Homology models of mu-opioid receptor with organic and inorganic cations at conserved aspartates in the second and third transmembrane domains

Metal ions affect ligand binding to G-protein-coupled receptors by as yet u nknown mechanisms. In particular, Na+ increases the affinity for antagonist s but decreases it for agonists. We had modeled the mu-opioid receptor (mu R) based on the low-resolution structure of rhodopsin by G. F. X. Schertler , C. Villa, and R. Henderson (1993, Nature 362, 770-772) and proposed that metal ions may be directly involved in the binding of ligands and receptor activation (B. S. Zhorov and V. S. Ananthanarayanan, 1998, J. Biomol. Struc t. Dyn. 15, 631-637). Developing this concept further, we present here homo logy models of mu R using as templates the structure of rhodopsin elaborate d by I. D. Pogozheva, A. L. Lomize, and H. I. Mosberg (1997, Biophys. J. 70 , 1963-1985) and J. M. Baldwin, G. F. X. Schertler, and V. M. Unger (1997, J. Mol. Biol, 272, 144-164). Using the Monte Carlo minimization (MCM) metho d, we docked the Na+-bound forms of mu R ligands: naloxone, bremazocine, an d carfentanyl. The resultant low-energy complexes showed that the two posit ive charges in the protonated metal-bound ligands interact with the two neg ative charges at Asp(3.32) and Asp(2.50) (for notations, see J. A. Balleste ros and H. Weinstein, 1995, Methods Neurosci. 25, 366-426). MCM computation on morphine docked inside the model of mu R by I. D. Pogozheva, A. L. Lomi ze, and H. I. Mosberg (1998, Biophys. J. 75, 612-634) yielded two binding m odes with the ligand's ammonium group salt-bridged either to Asp(3.32) (gen erally regarded as the ligand recognition site) or to Asp(2.50). The latter is the presumed site for Na+ ion, which is known to modulate ligand bindin g. Assuming that in the low-dielectric transmembrane region of mu R, organi c and inorganic cations would compete for Asp(3.32) and Asp(2.50), We propo se that ligand binding, as visualized in the above models, would first disp lace Na+ from Asp(3.32). A subsequent progress of the ligand toward Asp(2.5 0) would result in either the retention of Na+ at Asp(2.50) in the case of antagonists or the displacement of Na+ from Asp(2.50) in the case of agonis ts. The displaced Na+ would move toward the salt-bridged Asp(3.49)-Arg(3.50 ) and disengage the salt bridge. This, in turn, would result in conformatio nal changes at the cytoplasmic face of the receptor that facilitate the int eraction with the G-protein. (C) 2000 Academic Press.