Binding of bovine basic pancreatic trypsin inhibitor (Kunitz) to human urinary kallikrein and to porcine pancreatic beta-kallikrein A and B
The effect of pH and temperature on the association equilibrium constant (Ka) for bovine basic pancreatic trypsin inhibitor (BPTI, Kunitz inhibitor) binding to human urinary kallikrein and porcine pancreatic beta-kallikreins A and B has been investigated. Ka values decrease with decreasing pH, reflecting the acid-midpoint and pK shifts, upon BPTI binding, of a three-proton co-operative transition, between pH 3 and 5, and of a single ionizable group, between pH 5 and 9. At pH 8, the values of delta H degree (between 7 degrees C and 42 degrees C) and delta S degree (at 21 degrees C) for BPTI binding to the glandular kallikreins considered were determined. In particular, the delta H degree values have been found to be independent of temperature and the following values have been obtained by van't Hoff plots: +1.8 kcal/mol, +2.3 kcal/mol and +2.4 kcal/mol (1 kcal = 4184 J) for the inhibitor binding to human urinary kallikrein and porcine pancreatic beta-kallikreins A and B, respectively. Considering the known molecular structures of free porcine pancreatic beta-kallikrein A and BPTI, and of their complex, the stereochemistry of the enzyme : inhibitor contact regions was analysed for the three serine proteinases, in relation to their respective types of behaviour.
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