MOLECULAR DETERMINANTS OF RECOGNITION AND ACTIVATION AT THE MU-OPIOIDRECEPTOR BY MET-ENKEPHALIN-LIKE PEPTIDES

In this study, the techniques of computational chemistry were used to probe the origin of the differing pharmacological profiles found as a result of two simple modifications of the enkephalin analog Tyr-DAla-G ly-Phe-MetNH(2): the presence or absence of N-methylation of Phe(4) an d of the Met(5) residue. Although all four analogs have high mu-recept or affinity, their analgesic activity varies by a factor of 3000. Thus , they should share common determinants of mu-receptor recognition whi le differing in the ability to activate the receptor. To identify and characterize these determinants, a two-step procedure was used. In the first step, the energy conformational profile of each peptide was obt ained. The strategy used involved the iterative calculation of molecul ar dynamics trajectories at high and low temperatures, coupled to ener gy minimizations, allowing a thorough sampling of conformational space . In the second step, low-energy conformers of the four peptides were examined for the extent to which they fulfilled the requirements for m u-receptor recognition recently developed for nonpeptide analogs. In t hese studies, the amine nitrogen, a second proton-accepting moiety, an d an aromatic ring in a specific geometric arrangement were proposed a s the minimum components of a mu-pharmacophore for recognition. For al l four analogs, a unique low-energy conformer was found that contained these three recognition moieties in a geometric arrangement to intera ct with the same target binding site residues as in the nonpeptide ana logs. These results are consistent with the finding of high affinity f or all four peptides and provide common determinants of recognition of the mu-receptor by peptides and nonpeptides. When the four peptides w ere overlapped so that they could each interact with these three commo n recognition sites, the Phe(4) aromatic side chain was found to be a possible modulator of activation. For the parent pentapeptide, Tyr-DAl a-Gly-Phe-Met, with the lowest activity, there was poor overlap of the Phe(4) aromatic ring with the same ring in the other three analogs. T hese results implicate the Phe(4) ring in peptide activation of the mu -receptor. (C) 1993 John Wiley & Sons, Inc.