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Thermodynamics of the helix‐coil transition: Binding of S15 and a hybrid sequence, disulfide stabilized peptide to the S‐protein
Author(s) -
Bastos Margarida,
Pease Joseph H.B.,
Wemmer David E.,
Murphy Kenneth P.,
Connelly Patrick R.
Publication year - 2001
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/1097-0134(20010301)42:4<523::aid-prot100>3.0.co;2-b
Subject(s) - peptide , chemistry , enthalpy , helix (gastropod) , random coil , crystallography , thermodynamics , protein secondary structure , entropy (arrow of time) , ribonuclease , protein structure , circular dichroism , biochemistry , physics , rna , biology , ecology , snail , gene
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°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. Proteins 2001;42:523–530. © 2001 Wiley‐Liss, Inc.