environmental protection of the engineered yeast are explored, and the future research direction of yeast enriched in trace elements is discussed.

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.

Synthetic biology: a new frontier in food production

Concerns regarding food security arise from population growth, global warming, and reduction in arable land. With advances in synthetic biology, food production by microbes is considered to be a promising alternative that would allow rapid food production in an environmentally friendly manner. Moreover, synthetic biology can be adopted to the production of healthier or specifically designed food ingredients (e.g., high-value proteins, lipids, and vitamins) and broaden the utilization of feedstocks (e.g., methanol and CO₂), thereby offering potential solutions to high-quality food and the greenhouse effect. We first present how synthetic biology can facilitate the microbial production of various food components, and then discuss feedstock availability enabled by synthetic biology. Finally, we illustrate trends and key challenges in synthetic biology-driven food production.

Mechanisms Affecting the Biosynthesis and Incorporation Rate of Selenocysteine

Selenocysteine (Sec) is the 21st non-standard proteinogenic amino acid. Due to the particularity of the codon encoding Sec, the selenoprotein synthesis needs to be completed by unique mechanisms in specific biological systems. In this paper, the underlying mechanisms for the biosynthesis and incorporation of Sec into selenoprotein were comprehensively reviewed on five aspects: (i) the specific biosynthesis mechanism of Sec and the role of its internal influencing factors (SelA, SelB, SelC, SelD, SPS2 and PSTK); (ii) the elements (SECIS, PSL, SPUR and RF) on mRNA and their functional mechanisms; (iii) the specificity (either translation termination or translation into Sec) of UGA; (iv) the structure–activity relationship and action mechanism of SelA, SelB, SelC and SelD; and (v) the operating mechanism of two key enzyme systems for inorganic selenium source flow before Sec synthesis. Lastly, the size of the translation initiation interval, other action modes of SECIS and effects of REPS (Repetitive Extragenic Palindromic Sequences) that affect the incorporation efficiency of Sec was also discussed to provide scientific basis for the large-scale industrial fermentation for the production of selenoprotein.

Accumulation of Selenium in Candida utilis Growing in Media of Increasing Concentration of this Element

Selenium is considered an essential component of all living organisms. Studies on the enrichment of yeast cells with selenium, using the ability of cell biomass to bind this element, are being reported more and more. Yeast cultures were cultivated in YPD medium enriched with Na2SeO3 salts for 72 h at 28 °C on a shaker utilizing reciprocating motion. Selenium in cell biomass was determined with the use of ICP–MS. It was observed that the addition of selenium to the experimental medium (in the range of 4–100 mg/L) increased the content of this element in the yeast cell biomass. During the extension of cultivation time, the number of yeast cells and biomass yield exhibited a decreasing trend. Based on the obtained results, it was concluded that yeast cells exhibited the ability to accumulate selenium in both logarithmic and stationary growth phases. The dose of 20 and 30 mg/L of selenium in the culture medium meets the expectations in terms of both the content of selenium bound to yeast cells (1944 ± 110.8 μg/g dry weight) under 48-h cultivation. The obtained results confirmed that the Candida utilis ATCC 9950 strain exhibits the ability to bind selenium, which means that the biomass of these yeasts may be used as a natural source of selenium in the diet of humans and animals.

