Which amino acid is active at neutral pH?
Histidine
Leucine
Glycine
Lysine
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