<prompt> Which is 21st amino acid?AlanineCystineArginineSeleno cysteine  <response>: Selenocysteine is recently introduced as 21st amino acid. Selenocysteine occurs at the active site of several enzymes. Examples include Thioredoxin reductase Glutathione peroxidase which scavenges peroxides, De-iodinase that conves thyroxine to triiodothyronine Glycine reductase Selenoprotein P, a glycoprotein containing 10 selenocysteine residues, found in mammalian blood. It has an antioxidant function and its concentration falls in selenium deficiency.Ref: Textbook of medical biochemistry, MN Chatterji, 8th edition, page no: 80 