rodopsin

Rodopsin (also rhodopsin) is a light-sensitive G protein-coupled receptor found in the rod cells of the retina. It consists of the protein opsin bound to the chromophore 11-cis-retinal. In humans, the gene RHO encodes the opsin component.

Rhodopsin is a seven-transmembrane GPCR of about 348 amino acids. The chromophore 11-cis retinal forms a Schiff base with a lysine residue (Lys296) in the binding pocket. Photon absorption converts 11-cis retinal to all-trans retinal, initiating activation. The absorption maximum is near 498–505 nm.

Activation converts rhodopsin to metarhodopsin II, which activates the G protein transducin. Transducin then stimulates phosphodiesterase PDE6, reducing intracellular cGMP. Lower cGMP closes cyclic nucleotide–gated channels, hyperpolarizing the rod and decreasing glutamate release, thereby signaling light to bipolar cells.

After isomerization, all-trans retinal is released and regenerated to 11-cis retinal in the retinal pigment epithelium through the visual cycle, involving enzymes such as RPE65. Regenerated chromophore recombines with opsin to form functional rhodopsin again.

Rods support scotopic vision in dim light due to the high sensitivity of rod photoreceptors. Mutations in RHO or rhodopsin misfolding can cause inherited retinal diseases, including retinitis pigmentosa and congenital stationary night blindness.

Rhodes and rhodopsin were characterized as key molecules in vertebrate vision in the 20th century, and their chemical and signaling properties helped establish the framework of GPCR signaling and the visual cycle.