Comparison of the effects of selenomethionine and selenium-enriched yeast in the triple-transgenic mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a complex, multifactorial neurodegenerative disease that exhibits multiple pathogeneses and heterogeneity. Selenium (Se) is an essential trace element for human and animal nutrition. It has been shown that supplementation with two organic forms of Se, Se-enriched yeast (Se-yeast) and selenomethionine (Se-Met), could improve cognitive impairment, reverse synaptic deficits and mitigate tau pathology in triple-transgenic (3× Tg) AD mice. Se-yeast is well known for its high Se-Met content, which may mediate its anti-AD effects. In addition, a large amount of the physiological and biochemical mechanisms of these two Se drugs in the amelioration AD pathology remains unknown. In this study, the content of Se-yeast aside from Se was analyzed, and the effects of Se-Met and Se-yeast on 3× Tg-AD mice were investigated and compared. The results showed that both Se-Met and Se-yeast not only significantly increased the Se levels, enhanced the antioxidant capacity and improved the cognitive decline in the model, but also decreased the Aβ and tau pathologies in the brain tissue of the AD mice. Moreover, the ability of Se-Met to increase the Se levels in different tissues of the AD mice was more significant than that of Se-yeast. However, the positive effect of Se-yeast on improving the cognitive ability of the AD mice was better than that of Se-Met, likely due to the various elements, vitamins and other nutrients in Se-yeast. Collectively, these results suggest that Se-yeast has potential as a clinical health product or drug for AD but that Se-Met, as a pure organic Se compound, is more suitable for studying the therapeutic mechanism of Se because of its comprehensive effects on AD.

Precise genome-wide base editing by the CRISPR Nickase system in yeast

The CRISPR/Cas9 system has been applied to efficient genome editing in many eukaryotic cells. However, the bases that can be edited by this system have been limited to those within the protospacer adjacent motif (PAM) and guide RNA-targeting sequences. In this study, we developed a genome-wide base editing technology, "CRISPR Nickase system" that utilizes a single Cas9 nickase. This system was free from the limitation of editable bases that was observed in the CRISPR/Cas9 system, and was able to precisely edit bases up to 53 bp from the nicking site. In addition, this system showed no off-target editing, in contrast to the CRISPR/Cas9 system. Coupling the CRISPR Nickase system with yeast gap repair cloning enabled the construction of yeast mutants within only five days. The CRISPR Nickase system provides a versatile and powerful technology for rapid, site-specific, and precise base editing in yeast.

Binding and Conversion of Selenium in Candida utilis ATCC 9950 Yeasts in Bioreactor Culture

Selenium is considered an essential component of all living organisms. The use of yeasts as a selenium supplement in human nutrition has gained much interest over the last decade. The accumulation and biochemical transformation of selenium in yeast cells is particularly interesting to many researchers. In this article, we present the results of the determination of selenium and selenomethionine content in the biomass of feed yeast Candida utilis ATCC 9950 obtained from the culture grown in a bioreactor. The results indicated that C. utilis cells performed the biotransformation of inorganic selenium(IV) to organic derivatives (e.g., selenomethionine). Selenium introduced (20-30 mg Se4+∙L-1) to the experimental media in the form of sodium(IV) selenite (Na₂SeO₃) salt caused a significant increase in selenium content in the biomass of C. utilis,irrespective of the concentration. The highest amount of selenium (1841 μg∙gd.w.-1) was obtained after a 48-h culture in media containing 30 mg Se4+∙L-1. The highest content of selenomethionine (238.8 μg∙gd.w.-1) was found after 48-h culture from the experimental medium that was supplemented with selenium at a concentration of 20 mg Se4+∙L-1. Biomass cell in the cultures supplemented with selenium ranged from 1.5 to 14.1 g∙L-1. The results of this study indicate that yeast cell biomass of C. utilis enriched mainly with the organic forms of selenium can be a valuable source of protein. It creates the possibility of obtaining selenium biocomplexes that can be used in the production of protein-selenium dietary supplements for animals and humans.

The mechanism of improved intracellular organic selenium and glutathione contents in selenium-enriched Candida utilis by acid stress

