Biological and Conformational Examination of Stereochemical Modifications Using the Template Melanotropin Peptide, Ac-Nle-c[Asp-His-Phe-Arg-Trp- Ala-Lys]-NH<sub>2</sub>, on Human Melanocortin Receptors
作者:Carrie Haskell-Luevano、Gregory Nikiforovich、Shubh D. Sharma、Ying-Kui Yang、Chris Dickinson、Victor J. Hruby、Ira Gantz
DOI:10.1021/jm960845e
日期:1997.5.1
Examination of conformationally constrained melanotropin peptides (Ac-Nle(4)-c[Asp(5)-His-Phe(7)-Arg-Trp(9)-Ala-Lys]-NH2) on four human melanotropin receptors (hMC1R, hMC3R, hMC4R, and hMC5R) resulted in identifying the importance of ligand stereochemistry at positions 5, 7, and 9 for agonist binding affinity and receptor selectivity. A trend in ligand structure-activity relationships emerged for these peptides, with the hMC1R and hMC4R possessing similar tendencies, as did the hMC3R and hMC5R. alpha-MSH (Ac-Ser-Tyr-Ser-Met(4)-Glu-His-Phe(7)-Arg-Trp-Gly-Lys-Pro-Val-NH2), NDP-MSH (Ac-Ser-Tyr-Ser-Nle(4)-Glu-His-D-Phe(7)-Arg-Pro-Val-NH2), and MTII (Ac-Nle(4)-c[Asp(5),D-Phe(7),Lys(10)]-alpha-MSH(4-10)-NH2) were also examined at each of these melanocortin receptors. Interestingly, the linear NDP-MSH possessed greater binding affinity for the hMC3R and hMC5R than did the cyclic analogue MTII. The peptide Ac-Nle-c[Asp-His-Phe-Arg-D-Trp(9)-Ala-Lys]-NH2 demonstrated the greatest differentiation in binding affinity between the hMC1R and hMC4R (78-fold). Analogue Ac-Nle-c[Asp-His-Phe(7)-Arg-Trp-Ala-Lys]-NH2 resulted in micromolar binding affinity (or greater) at the hMC3R and hMC5R, demonstrating the importance of D-Phe(7) for ligand binding potency at these receptors. Ac-c[Asp-His-Phe-Arg-Trp-Ala-Lys]-NH2 resulted in loss of binding affinity at the hMC5R, implicating the importance of Nle(4) (or a hydrophobic residue in this position) for binding to this receptor. Ac-Nle-c[D-Asp(5)-His-Phe-Arg-Trp-Ala-Lys]-NH2 was unable to competitively displace [I-125]NDP-MSH binding at micromolar concentrations on the hMC3R and hMC5R, suggesting the importance of chirality of Asp(5) either for ligand-receptor interactions or for orientation of the side chain lactam bridge and the structural integrity of the peptide conformation. Energy calculations performed for these peptides resulted in the identification of a low-energy ligand conformer family that is common to all the ligands. The differences in ligand binding affinities observed in this study are postulated to be a result of different ligand-receptor complexed interactions and not solely to the ligand structure.