An unusual amino acid of proteins, the analogue of cysteine in which selenium replaces sulphur. Involved in the catalytic mechanism of selenoenzymes such as glutathione peroxidase. May be cotranslationally coded by a special opal suppressor tRNA that recognizes certain UGA nonsense codons. Selenoproteins contain selenocysteine. Selenoprotein N (590 aa) is an endoplasmic reticulum glycoprotein with a single selenocysteine residue. It is highly expressed in myoblasts and down-regulated in differentiating myotubes. Mutations in the gene lead to rigid-spine muscular dystrophy. Selenoprotein P (381 aa) is a plasma heparin-binding protein that appears to be associated with endothelial cells and has been implicated as an oxidant defence. It has multiple selenocysteine residues. Mitochondrial capsule selenoprotein (116 aa) is important for the maintenance and stabilization of the crescent structure of the sperm mitochondria. Selenoprotein Z (524 aa) is thioredoxin reductase 2 (EC 126.96.36.199). Various other selenoproteins are known but are of uncertain function.
Subjects: Medicine and Health — Chemistry.