The selenophosphate synthetase family: A review

SEPHS1 Gene: A new master key for neurodevelopmental disorders

The SEPHS1 (Selenophosphate Synthetase 1) gene encodes a critical enzyme for synthesizing selenophosphate, the active donor of selenium (Se) necessary for selenoprotein biosynthesis. Selenoproteins are vital for antioxidant defense, thyroid hormone metabolism, and cellular homeostasis. Mutations in SEPHS1 gene, are associated with neurodevelopmental disorders with developmental delay, poor growth, hypotonia, and dysmorphic features. Due to Se's critical role in brain development and function, SEPHS1 gene has taken center stage in neurodevelopmental research. This review explores the structure and function of the SEPHS1 gene, its role in neurodevelopment, and the implications of its dysregulation for neurodevelopmental disorders. Therapeutic strategies, including Se supplementation, gene therapy, and targeted therapies, are discussed as potential interventions to address SEPHS1 associated neurodevelopmental dysfunction. The study's findings reveal how SEPHS1 mutations disrupt neurodevelopment, emphasizing the gene's intolerance to loss of function. Future research should focus on functional characterization of SEPHS1 variants, broader genetic screenings, and therapeutic developments.

Enzymatic strategies for selenium incorporation into biological molecules

The trace element selenium (Se) is essential to the physiology of most organisms on the planet. The most well documented of Se's biological forms are selenoproteins, where selenocysteine often serves as the catalytic center for crucial redox processes. Se is also found in several other classes of biological molecules, including nucleic acids, sugars, and modified amino acids, although its role in the function of these metabolites is less understood. Despite its prevalence, only a small number of Se-specific biosynthetic pathways have been discovered. Around half of these were first characterized in the past three years, suggesting that the selenometabolome may be more diverse than previously appreciated. Here, we review the recent advances in our understanding of this intriguing biochemical space, and discuss prospects for future discovery efforts.

PRDX6 augments selenium utilization to limit iron toxicity and ferroptosis

Ferroptosis is a form of regulated cell death induced by iron-dependent accumulation of lipid hydroperoxides. Selenoprotein glutathione peroxidase 4 (GPX4) suppresses ferroptosis by detoxifying lipid hydroperoxides via a catalytic selenocysteine (Sec) residue. Sec, the genetically encoded 21st amino acid, is biosynthesized from a reactive selenium donor on its cognate tRNA[Ser]Sec. It is thought that intracellular selenium must be delivered 'safely' and 'efficiently' by a carrier protein owing to its high reactivity and very low concentrations. Here, we identified peroxiredoxin 6 (PRDX6) as a novel selenoprotein synthesis factor. Loss of PRDX6 decreases the expression of selenoproteins and induces ferroptosis via a reduction in GPX4. Mechanistically, PRDX6 increases the efficiency of intracellular selenium utilization by transferring selenium between proteins within the selenocysteyl-tRNA[Ser]Sec synthesis machinery, leading to efficient synthesis of selenocysteyl-tRNA[Ser]Sec. These findings highlight previously unidentified selenium metabolic systems and provide new insights into ferroptosis.

Asgard archaeal selenoproteome reveals a roadmap for the archaea-to-eukaryote transition of selenocysteine incorporation machinery

Selenocysteine (Sec) is encoded by the UGA codon that normally functions as a stop signal and is specifically incorporated into selenoproteins via a unique recoding mechanism. The translational recoding of UGA as Sec is directed by an unusual RNA structure, the SECIS element. Although archaea and eukaryotes adopt similar Sec encoding machinery, the SECIS elements have no similarities to each other with regard to sequence and structure. We analyzed >400 Asgard archaeal genomes to examine the occurrence of both Sec encoding system and selenoproteins in this archaeal superphylum, the closest prokaryotic relatives of eukaryotes. A comprehensive map of Sec utilization trait has been generated, providing the most detailed understanding of the use of this nonstandard amino acid in Asgard archaea so far. By characterizing the selenoproteomes of all organisms, several selenoprotein-rich phyla and species were identified. Most Asgard archaeal selenoprotein genes possess eukaryotic SECIS-like structures with varying degrees of diversity. Moreover, euryarchaeal SECIS elements might originate from Asgard archaeal SECIS elements via lateral gene transfer, indicating a complex and dynamic scenario of the evolution of SECIS element within archaea. Finally, a roadmap for the transition of eukaryotic SECIS elements from archaea was proposed, and selenophosphate synthetase may serve as a potential intermediate for the generation of ancestral eukaryotic SECIS element. Our results offer new insights into a deeper understanding of the evolution of Sec insertion machinery.

Selenoproteins in Health

Selenium (Se) is a naturally occurring essential micronutrient that is required for human health. The existing form of Se includes inorganic and organic. In contrast to the inorganic Se, which has low bioavailability and high cytotoxicity, organic Se exhibits higher bioavailability, lower toxicity, and has a more diverse composition and structure. This review presents the nutritional benefits of Se by listing and linking selenoprotein (SeP) functions to evidence of health benefits. The research status of SeP from foods in recent years is introduced systematically, particularly the sources, biochemical transformation and speciation, and the bioactivities. These aspects are elaborated with references for further research and utilization of organic Se compounds in the field of health.

Simultaneous DHA and organic selenium production by Schizochytrium sp.: a theoretical basis

Docosahexaenoic acid (DHA) and selenium (Se) are nutrients that confer several health benefits to both humans and animals. Widespread use of DHA in milk powder and health products requires large-scale mass production via Schizochytrium sp., while Se intended for human consumption is produced as organic Se via yeast. However, producing these nutrients on an industrial scale is constrained by various factors. We found that supplementing Schizochytrium sp. with Na2SeO3 (0.5 mg/L) improves its biomass and DHA production and also provides organic Se. De novo assembled transcriptome and biochemical indicators showed that Na2SeO3 promotes forming acetyl coenzyme A and L-cysteine via the glycerol kinase and cysteine synthase pathways, promoting DHA synthesis through the polyketide synthase pathway. However, high doses of Na2SeO3 (5 mg/L) limited the biomass of Schizochytrium sp. and DHA content. This study provided a theoretical basis for the simultaneous production of organic Se and DHA via Schizochytrium sp.

Biological and Catalytic Properties of Selenoproteins

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