This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'Which amino acid is active at neutral pH? \nSelect and explain. \nA. Histidine, B. Leucine, C. Glycine, D. Lysine', 'role': 'user'}, {'content': "Answer: A \nExplanation: Histidine plays unique roles in enzymatic catalysis. The pKa\xa0of its imidazole proton permits histidine to function at neutral pH as either a base or an acid\xa0catalyst\xa0 without\xa0 the\xa0 need\xa0 for\xa0 any\xa0 environmentally\xa0 induced\xa0shift.\nHarper's Illustrated Biochemistry 30 th edition page no 22", 'role': 'assistant'}]