Denaturation of free and complexed bovine trypsinogen with the calcium ion, dipeptide Ile-Val and basic pancreatic trypsin inhibitor (Kunitz).

Thermal and chemical denaturation has been used to probe changes in the thermodynamic stability of trypsinogen upon complexation with calcium ion and with ligands, which induce the conformational transition of the zymogen to the trypsin-like form. Chemical and thermal unfolding curves of ligand-free trypsinogen at pH 5.8 are cooperative and yielded the following stability parameters: the free energy change of denaturation delta Gden = 44.8 kJ/mol, the denaturation temperature = 65.7 degrees C, the enthalpy change of denaturation delta Hden at the denaturation temperature Tden = 607 kJ/mol and the heat capacity change of denaturation delta Cp,den = 12.4 kJ.mol-1.K-1. Fast phases of both unfolding and refolding of trypsinogen proceed on a time scale of seconds and fit to a single exponential. At pH 5.8, the calcium ion increases the conformational stability delta Gden by 7.1 kJ/mol, Tden by 2.6 K and delta Hden by 80 kJ/mol, but does not induce any substantial structural change in the trypsinogen molecule, as revealed by 1H-NMR spectra. The trypsin-like form of trypsinogen, generated by complexation of the zymogen with the dipeptide Ile-Val and/or basic pancreatic trypsin inhibitor (Kunitz), is characterized by increase of delta Hden by 134 kJ/mol and Tden by 2.5 K, which may be attributed to the additional energy required to disrupt the rigidified activation domain in the complexed trypsinogen.