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 can protonate & deprotonate at neutral pH \nSelect and explain. \nA. Histidine, B. Leucine, C. Hemocysteine, D. Arginine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Histidine is the only amino acid that has a functional group (imidazole) that titrates in the physiological pH range.\nDepending upon localized pH of its environment, histidine can serve both proton donor (deprotonation) and acceptor (protonation).', 'role': 'assistant'}]