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 is active at neutral pH? 
Select and explain. 
A. Histidine, B. Leucine, C. Glycine, D. Lysine
A: Histidine
Answer: A: Histidine 
Explanation: Histidine plays unique roles in enzymatic catalysis. The pKa of its imidazole proton permits histidine to function at neutral pH as either a base or an acid catalyst  without  the  need  for  any  environmentally  induced shift.
Harper's Illustrated Biochemistry 30 th edition page no 22