Which is 21st amino acid?

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