Cofactor for glutathione peroxidase

Selenocysteine is abbreted as SeCys. It is seen at the active site of the following enzymes: a) Thioredoxin reductase; b) Glutathione peroxidase, which scavenges peroxides; c) 5&;- De-iodinase that removes iodine from thyroxine to make triiodothyronine and d) Selenoprotein P, a glycoprotein seen in mammalian blood. Replacement of SeCys by Cys will lead to decreased activity of these enzymes. Deficiency of some of these enzymes with the anti-oxidant function may be related to atherosclerosis. Concentration falls in selenium deficiency. Its structure is COOH-CHNH2-CH2-SeH. It is directly incorporated into proteins during the translation process. Biosynthesis of selenocysteine is by replacing the oxygen of serine by selenium; this is done by two steps: Se + ATP - Se-P + AMP + Pi Serine + Se-P - SeCys + Pi Its genetic code is UGA. The tRNA-Sec is the specific tRNA, inseing SeCys. However, normally UGA codon is a stop signal. UGA code for SeCys is identified by a SeCys inseion element in the mRNA, with the help of a specific elongation factor. The tRNA-Sec is first charged with serine, then it is conveed into SeCys. This is then inseed into the correct position when protein is synthesized.Ref: DM Vasudevan, page no: 187