Potentiometric Titration and Conformational Change in Pepsinogen

Potentiometric titrations of pepsinogen as functions of ionic strength and temperature were studied. It was found that the ionic strength has a minor effect on the titration behavior of the protein. The maximum values of the buffering capacity in the pH range of 6.0 to 9.0 were found at pH values corresponding to those at which a conformational change of the protein occurs as determined by optical rotatory dispersion measurements. The change of the pH transition with temperature gave a transitional enthalpy of 50 kcal per mole of hydrogen ions liberated in the conformational change. It was shown that the transition occurs in the range of ionization of the three histidines of pepsinogen.