HEAT-CAPACITY CHANGES FOR PROTEIN PEPTIDE INTERACTIONS IN THE RIBONUCLEASE-S SYSTEM

Two fragments of pancreatic ribonuclease A, a truncated version of S-peptide (residues 1-15) and S-protein (residues 21-124), combine to give a catalytically active complex designated ribonuclease S. We have substituted the wild-type residue Met- 1 3 with six other hydrophobic residues ranging in size from alanine to phenylalanine and have determined the thermodynamic parameters associated with binding of these analogues to S-protein by titration calorimetry in the temperature range 5-25-degrees-C. The heat capacity change (DELTA-C(p)) associated with binding was obtained from a global analysis of the temperature dependences of the free energies and enthalpies of binding. The DELTA-C(p)'s were not correlated in any simple fashion with the nonpolar surface area (DELTA-A(np)) buried upon binding.