Abstract
Selenium is an essential micronutrient for man and animals. The role of selenium has been attributed largely to its presence in selenoproteins as the 21st amino acid, selenocysteine (Sec, U). Sec is encoded by TGA in DNA. A unique mechanism is used to decode the UGA codon in mRNA to co-translationally incorporate Sec into the growing polypeptide because there is no free pool of Sec. In the human genome, 25 genes for selenoproteins have been identified. Selenoproteins such as glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases are involved in redox reactions, and Sec is an active-site residue essential for catalytic activity. Selenoproteins have biological functions in oxidoreductions, redox signaling, antioxidant defense, thyroid hormone metabolism, and immune responses. They thus possess a strong correlation with human diseases such as cancer, Keshan disease, virus infections, male infertility, and abnormalities in immune responses and thyroid hormone function.
Highlights
Selenium was discovered by the Swedish chemist Jons Jacob Berzelius in 1817 and has been recognized as an essential trace element for many life forms including man since 1957 [1, 2]
Sec2 is co-translationally incorporated within the growing polypeptide chain by an unusually complex machinery first described in Escherichia coli [3] and well characterized in mammalian cells [1, 2, 4]
One reason for no free Sec pool is the risk of misincorporation of this highly reactive amino acid in the place of Cys. Another reason is that Sec, as well as other selenium compounds like selenite, reacts with oxygen and mammalian thioredoxin and thioredoxin reductase, resulting in rapid NADPH oxidation and reactive oxygen species formation [5, 6]
Summary
The incorporation of selenium as Sec into a selenoprotein requires a specific mechanism to decode the UGA codon in mRNA, which normally operates in translation termination [4]. Selenite and selenate from food and water are used by mammalian cells as selenium sources, and selenite is reduced to selenide by the glutathione-glutaredoxin and thioredoxin systems [5, 6]. The selenoprotein synthesis machinery including a specific secondary structure in the 3Ј-untranslated region of selenoprotein mRNAs termed a SECIS element, Sec-specific elongation factor, Sec-tRNASec, SBP2 (SECIS-binding protein 2), ribosomal protein L30, 43-kDa RNA-binding protein, soluble liver antigen protein, and SPS1 work in concert to incorporate the Sec into a nascent polypeptide at the site encoded by the UGA codon in mammalian cells [1, 4]
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