Which amino acid is active at neutral pH?: (A) Histidine, (B) Leucine, (C) Glycine, (D) Lysine
Answer is A. 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