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Rhodopsin (also known as visual purple) is a light-sensitive receptor protein. 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 the protein moiety opsin and a reversibly covalently bound cofactor, retinal. Opsin, a bundle of seven transmembrane helices connected to each other by polypeptide loops, binds retinal (a photoreactive chromophore), which is located in a central pocket on the seventh helix at a lysine residue. The retinal lies horizontally with relation to the membrane. Each outer segment disc contains thousands of visual pigment molecules. About half the opsin is embedded within the lipid bilayer. 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.〔 Rhodopsin of the rods most strongly absorbs green-blue light and, therefore, appears reddish-purple, which is why it is also called "visual purple". It is responsible for ''monochromatic'' vision in the dark.〔 Several closely related opsins exist that differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. Humans have four different other opsins besides rhodopsin. The photopsins are found in the different types of the cone cells of the retina and are the basis of color vision. They have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light. The remaining opsin (melanopsin) is found in photosensitive ganglion cells and absorbs blue light most strongly. In rhodopsin, the aldehyde of retinal is covalently linked to the amino group of a lysine residue on the protein in a protonated Schiff base (-NH+=CH-). When rhodopsin absorbs light, its retinal cofactor isomerizes from the 11-cis to the all-trans configuration, and the protein subsequently undergoes a series of relaxations to accommodate the altered shape of the isomerized cofactor. The intermediates formed during this process were first investigated in the laboratory of George Wald, who received the Nobel prize for this research in 1967. The photoisomerization dynamics has been subsequently investigated with time-resolved IR spectroscopy and UV/Vis spectroscopy. A first photoproduct called photorhodopsin forms within 200 femtoseconds after irradiation, followed within picoseconds by a second one called bathorhodopsin with distorted all-trans bonds. This intermediate can be trapped and studied at cryogenic temperatures, and was initially referred to as prelumirhodopsin. In subsequent intermediates lumirhodopsin and metarhodopsin I, the Schiff's base linkage to all-trans retinal remains protonated, and the protein retains its reddish color. The critical change that initiates the neuronal excitation involves the conversion of metarhodopsin I to metarhodopsin II, which is associated with deprotonation of the Schiff's base and change in color from red to yellow. The structure of rhodopsin has been studied in detail via x-ray crystallography on rhodopsin crystals. Several models (e.g., the ''bicycle-pedal mechanism'', ''hula-twist mechanism'') attempt to explain how the retinal group can change its conformation without clashing with the enveloping rhodopsin protein pocket. Recent data support that it is a functional monomer- as opposed to a dimer- which was the paradigm of G-protein-coupled receptors for many years. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「rhodopsin」の詳細全文を読む スポンサード リンク
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