Threonine6-bradykinin: structural characterization in the presence of micelles by nuclear magnetic resonance and distance geometry.

The conformation of the natural peptide [Thr6]-bradykinin, Arg1-Pro2-Pro3-Gly4-Phe5-Thr6-Pro7-Phe8-Arg9, is investigated by NMR spectroscopy and computer simulations in an aqueous solution of sodium dodecyl sulfate micelles. The structural analysis of the peptide is of particular interest since it displays a different biological profile from bradykinin despite the high sequence homology (only one conservative substitution: Ser6/Thr6) and the fact that both peptides bind and activate common receptors. The SDS micelles provide a model system for the membrane-interface environment the peptide experiences when interacting with the membrane-embedded receptor and allow for the conformational examination of the peptide using high-resolution NMR techniques. The NMR spectra show that the micellar system induces a secondary structure in the otherwise inherently flexible peptide (as observed in benign aqueous solution). The distance geometry calculations indicate a beta-turn of type I about residues 7-8 as the preferred conformation. The results of ensemble calculations reveal conformational changes occurring rapidly on the NMR time scale and allow for the identification of three different families of conformations that average to reproduce the NMR observables. The three families differ in the type of conformation adopted at the C-terminus: type I beta-turn, type II beta-turn and a third conformation, intermediate between the two beta-turns. The structural results support the hypothesis of the determining role of the C-terminal conformation for biological activity and can provide an explanation of the different activities observed for bradykinin and [Thr6]-bradykinin.