Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction in rods. Rhodopsin mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, after which the rods are more sensitive. Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness. Names Rhodopsin was discovered by Franz Christian Boll in 1876. The name rhodospsin derives from Ancient Greek () for "rose", due to its pinkish color, and () for "sight". It was coined in 1878 by the German physiologist Wilhelm Friedrich Kühne (1837-1900). When George Wald discovered that rhodopsin is a holoprotein, consisting of retinal and an apoprotein, he called it opsin, which today would be described more narrowly as apo-rhodopsin. Today, the term opsin refers more broadly to the class of G-protein-coupled receptors that bind retinal and as a result become a light sensitive photoreceptor, including all closely related proteins. When Wald and colleges later isolated iodopsin from chicken retinas, thereby discovering the first known cone opsin, they called apo-iodopsin photopsin (for its relation to photopic vision) and apo-rhodopsin scotopsin (for its use in scotopic vision). General Rhodopsin is a protein found in the outer segment discs of rod cells. It mediates scotopic vision, which is monochromatic vision in dim light. Rhodopsin most strongly absorbs green-blue light (~500 nm) and appears therefore reddish-purple, hence the archaic term "visual purple". Several closely related opsins differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. Humans have, including rhodopsin, nine opsins, as well as cryptochrome (light-sensitive, but not an opsin). Structure Rhodopsin, like other opsins, is a G-protein-coupled receptor (GPCR). GPCRs are chemoreceptors that embed in the lipid bilayer of the cell membranes and have seven transmembrane domains forming a binding pocket for a ligand. The ligand for rhodopsin is the vitamin A-based chromophore 11-cis-retinal, which lies horizontally to the cell membrane and is covalently bound to a lysine residue (lys296) in the seventh transmembrane domain through a Schiff-base. However, 11-cis-retinal only blocks the binding pocket and does not activate rhodopsin. It is only activated when 11-cis-retinal absorbs