Structural energetics of peptide recognition: Angiotensin II/antibody binding

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Abstract
The ability to predict the strength of the association of peptide hormones or other ligands with their protein receptors is of fundamental importance in the fields of protein engineering and rational drug design. To form a tight complex between a flexible peptide hormone and its receptor, the large loss of configurational entropy must be overcome. Recently, the crystallographic structure of the complex between angiotensin II and the Fab fragment of a high affinity monoclonal antibody has been determined (Garcia, K. C., Ronco, P. M., Verroust, P. J., Brünger, A. T., Amzel, L. M. Three‐dimensional structure of an angiotensin II–Fab complex at 3 Å: Hormone recognition by an anti‐idiotypic antibody. Science 257:502–507, 1992). In this paper we present a study of the thermodynamics of the association by high sensitivity isothermal titration calorimetry. The results of the experiments indicate that at 30°C the binding is characterized by (1) a ΔH of −8.9 ± 0.7 kcal mol−1, (2) a ΔCp of −240 ± 20 cal K−1 mol−1, and (3) the release of 1.1 ± 0.1 protons per binding site in the pH range 6.0–7.3. Using these values and the previously determined binding constant in phosphate buffer, ΔG at 30°C is estimated as −11 kcal mol−1 and ΔS as 6.9 cal K−1 mol−1. The calorimetric data indicate that binding is favored both enthalpically and entropically. These results have been complemented by structural thermodynamic calculations. The calculated and experimentally determined thermodynamic quantities are in good agreement. Entropically, the loss of configurational entropy is more than compensated by the entropy gain from solvent release associated with the hydrophobic effect. Enthalpically, binding is favored by polar interactions (hydrogen bonding). Consequently, the problem of binding flexible hormones is solved in much the same way as the folding of an unstructured polypeptide chain into a globular protein.