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
Maximum influence of local milieu on ionization is on 
Select and explain. 
A. Histidine, B. Protine, C. Glycine, D. Lysine
A: Histidine
Answer: A: Histidine 
Explanation: Depending upon localized pH of its environment (local milieu), histidine can serve both proton donor and acceptor.