THERMODYNAMICS OF PROTEIN PEPTIDE INTERACTIONS IN THE RIBONUCLEASE-S SYSTEM STUDIED BY TITRATION CALORIMETRY

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. Residue 13 in the peptide is methionine. According to the X-ray structure of the complex of S-protein and S-peptide (1–20), this residue is almost fully buried. We have substituted Met-13 with seven other hydrophobic residues ranging in size from glycine to phenylalanine and have determined the thermodynamic parameters associated with the binding of these analogues to S-protein by titration calorimetry at 25°C. These data should provide useful quantitative information for evaluating the contribution of hydrophobic interactions in the stabilization of protein structures. © 1990, American Chemical Society. All rights reserved.