Binding of the Bovine and Porcine Pancreatic Secretory Trypsin Inhibitor (Kazal) To Human Leukocyte Elastase: A Thermodynamic Study

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Abstract
The effect of pH and temperature on the apparent association equilibrium constant (Ka) for the binding of the bovine and porcine pancreatic secretory trypsin inhibitor (Kazal-type inhibitor, PSTI) to human leukocyte elastase has been investigated. At pH8.0, values of the apparent thermodynamic parameters for human leukocyte elastase: Kazal-type inhibitor complex formation are: bovine PSTT – Ka = 6.3 × 104M−1, δ5G° = -26.9kJ/mol, δH° = +11.7kJ/mol, and δS° = +1.3 × 102 entropy units; porcine PSTI –Ka = 7.0 × 103M−1,δG° = -21.5kJ/mol, δH° = +13.0kJ/mol, and δS° = +1.2 × 102 entropy units (values of Ka δG° and δS° were obtained at 21.0°C; values of δH° were temperature independent over the range (between 5.0°C and 45.0°C) explored). On increasing the pH from 4.5 to 9.5, values of Ka for bovine and porcine PSTI binding to human leukocyte elastase increase thus reflecting the acidic pK-shift of the His57 catalytic residue from ⋍7.0, in the free enzyme, to ⋍5.1, in the serine proteinase: inhibitor complexes. Thermodynamics of bovine and porcine PSTI binding to human leukocyte elastase has been analyzed in parallel with that of related serine (pro)enzyme/Kazal-type inhibitor systems. Considering the known molecular models, the observed binding behaviour of bovine and porcine PSTI to human leukocyte elastase was related to the inferred stereochemistry of the serine proteinase/inhibitor contact region(s).