This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
Which amino acid can protonate and deprotonate at neutral pH? 
Select and explain. 
A. Histidine, B. Leucine, C. Glycine, D. Arginine
A: Histidine
Answer: A: Histidine 
Explanation: Ans. A. HistidineAmino acid which can protonate and deprotonate means those which can act as buffer.Amino acid whose pKa=pH of the medium has maximum buffering capacity.pKa of imidazole group of histidine is 6.5 - 7.4.At pH=7, Imidazole group of histidine can act as buffer.