Order of Message and Address Domain Engagement Determines Productive β-Endorphin Binding to the μ-Opioid Receptor

ABSTRACT Understanding β -endorphin binding to the μ-opioid receptor ( μ OR) is crucial for designing safer analgesics. The peptide comprises a message domain mediating activation and an address domain conferring selectivity. Using 1,000 independent CABS-dock simulations, without prior binding-site knowledge, we analysed binding trajectories to compare alternative binding pathways. Message-first binding is most frequently sampled but rarely reaches native-like structures (5.0%). In contrast, address-first binding occurs less often yet shows a 3.8-fold higher success rate (18.8%, p < 0.001). These results refine the message–address model and suggest that early address-domain engagement promotes productive μOR binding.