Batch culture of Candida utilis CCTCC M 209298 for the preparation of selenium (Se)-enriched yeast was carried out under different pH conditions, and maximal intracellular organic Se and glutathione (GSH) contents were obtained in a moderate acid stress environment (pH 3.5). In order to elucidate the physiological mechanism of improved performance of Se-enriched yeast by acid stress, assays of the key enzymes involved in GSH biosynthesis and determinations of energy supply and regeneration were performed. The results indicated that moderate acid stress increased the activity of γ-glutamylcysteine synthetase and the ratios of NADH/NAD⁺ and ATP/ADP, although no significant changes in intracellular pH were observed. In addition, the molecular mechanism of moderate acid stress favoring the improvement of Se-yeast performance was revealed by comparing whole transcriptomes of yeast cells cultured at pH 3.5 and 5.5. Comparative analysis of RNA-Seq data indicated that 882 genes were significantly up-regulated by moderate acid stress. Functional annotation of the up-regulated genes based on gene ontology and the Kyoto Encyclopedia of Genes and Genome (KEGG) pathway showed that these genes are involved in ATP synthesis and sulfur metabolism, including the biosynthesis of methionine, cysteine, and GSH in yeast cells. Increased intracellular ATP supply and more amounts of sulfur-containing substances in turn contributed to Na₂SeO₃ assimilation and biotransformation, which ultimately improved the performance of the Se-enriched C. utilis.

Effects of Selenium on Morphological Changes in Candida utilis ATCC 9950 Yeast Cells

This paper presents the results of microscopic examinations of the yeast cells cultured in yeast extract-peptone-dextrose (YPD) media supplemented with sodium selenite(IV). The analysis of the morphological changes in yeast cells aimed to determine whether the selected selenium doses and culturing time may affect this element accumulation in yeast cell structures in a form of inorganic or organic compounds, as a result of detoxification processes. The range of characteristic morphological changes in yeasts cultivated in experimental media with sodium selenite(IV) was observed, including cell shrinkage and cytoplasm thickening of the changes within vacuole structure. The processes of vacuole disintegration were observed in aging yeast cells in culturing medium, which may indicate the presence of so-called ghost cells lacking intracellular organelles The changes occurring in the morphology of yeasts cultured in media supplemented with sodium selenite were typical for stationary phase of yeast growth. From detailed microscopic observations, larger surface area of the cell (6.03 μm(2)) and yeast vacuole (2.17 μm(2)) were noticed after 24-h culturing in the medium with selenium of 20 mg Se(4+)/L. The coefficient of shape of the yeast cells cultured in media enriched with sodium selenite as well as in the control YPD medium ranged from 1.02 to 1.22. Elongation of cultivation time (up to 48 and 72 h) in the media supplemented with sodium selenite caused a reduction in the surface area of the yeast cell and vacuole due to detoxification processes.

Biochemical Comparison of Commercial Selenium Yeast Preparations

The trace mineral selenium (Se) is an essential element for human and animal nutrition. The addition of Se to the diet through dietary supplements or fortified food/feed is increasingly common owing to the often sub-optimal content of standard diets of many countries. Se supplements commercially available include the inorganic mineral salts such as sodium selenite or selenate, and organic forms such as Se-enriched yeast. Today, Se yeast is produced by several manufacturers and has become the most widely used source of Se for human supplementation and is also widely employed in animal nutrition where approval in all species has been granted by regulatory bodies such as the European Food Safety Authority (EFSA). Characterisation and comparison of Se-enriched yeast products has traditionally been made by quantifying total selenomethionine (SeMet) content. A disadvantage of this approach, however, is that it does not consider the effects of Se deposition on subsequent digestive availability. In this study, an assessment was made of the water-soluble extracts of commercially available Se-enriched yeast samples for free, peptide-bound and total water-soluble SeMet. Using LC-MS/MS, a total of 62 Se-containing proteins were identified across four Se yeast products, displaying quantitative/qualitative changes in abundance relative to the certified reference material, SELM-1 (P value <0.05; fold change ≥2). Overall, the study indicates that significant differences exist between Se yeast products in terms of SeMet content, Se-containing protein abundance and associated metabolic pathways.

