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

Endogenous opioid peptides consist of a conserved amino acid residue of Phe³ and Phe⁴, although their binding modes for opioid receptors have not been elucidated in detail. Endomorphin-2, which is highly selective and specific for the μ opioid receptor, possesses two Phe residues at the consecutive positions 3 and 4. In order to clarify the role of Phe³ and Phe⁴ in binding to the μ receptor, we synthesized a series of analogs in which Phe³ and Phe⁴ were replaced by various amino acids. It was found that the aromaticity of the Phe-β-phenyl groups of Phe³ and Phe⁴ is a principal determinant of how strongly it binds to the receptor, although better molecular hydrophobicity reinforces the activity. The receptor binding subsites of Phe³ and Phe⁴ of endomorphin-2 were found to exhibit different structural requirements. The results suggest that [Trp³]endomorphin-2 (native endomorphin-1) and endomorphin-2 bind to different receptor subclasses.