A novel receptor activation mechanism revealed by modeling n-OPIOID receptor-ligand complexes

Recent studies on G-protein coupled receptors (GPCRs) have identified several acidic and polar residues as crucial for ligand binding and receptor activation. Based on our work on interaction of ligands and channel proteins with metal ions (eg. Ca2' ), we have invoiced the binding of organic and metal cations to the above residues as an important aspect of receptor-ligand interaction. Using the Monte Carlo with energy minimization technique, we modeled the n-opioid receptor starting with rhodopsin structure and using Ca2'-bound morphine and enkephalin as docking ligands. In the narrow inverted-cone interface of helices H2-H3-H7, the ammonium group of the ligand is bound at the conserved aspartic acid residue D3 32 while Ca ²⁺ is bound simultaneously to the ligand oxygens and D3.32. A "gate" of conserved nonpolar residues is found beneath the bound ligand. The H2-H3H7 interface from the "gate" to the cytoplasmic part is lined with conserved polar residues, N7.45, S7.46, N7.49, D3.49 and R3.50. The last two form a salt-bridge. We hypothesize that ligand binding displaces a Na* ion bound to D2.50; the movement of Na* is regulated by the "gate"; the displaced Na would break the D 3.49 - R3.50 salt-bridge and expose R 3.50 for G-protein coupling Supported by MRC, Canada.