Hydrophobic interactions are the driving force for the binding of peptide mimotopes and Staphylococcal protein A to recombinant human IgG1

We studied the interaction of several nonapeptide mimotopes of different sequence and Staphylococcal protein A (SpA) with a recombinant human IgG1 antibody using isothermal titration calorimetry (ITC). The amino acid primary structure of the peptides was varied in order to identify the specific antibody-peptide binding sites. Additionally, the influence of temperature and salt concentration was investigated. An attempt was made to elucidate the structural changes upon complex formation using the determined thermodynamic parameters. The amino acid composition of the mimotopes determined their binding affinity. The binding constant Ka of the mimotopes was in the range 1 x 10(4) to 1 x 106 M-1. The binding constant of SpA was on the average about three orders of magnitude higher than that of the peptides. The binding constant of the peptides and of SpA decreased with temperature and the binding process was connected with negative changes in enthalpy, entropy, and heat capacity. The binding of the mimotopes to the Fab part of the IgG1 antibody and binding of SpA to the Fe part of the IgG1 antibody were mainly driven by hydrophobic effects and associated with a relatively