Three-dimensional molecular models of the hMC1R melanocortin receptor: complexes with melanotropin peptide agonists

Three-dimensional molecular models of the human melanocortin receptor (hMC1R) have been developed based upon the electron cryo-microscopic structure of bacteriorhodopsin and the electron density footprint of bovine rhodopsin. alpha-Melanocyte-stimulating hormone, Ac-Ser-Tyr-Ser-Met4-Glu-His-Phe7-Arg-Trp-Gly-Lys-Pro-Val-NH2 (alpha-MSH, alpha-melanotropin), and the superpotent, prolonged acting agonists, Ac-Ser-Tyr-Ser-Nle4-Glu-His-DPhe7-Arg-Trp-Gly-Lys-Pro-Val-NH2 (NDP-MSH) and Ac-Nle4-c[Asp5-His6-DPhe7-Arg8-Trp9-Lys10]-NH2 (MTII), have been modeled into the proposed binding sites with specific ligand-receptor interactions identified. The melanotropin sidechain pharmacophores, DPhe7 and Trp9, are proposed to interact with a hydrophobic network of receptor aromatic residues in transmembrane regions 4, 5, 6, and 7. In addition, a hydrophilic network involving the ligand Arg8 and polar receptor residues located in transmembrane regions 2 and 3 were identified. Biological studies on alpha-MSH, NDP-MSH, MTII, and related peptides have been correlated with the proposed hMC1R model in terms of agonism, affinity, and prolongation. Finally, limited MC1R mutagenesis studies comparing alpha-MSH and NDP-MSH are interpreted within the context of the proposed hMC1R models.