Rhodopsin

Rhodopsin (also known as visual purple) is a light-sensitive receptor protein involved in visual phototransduction. It is named after ancient Greek ῥόδον (rhódon) for “rose”, due to its pinkish color, and ὄψις (ópsis) for “sight”. Rhodopsin is a biological pigment found in the rods of the retina and is a G-protein-coupled receptor (GPCR). Rhodopsin is extremely sensitive to light, and thus enables vision in low-light conditions. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 45 minutes.

Rhodopsin was discovered by Franz Christian Boll in 1876.

Structure

Rhodopsin consists of a protein moiety also called scotopsin, which binds covalently a cofactor called retinal. Scotopsin is an opsin. Opsins are G protein coupled receptors and have seven transmembrane domains. The seven transmembrane domains form a pocket, where the retinal (as photoreactive chromophore) binds to a lysine residue in the seventh transmembrane domain. The retinal lies horizontally to the cell membrane. And the cell membrane lipid bilayer embeds half of the rhodopsin. Thousands of rhodopsin molecules are found in each outer segment disc of the host rod cell. Retinol is produced in the retina from Vitamin A, from dietary beta-carotene. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change (bleaching) in opsin, continuing with metarhodopsin II, which activates the associated G protein transducin and triggers a Cyclic Guanosine Monophosphate, second messenger, cascade.