Molecular and conformational determinants of pituitary adenylate cyclase-activating polypeptide (PACAP) for activation of the PAC1 receptor.

PAC1 receptor is abundant in the CNS and plays an important role in neuronal survival. To identify the molecular determinants and the conformational components responsible for the activation of the PAC1 receptor, we performed a SAR study focusing on the N-terminal domain of its endogenous ligand, PACAP. This approach revealed that residues Asp(3) and Phe(6) are key elements of the pharmacophore of the PAC1 receptor. This study, supported by NMR structural analyses, suggests that the N-terminal tail of PACAP (residues 1 to 4) adopts a specific conformation similar to a turn when it activates the PAC1 receptor. Moreover, the integrity of the alpha-helix conformation observed at positions 5 to 7 appears crucial to allow the binding of PACAP. Characterization of analogues led to the identification of several superagonists, such as [Bip(6)]PACAP27, and of a new potent PAC1 receptor antagonist, [Sar(4)]PACAP38. The bioactive conformation inferred from this SAR study could constitute an appropriate molecular scaffold supporting the design of nonpeptidic PAC1 receptor agonists.