Structure-function studies on the cyclic peptide MT-II, lactam derivative of alpha-melanotropin

The alanine-substituted and the retro, enantio, and retro-enantio analogs of MT-II, a potent agonist at melanocortin (MC) receptors, were prepared by solid-phase synthesis and evaluated for their ability to bind and activate human MC3, MC4, and MC5 receptors. Replacement of His with Ala resulted in [Ala6]-MT-II with affinity and agonist potency at human MC3, MC4, and MC5 receptors similar to MT-II. Substitution of Arg with Ala gave compound 100-fold less potent than MT-II, but replacement of Phe or Trp with Ala led to inactive compounds (at the micromolar concentrations). The significant drop of potency of the retro, enantio, and retro-enantio analogs of MT-II, demonstrated a crucial role of side-chain topology, and to a lesser degree, of peptide backbone in interactions of MT-II with the melanocortin receptors. The nuclear magnetic resonance analysis of MT-II suggested involvement of Phe and Arg residues in H-bonds stabilizing the bent conformations of the peptide backbone.