Differential receptor binding characteristics of consecutive phenylalanines in micro-opioid specific peptide ligand endomorphin-2

Endogenous opioid peptides consist of a conserved amino acid residue of Phe(3) and Phe(4), although their binding modes for opioid receptors have not been elucidated in detail. Endomorphin-2, which is highly selective and specific for the mu opioid receptor, possesses two Phe residues at the consecutive positions 3 and 4. In order to clarify the role of Phe(3) and Phe(4) in binding to the mu receptor, we synthesized a series of analogs in which Phe(3) and Phe(4) were replaced by various amino acids. It was found that the aromaticity of the Phe-beta-phenyl groups of Phe(3) and Phe(4) is a principal determinant of how strongly it binds to the receptor, although better molecular hydrophobicity reinforces the activity. The receptor binding subsites of Phe(3) and Phe(4) of endomorphin-2 were found to exhibit different structural requirements. The results suggest that [Trp(3)]endomorphin-2 (native endomorphin-1) and endomorphin-2 bind to different receptor subclasses.