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': 'Maximum influence of local milieu on ionization is on \nSelect and explain. \nA. Histidine, B. Protine, C. Glycine, D. Lysine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Depending upon localized pH of its environment (local milieu), histidine can serve both proton donor and acceptor.', 'role': 'assistant'}]