Preferred conformation of endomorphin-1 in aqueous and membrane-mimetic environments

The newly discovered endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are potent opioid peptides with the highest affinity and selectivity for the mu receptor among all known endogenous ligands. To investigate a possible correlation between these biological properties and the conformational preferences of the small peptides, a comparative structu ral analysis was performed of endomorphin-1 in aqueous buffer and in membra ne-mimicking SDS and AOT normal and reverse micelles by the use of CD, FT-I R, fluorescence and H-1-NMR spectroscopy. It is well established for opioid peptides that, independently of the receptor selectivity, the Tyr1 residue plays the role of the primary pharmacophore and that the orientation of th e second aromatic pharmacophore relative to the tyrosine side-chain dictate s the mu or F-receptor selectivity. By varying the environment of endomorph in-1 from water to the amphipathic SDS micelles and even more efficiently t o the AOT reverse micelles, the display of the aromatic side-chains changes from an interaction of the Tyr1 and Phe4 residues to a switch of the Trp3 indole group into close contact with the phenolic moiety to prevent this ty pe of interaction and to force an orientation of the Phe4 side-chain into t he opposite direction. This conformational switch is accompanied by a stabi lization of the cis-Pro2 isomer and the resulting spatial array of the phar macophoric groups correlate well with the structural model of mu receptor-b ound opioid peptides. The results indicate that AOT reverse micelles with a w(o) of 10, where almost exclusively ordered water is secluded in the cavi ty, constitute with their electrostatic and hydrophobic potential an excell ent mimetic of amphipathic surfaces as present on lipid bilayers and on lig and-recognition and ligand-binding sites of proteins. (C) 1999 Academic Pre ss.