Rhodopsin is the visual photoreceptor responsible for black and white vision in low light that is found in the rod cells of the human retina. It is a 40 kDa eukaryotic membrane protein that belongs to the large, pharmaceutically important family of G protein-coupled receptors (GPCRs). These receptors have a common architecture consisting of seven transmembrane helices. Activation of rhodopsin by light is initiated by isomerization of its photoreactive 11-cis retinylidene chromophore. The chromophore is covalently bound within the bundle of transmembrane helices through a protonated Schiff’s base linkage to Lys296. The crystal structure of rhodopsin in the dark, inactive state has previously been solved to high resolution. However, no high-resolution structural data are available for metarhodopsin II, the active state of rhodopsin. My thesis work describes how structural constraints obtained by solid-state Nuclear Magnetic Resonance (NMR) spectroscopic measurements of the metarhodopsin II intermediate are combined with restrained molecular dynamics simulations to understand how rhodopsin converts light into a chemical signal.

Retinal isomerization leads to steric strain within the retinal binding site between the retinal β-ionone ring and helix 5 (H5), and between the retinal C19/C20 methyl groups and the second extracellular loop (EL2). These interactions lead to a rearrangement of hydrogen bonding networks involving H5 and EL2 triggering the motion of EL2 away from the retinal, deprotonation of the Schiff base nitrogen and protonation of Glu113. Displacement of EL2 is coupled to the motion of H5 in metarhodopsin II. Motion of the β-ionone ring is coupled to the motion of Trp265 on H6, which triggers a shift of H6 and H7 into active conformations and rearrangement of the hydrogen bonding network centered on the conserved NPxxY motif on H7. Motion of H5, H6 and H7, in turn, is coupled to the rearrangement of electrostatic interactions involving the conserved ERY motif at the cytoplasmic end of H3, exposing the G-protein binding site on the cytoplasmic surface of the protein. The location of the retinal and structural reorganization of the protein upon activation provides a blue print for understanding the action of agonists and antagonists in the large family of class A GPCRs.