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