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
Which amino acid can protonate & deprotonate at neutral pH 
Select and explain. 
A. Histidine, B. Leucine, C. Hemocysteine, D. Arginine
A: Histidine
Answer: A: Histidine 
Explanation: Histidine is the only amino acid that has a functional group (imidazole) that titrates in the physiological pH range.
Depending upon localized pH of its environment, histidine can serve both proton donor (deprotonation) and acceptor (protonation).