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D-configuration is essential for high-affinity binding Arg : The guanidinium group forms a critical salt bridge with Asp122 and Asp126 in transmembrane helix 3 of MC4R Trp : The indole ring system provides additional hydrophobic and -stacking interactions within the binding cleft Physicochemical Properties Structural Comparison: -MSH MT-II PT-141 The evolutionary design pathway from the endogenous hormone to the clinical compound involved progressive optimization: -MSH (13 amino acids, linear): Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH 2 Half-life: approximately 3-5 minutes Non-selective for MC1R-MC5R Rapidly degraded by serum proteases Melanotan II (7 amino acids, cyclic): Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH 2 Half-life: approximately 1-2 hours Potent at MC1R, MC3R, MC4R, MC5R Strong tanning and pro-sexual effects PT-141 / Bremelanotide (7 amino acids, cyclic): Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH Half-life: approximately 2.7 hours Preferential MC4R agonism for sexual function Reduced melanogenic activity compared to MT-II C-terminal free acid corresponds to the active metabolite of MT-II Detailed Mechanism of Action MC4R Binding and Signal Transduction PT-141 exerts its primary biological effects through agonism of the melanocortin-4 receptor (MC4R), a 332-amino acid class A (rhodopsin family) G-protein coupled receptor encoded by the MC4R gene on chromosome 18q21.32
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Some patients do make this switch under medical supervision
