This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
[{'content': '21st Amino acid is \nSelect and explain. \nA. Selenocysteine, B. Valine, C. Tyrosine, D. Hydroxyproline', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Selenocysteine, the 21st Amino AcidSelenocysteine is present at the active site of several human enzymes that catalyze redox reactions. Examples include thioredoxin reductase, glutathione peroxidase, and the deiodinase that conves thyroxine to triiodothyronine.Ref: Harper&;s Biochemistry; 30th edition; Chapter 27 Biosynthesis of the Nutritionally Nonessential Amino Acids', 'role': 'assistant'}]