Micelle-bound conformations of a bombesin/gastrin releasing peptide receptor agonist and an antagonist by two-dimensional NMR and restrained molecular dynamics.

Two nonapeptide analogs of the carboxyl termini of bombesin (Bn) and gastrin releasing peptide (GRP) have been synthesized. Despite the small difference in chemical composition between these peptides, one was a potent agonist and the other a potent antagonist of the Bn/GRP receptor in murine pancreas. All protons of both peptides, in dodecylphosphocholine micelles, were assigned by two-dimensional nuclear magnetic resonance spectroscopy. Interproton distance were derived from cross-peak volumes in nuclear Overhauser enhancement spectra. Conformations of both peptides were derived by distance-restrained molecular dynamics simulations using the interproton distances as constrains. The agonist conformation resembled a relaxed helix formed by three connected turns. The two N-terminal turns were similar for both peptides. The third turn of the agonist, at the carboxyl terminus, was absent in the antagonist. One interproton distance at the carboxyl terminus of the antagonist indicates that the chemical group connecting the last two residues of this peptide mimics a cis peptide bond geometry.