DIFFERENTIAL SCANNING CALORIMETRIC STUDY OF CARBOXYPEPTIDASE-B, PROCARBOXYPEPTIDASE-B AND ITS GLOBULAR ACTIVATION DOMAIN

High-sensitivity differential scanning calorimetry has been applied to the study of porcine pancreatic carboxypeptidase B, the proenzyme and its 81-residue activation domain. The thermal study has been carried out over a range of scan rates, ionic strengths and pH values. The thermal unfolding of the isolated activation domain has been found to be reversible and corresponds to that of a typical compact globular structure, with melting temperatures higher than those of the enzyme and proenzyme. Both proteins, on the other hand, undergo an irreversible, highly scan-rate-dependent thermal denaturation under all the experimental conditions investigated. The denaturation of the enzyme at pH 7.5 and the proenzyme at pH 7.5 and 9.0 follows the two-state irreversible model [Sanchez-Ruiz, J. M., Lopez-Lacomba, J. L., Cortijo, M. & Mateo, P. L. (1988) Biochemistry 27, 1648-1652]. Thus the kinetic constants and activation parameters of the denaturation process could be obtained and compared to those for other proteins, particularly those of the closely related carboxypeptidase A system.