Computational insights into the G-protein-biased activation and inactivation mechanisms of the mu opioid receptor

The mu opioid receptor (OR), a member of the class A subfamily of G-protein coupled receptors (GPCRs), is a major target for the treatment of pain. G-protein biased mu-OR agonists promise to be developed as analgesics. Thus, TRV130, the first representative mu-OR ligand with G-protein bias, has entered into phase III clinical trials. To identify the detailed G-protein-biased activation and inactivation mechanisms of the mu-OR, we constructed five mu-OR systems that were in complexes with the G-protein-biased agonists TRV130 and BU72, the antagonists beta-FNA and naltrexone, as well as the free receptor. We performed a series of conventional molecular dynamics simulations and analyses of G-protein-biased activation and inactivation mechanisms of mu-OR. Our results, together with previously reported mutation results, revealed the operating mode of the activation switch composed of residues W-6.48 and Y-7.43 (Ballesteros/Weinstein numbering), the activity of which was responsible for down-and up-regulation, respectively, of the beta-arrestin signaling, which in turn affected G-protein-biased activation of mu-OR. TRV130 was found to stabilize W-6.48 by interacting with Y-7.43. In addition, we obtained useful information regarding mu-OR-biased activation, such as strong stabilization of W-7.35 through a hydrophobic ring interaction in the TRV130 system. These findings may facilitate understanding of mu-OR biased activation and the design of new biased ligands for GPCRs.