Solution Structures of Cyclic Melanocortin Agonists and Antagonists by NMR

The melanocortin receptors are involved in many physiological functions, including pigmentation, sexual function, feeding behavior, and energy homeostasis, making them potential targets for drugs to treat obesity, sexual dysfunction, etc. Understanding the conformational basis of the receptor-ligand interactions is crucial to the design of potent and selective ligands for these receptors. The solution structures of the cyclic melanocortin agonists, partial agonist, and antagonists MTII, VJH085, SHU9II9, MK5, and MK9 were determined by two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy at pH 4.5 and 25 °C in water (90% H₂O/10% D₂O). The overall backbone structures of these cyclic α-melanocyte-stimulating hormone (α-MSH) analogues around the message sequence (His⁶-D-Phe ⁷/D-Nal(2′)⁷-Arg⁸-Trp⁹) were similar and reasonably well defined. β-Turns spanning His⁶ and D-Phe⁷/D-Nal(2′)⁷ were identified in all analogues, and an amphiphilic molecular surface was obtained for the message sequence residues in most structures within the NMR ensembles. The β-turn, which most closely resembles a type II β-turn, leads to stacking between the aromatic rings of His⁶ and D-Phe⁷ in MTII and VJH085. However, no aromatic stacking between His⁶ and D-Nal(2′) ⁷ was found in structures of the D-Nal(2′) ⁷-containing analogues. The difference in the side-chain dispositions of His⁶ and D-Nal(2′)⁷ may be responsible for the reduced potency or antagonist activity of the D-Nal(2′)⁷-containing analogues. In addition, our results suggest that the side-chain orientations may also modulate the receptor selectivity. The information found in this study will be useful for the further design of ligands for melanocortin receptors.