Structural and Dynamic Insights into the Biased Signaling Mechanism of the Human Kappa Opioid Receptor

The κ-opioid receptor (KOR) is a member of the G protein-coupled receptor (GPCR) family, responsible for modulating cellular responses through transducers such as G proteins and arrestins. G protein-biased KOR agonists hold promise due to their potential to mitigate side effects such as drug aversion and sedation while preserving analgesic and antipruritic effects. Here, we shed light on the structural dynamics of the human KOR-G _i signaling complex bound with either nalfurafine (a G-protein-biased agonist) or U-50,488H (a balanced agonist) using cryo-electron microscopy (cryo-EM). Cryo-EM structures of the KOR-G _i signaling complexes identify the ligand binding mode in the activated state. Vibrational spectroscopy analysis reveals changes in the ligand-binding pocket upon binding to these ligands. Cell-based mutant experiments pinpoint four amino acids (K227 ^5.40 , C286 ^6.47 , H291 ^6.52 , and Y312 ^7.34 ; Ballesteros–Weinstein numbering is shown in superscript) that play crucial roles in arrestin recruitment. Among these four amino acids, H291 ^6.52 and Y312 ^7.34 are also implicated in G-protein coupling. Our findings pave the way for targeting specific residues in the KOR ligand-binding pocket to enhance KOR-mediated therapeutic effects while mitigating unwanted side effects.