A calorimetric comparison of trypsin and its anhydro modification in complex formation with Kunitz soybean inhibitor

The heat of complex formation between native trypsin or its anhydro modification (Ako, H., Foster, R. J., and Ryan, C. J. (1972) Biochem. Biophys. Res Commun. 47, 1402--1407) with native or cleaved Kunitz soybean inhibitor (Finkenstadt, W. R., and Laskowski, M., Jr. (1965) J. Biol. Chem. 240, 962--963) has been observed from pH 3.5 to 7.5. Steep dependence of reaction heat upon pH between pH 3.5 and 4.25 is observed with native inhibitor in reaction with both trypsin and anhydrotrypsin. The character of this heat-pH relationship is consistent with a cooperative process involving three to four ionizable groups. Above pH 4.25, trypsin and anhydrotrypsin in reaction with native inhibitor have quite different pH dependencies. Native trypsin shows an apparent pK near pH 5, whereas anhydrotrypsin + inhibitor reaction heat remains nearly constant from pH 4.5 to 6, and shows an apparent pK near pH 7.0. Above pH 4, the reaction heat-pH relations for native or cleaved inhibitor in reaction with trypsin are only slightly different (Barnhill, M. T., Jr., and Trowbridge, C. G. (1975) J. Biol. Chem. 250, 5501--5507). On the other hand, substitution of cleaved for native inhibitor in reaction with anhydrotrypsin causes a greatly reduced reaction heat, with no clearly developed pH dependence from pH 5.5 to 7.5. The reaction heat-pH relationships are analyzed thermodynamically in terms of the temperature coefficient of a pH-dependent equilibrium constant. It is clear that conversion of the active site serine side chain of trypsin to dehydroalanine changes the pH dependence of its reaction heat with soybean inhibitor, and that the complex formed is sensitive to the state of the scissile inhibitor bond. These differences provide a comparison of a protein-protein association with and without covalent bond formation between reactants.