Figure 5.

Stereoview of peptide agonist NDP-MSH (purple) inside the binding pocket of the hMC4R active conformation model. The mutated hMC4R residues and residues forming H-bonds with peptide are shown in the licorice representation colored by element. THIQ is shown in thin purple line for comparison. The receptor-bound conformation of NDP-MSH represents a β-hairpin-like structure with a reverse turn spanning His⁶ and D-Phe⁷, aromatic rings of D-Phe⁷ and Trp⁹ forming stacking interactions, and polar residues Glu⁵ and Arg⁸ located on different sides of the β-hairpin. Inside the binding pocket, NDP-MSH forms at least nine H-bonds with polar groups of the receptor (shown by dashed lines): Tyr² hydroxyl with Thr¹¹⁸ hydroxyl, carboxylate group of Asp⁵ and backbone carbonyl of Trp⁹ with His²⁶⁴, imidazole of His⁶ with Glu¹⁰⁰, guanidinium group of Arg⁸ with Asp¹²² and Asp¹²⁶ carboxylate groups and hydroxyl of Thr¹¹⁸, backbone amide group of D-Phe⁷ with Asp¹²⁶ carboxylate, backbone carbonyl of Gly¹⁰ with Tyr²⁶⁸ hydroxyl, and amine group of Lys¹¹ with Asp¹⁸⁹ carboxylate. Important hydrophobic interactions are formed between aromatic rings of D-Phe⁷ and Trp⁹ of peptide agonist and non-polar binding pocket residues , Ile¹²⁹, Leu¹³³, Phe¹⁸⁴, Leu²⁶⁵, Trp²⁵⁸, Phe²⁶¹, Leu²⁸⁸.