Structural energetics of serine protease inhibition*

We have investigated the binding of the serine protease inhibitor, turkey ovomucoid third domain (OMTKY3), to the serine protease, porcine pancreatic elastase (PPE), using isothermal titration calorimetry and structural energetics calculations. The calculations predict that the binding at 25 8C is characterized by a negligible DH 8, a large and positive DS 8, and a large and negative DCp, resulting in a large and favorable DG 8. The experimental results indicate a significant contribution to the binding energetics from a change in the pKa of an ionizable group, presumably His57 of PPE. The resulting proton linkage is manifest in the observed DH 8 and DCp of binding. However, a global analysis of binding data as a function of pH, buffer, and temperature yields the intrinsic binding energetics as well as the energetics of proton binding to the ionizable group in the free and bound PPE. The experimentally determined intrinsic energetics and the calculated values are in very good agreement, suggesting that the structural energetics calculations may be a useful tool for understanding protein–protein interactions in solution.