BINDING OF BOVINE BASIC PANCREATIC TRYPSIN-INHIBITOR (KUNITZ) AS WELLAS 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 paramete rs for the binding of the bovine basic pancreatic trypsin inhibitor (K unitz inhibitor; BPTI) as well as bovine and porcine pancreatic secret ory trypsin inhibitor (Kazal inhibitor; bovine and porcine PSTI, respe ctively) to human cathepsin G (EC 3.4.21.20) has been investigated. Th e affinity of the macromolecular inhibitors examined for cathepsin G i s characterized by an endothermic, entropy-driven, behaviour, and show s the following trend: BPTI > bovine PSTI > porcine PSTI. The affinity difference of BPTI as well as of bovine and porcine PSTI for cathepsi n G is mostly accounted for by changes in the values of the apparent d issociation rate constant for the proteinase:inhibitor complex destabi lization. On increasing the pH from 4.5 to 9.5 (at 25.0-degrees-C), th e 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 almost-equal-to 6.9 in the free enzyme to almost-equal-t o 5.0 in the serine proteinase:inhibitor complexes. The BPTI as well a s the bovine and porcine PSTI binding properties of cathepsin G have b een analyzed in parallel with those of related serine (pro)enzyme/macr omolecular inhibitor systems. Considering the known molecular models, the observed binding behaviour of BPTI as well as that of bovine and p orcine PSTI to cathepsin G has been related to the inferred stereochem istry of the serine proteinase/inhibitor contact region(s).