title: Thermodynamics of the helix-coil transition: Binding of S15 and a hybrid sequence, disulfide stabilized peptide to the S-protein.
abstract: Pancreatic ribonuclease A may be cleaved to produce two fragments: the S-peptide (residues 1-20) and the S-protein (residues 21-124). The S-peptide, or a truncated version designated as the S15 peptide (residues 1-15), combines with the S-protein to produce catalytically active complexes. The conformation of these peptides and many of their analogues is predominantly random coil at room temperature; however, they populate a significant fraction of helical form at low temperature under certain solution conditions. Moreover, they adopt a helical conformation when bound to the S-protein. A hybrid sequence, disulfide-stabilized peptide (ApaS-25), designed to stabilize the helical structure of the S-peptide in solution, also combines with the S-protein to yield a catalytically active complex. We have performed high-precision titration microcalorimetric measurements to determine the free energy, enthalpy, entropy, and heat capacity changes for the binding of ApaS-25 to S-protein within the temperature range 5-25 degrees C. The thermodynamic parameters for both the complex formation reactions and the helix-to-coil transition also were calculated, using a structure-based approach, by calculating changes in accessible surface area and using published empirical parameters. A simple thermodynamic model is presented in an attempt to account for the differences between the binding of ApaS-25 and the S-peptide. From this model, the thermodynamic parameters of the helix-to-coil transition of S15 can be calculated.
authors.affiliation: CIQ(UP) Department of Chemistry, Faculty of Sciences, University of Porto, Portugal. mbastos@fc.up.pt
authors.forename: M
authors.initials: M
authors.lastname: Bastos
authors.forename: J H
authors.initials: JH
authors.lastname: Pease
authors.forename: D E
authors.initials: DE
authors.lastname: Wemmer
authors.forename: K P
authors.initials: KP
authors.lastname: Murphy
authors.forename: P R
authors.initials: PR
authors.lastname: Connelly
mesh_terms: D000596:Amino Acids; D002151:Calorimetry / Q000379:methods; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D008715:Methionine; D010446:Peptide Fragments / Q000737:chemistry*; D011487:Protein Conformation; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*; D012260:Ribonucleases / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000596:Amino Acids; D010446:Peptide Fragments; C060871:ribonuclease A S-peptide; C023560:ribonuclease S-peptide; D008715:Methionine; D012260:Ribonucleases; D012259:Ribonuclease, Pancreatic