Binding of bovine basic pancreatic trypsin inhibitor (Kunitz) as well as bovine and porcine pancreatic secretory trypsin inhibitor (Kazal) to human cathepsin G: a kinetic and thermodynamic study.

The effect of pH and temperature on kinetic and thermodynamic parameters for the binding of the bovine basic pancreatic trypsin inhibitor (Kunitz inhibitor; BPTI) as well as bovine and porcine pancreatic secretory trypsin inhibitor (Kazal inhibitor; bovine and porcine PSTI, respectively) to human cathepsin G (EC 3.4.21.20) has been investigated. The affinity of the macromolecular inhibitors examined for cathepsin G is characterized by an endothermic, entropy-driven, behaviour, and shows the following trend: BPTI > bovine PSTI > porcine PSTI. The affinity difference of BPTI as well as of bovine and porcine PSTI for cathepsin G is mostly accounted for by changes in the values of the apparent dissociation rate constant for the proteinase:inhibitor complex destabilization. On increasing the pH from 4.5 to 9.5 (at 25.0 degrees C), the affinity of BPTI, as well as bovine and porcine PSTI for cathepsin G increases thus reflecting the acidic-pK shift of the His-57 catalytic residue from approximately 6.9 in the free enzyme to approximately 5.0 in the serine proteinase:inhibitor complexes. The BPTI as well as the bovine and porcine PSTI binding properties of cathepsin G have been analyzed in parallel with those of related serine (pro)enzyme/macromolecular inhibitor systems. Considering the known molecular models, the observed binding behaviour of BPTI as well as that of bovine and porcine PSTI to cathepsin G has been related to the inferred stereochemistry of the serine proteinase/inhibitor contact region(s).