6-s-cis Conformation and Polar Binding Pocket of the Retinal Chromophore in the Photoactivated State of Rhodopsin

The visual pigment rhodopsin is unique among the G protein-coupled receptors in having an 11-cis retinal chromophore covalently bound to the protein through a protonated Schiff base linkage. The chromophore locks the visual receptor in an inactive conformation through specific steric and electrostatic interactions. This efficient inverse agonist is rapidly converted to an agonist, the unprotonated Schiff base of all-trans retinal, upon light activation. Here, we use magic angle spinning NMR spectroscopy to obtain the C-13 chemical shifts (C5-C20) of the all-trans retinylidene chromophore and the N-15 chemical shift of the Schiff base nitrogen in the active metarhodopsin 11 intermediate. The retinal chemical shifts are sensitive to the conformation of the chromophore and its molecular interactions within the protein-binding site. Comparison of the retinal chemical shifts in metarhodopsin 11 with those of retinal model compounds reveals that the Schiff base environment is polar. In particular, the (13)C15 and N-15 epsilon chemical shifts indicate that the C=N bond is highly polarized in a manner that would facilitate Schiff base hydrolysis. We show that a strong perturbation of the retinal (13)C12 chemical shift observed in rhodopsin is reduced in wild-type metarhodopsin 11 and in the E181Q mutant of rhodopsin. On the basis of the T-1 relaxation time of the retinal (13)C18 methyl group and the conjugated retinal (11)C5 and (13)C8 chemical shifts, we have determined that the conformation of the retinal C6-C7 single bond connecting the beta-ionone ring and the retinylidene chain is 6-s-cis in both the inactive and the active states of rhodopsin. These results are discussed within the general framework of ligand-activated G protein-coupled receptors.