Accumulation and metabolism of selenium by yeast cells

This paper examines the process of selenium bioaccumulation and selenium metabolism in yeast cells. Yeast cells can bind elements in ionic from the environment and permanently integrate them into their cellular structure. Up to now, Saccharomyces cerevisiae, Candida utilis, and Yarrowia lipolytica yeasts have been used primarily in biotechnological studies to evaluate binding of minerals. Yeast cells are able to bind selenium in the form of both organic and inorganic compounds. The process of bioaccumulation of selenium by microorganisms occurs through two mechanisms: extracellular binding by ligands of membrane assembly and intracellular accumulation associated with the transport of ions across the cytoplasmic membrane into the cell interior. During intracellular metabolism of selenium, oxidation, reduction, methylation, and selenoprotein synthesis processes are involved, as exemplified by detoxification processes that allow yeasts to survive under culture conditions involving the elevated selenium concentrations which were observed. Selenium yeasts represent probably the best absorbed form of this element. In turn, in terms of wide application, the inclusion of yeast with accumulated selenium may aid in lessening selenium deficiency in a diet.

Effects of supplementation with selenium, as selenized yeast, in a healthy male population from New Zealand

Selenium (Se) supplementation was tested in a group of healthy men from Auckland, New Zealnd with selenized yeast (Selplex, 200 μg/day) as the supplementation mode. A set of biomarkers, including DNA damage levels and seleno-antioxidant enzyme levels, were evaluated at pre- and postsupplementation time points. Supplementation produced significant increases in serum Se levels, red blood cell (RBC) thioredoxin reductase (TR) activity and peroxide-induced DNA damage, when the mean baseline serum Se level was 110 ng/ml. Those with higher baseline serum Se levels gained less serum Se and showed a significant reduction of RBC glutathione peroxidase (GPx) activity by supplementation. The optimum benefits of supplementation on DNA stability are observed when the serum Se level reaches between >120 and <160 ng/ml. However, the most significant observation was that those with highest baseline DNA damage benefit the most from Se supplementation, whereas those having lower baseline DNA damage are disadvantaged. A dose of 200 μg/day selenized yeast was also shown to be a safer supplementation option compared to a similar dose of selenomethionine (SeMet). This study highlights the requirement for prestratification of a population by standing serum Se level and baseline DNA damage level, before any Se supplementation is carried out.

Metabolic engineering of Saccharomyces cerevisiae: a key cell factory platform for future biorefineries

Metabolic engineering is the enabling science of development of efficient cell factories for the production of fuels, chemicals, pharmaceuticals, and food ingredients through microbial fermentations. The yeast Saccharomyces cerevisiae is a key cell factory already used for the production of a wide range of industrial products, and here we review ongoing work, particularly in industry, on using this organism for the production of butanol, which can be used as biofuel, and isoprenoids, which can find a wide range of applications including as pharmaceuticals and as biodiesel. We also look into how engineering of yeast can lead to improved uptake of sugars that are present in biomass hydrolyzates, and hereby allow for utilization of biomass as feedstock in the production of fuels and chemicals employing S. cerevisiae. Finally, we discuss the perspectives of how technologies from systems biology and synthetic biology can be used to advance metabolic engineering of yeast.

The interplay between sulphur and selenium metabolism influences the intracellular redox balance in Saccharomyces cerevisiae

Selenium (Se) is an essential element for most eukaryotic organisms, including humans. The balance between Se toxicity and its beneficial effects is very delicate. It has been demonstrated that a diet enriched with Se has cancer prevention potential in humans. The most popular commercial Se supplementation is selenized yeast, which is produced in a fermentation process using an inorganic source of Se. Here, we show that the uptake of Se, Se toxic effects and intracellular Se-metabolite profile are largely influenced by the level of sulphur source supplied during the fermentation. A Yap1-dependent oxidative stress response is active when yeast actively metabolizes Se, and this response is linked to the generation of intracellular redox imbalance. The redox imbalance derives from a disproportionate ratio between the reduced and oxidized forms of glutathione and also from the influence of Se metabolism on the central carbon metabolism. The observed increase in glycerol production rate, concomitant with the inhibition of ethanol formation in the presence of Se, can be ascribed to the occurrence of redox imbalance that triggers glycerol biosynthesis to replenish the pool of NAD(+) .