Selenocysteine is a pa of: (A) NADP re ductase, (B) NADPH dehydrogenase, (C) Thioredoxin reductase, (D) Pyruvate dehydeogenase
Answer is C. 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 with 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 + PiRef: DM Vasudevan, Page no: 187