Which of the following aminoacids is a component of Thioredoxin reductase-

Ans. is 'a' i.e., Selenocysteine* The 21st proteinogenic aminoacid is selenocysteine (It is represented as sec or as U).* Selenocysteine is a component of:# Thioredoxin reductase# 5' deiodinase# Glutathione peroxidase* Selenocysteine is coded by one of the stop codons UGA, if it is followed by selenocysteine insertion element in the 3' untranslated region of mRNA* tRNAsec (tRNA which is charged with selenocysteine) is first attached to serine by seryl tRNA synthetase or seryl tRNA ligase. But this tRNA is not identified by EF1A (the translation factor which attaches every aminoacyltRNA to A site of ribosome during chain elongation of protein synthesis).* Serine attached tRNAsec is converted into selenocysteine attached tRNAsec by selenocysteine synthetase (Pyridoxal phosphate dependent enzyme) and is guided by EFsec to A site of 60 s ribosome* Please note the 22nd aminoacid is pyrrolysine (represented as pyl or O) is also coded by amber codon UAG. It is not found in human proteins.