Selenoproteine

Selenoproteins are a family of proteins that incorporate the amino acid selenocysteine, the 21st amino acid, into their polypeptide chains. Selenocysteine is encoded by the UGA codon, which normally signals stop. In selenoprotein synthesis, UGA is recoded as Sec with the help of a specific RNA structure called the SECIS element located in the 3' untranslated region of the mRNA, and a dedicated set of translation factors and tRNA, including the Sec tRNA, SBP2 (SECIS binding protein 2), SecS (selenocysteine synthase), and selenophosphate synthetase (SPS2), which supplies selenium in the form of selenophosphate.

Selenoproteins perform a range of redox and metabolic functions. Well-known examples include the glutathione peroxidases (GPX1–GPX4) and thioredoxin reductases (TXNRD1–TXNRD3), which contribute to antioxidant defense and redox signaling, as well as the deiodinases (DIO1–DIO3) that regulate thyroid hormone activation and inactivation. Other selenoproteins have diverse roles in various cellular compartments, including selenium transport (selenoprotein P, SELENOP) and functions in the endoplasmic reticulum and mitochondria (such as SELENOK, SELENOS, SELENOT, and SELENON).

Biological significance varies by species and tissue, but many selenoproteins are essential for normal development and physiology. In mammals, dietary selenium supports proper selenoprotein synthesis; deficiency can compromise antioxidant capacity and thyroid hormone metabolism, while dysregulation of selenoproteins is associated with various diseases.