<prompt> Most stable amino acid at physiological pH isHistidineLysineArginineGlycine  <response>: 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. Harpers Illustrated Biochemistry 30 th edition page no 22 