This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
[{'content': 'Which is 21st amino acid? \nSelect and explain. \nA. Alanine, B. Cystine, C. Arginine, D. Seleno cysteine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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', 'role': 'assistant'}]