Meta-Biol

• Pathways

The visual pigment rhodopsin consists of a seven transmembrane helix protein, opsin (RHO), to which an 11-cis-retinal (11cRAL) chromophore is bound as a protonated Schiff base (Hargrave et al. 1983, Nathans & Hogness 1984, Ovchinnikov et al. 1983). The covalent bond between opsin and its 11-cis-retinyl (11c-retinyl) ligand, which is unique among G protein coupled receptors, helps to confer extraordinary stability in darkness (Baylor et al. 1984). 11cRAL is an inverse agonist, that quenches the weak ability of opsin to activate transducin G protein (Gt). Upon photon absorption, the bound 11c-retinyl group isomerizes in a few hundred femtoseconds (Schoenlein et al. 1991) and with a high quantum efficiency of 0.7 (Dartnall 1968) to the bound all-trans-retinyl (at-retinyl) isomer. Then in the next few milliseconds, opsin undergoes a rearrangement in structure that renders it catalytically active (MII aka metarhodopsin II or R*) (Emeis et al. 1982). The isomerisation is a very fast photochemical process (femtoseconds) followed by slower events (Smith 2010).

Mutations in RHO can give rise to autosomal dominant or recessive forms of retinitis pigmentosa or autosomal dominant congenital stationary night blindness (https://sph.uth.edu/retnet/). Retinitis pigmentosa is a progressive form of blindness marked by an initial degeneration of rods, followed by the secondary loss of cones.