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Bovine Rhodopsin - General Information
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Rhodopsin is found in both prokaryotes and eukaryotes. Rhodopsin is a biological pigment that is found in the rods of the retina and is a G-protein-coupled receptor (GPCR). Rhodopsin of the rods is known for absorbing large amounts of green-blue light and therefore, appears reddish-purple; which is why it is also called "visual purple". Rhodopsin is responsible for monochromatic vision in the dark. Rhodopsin is very sensitive to light which is why it allows for vision in low-light conditions. However, when rhodopsin is exposed to light it immediately gets bleached and thus, vision will be inhibited. In order for the rods to continue functioning and for vision to be possible after the bleaching of rhodopsin, we must regenerate rhodopsin at a rate that keeps pace with bleaching. In humans, rhodopsin usually regenerates after about 30 minutes.
Structure and Function
Rhodopsin consists of two major structural components: Retinal and Scotopsin
Retinal
is produced in the retina from vitamin A specifically from dietary beta-carotene and it is bound to proteins called opsins such as Rhodopsin which is the chemical basis of animal vision. When rhodopsin absorbs light, its retinal cofactor isomerizes from the 11-cis to the all-trans configuration. This is what causes the photobleaching of Rhodopsin and stops it from fuction in correctly. In rhodopsin, the aldehyde group of retinal is covalently linked to the amino group of a lysine residue on the protein called . This lysine is in a protonated Schiff base (-NH+=CH-) which is where it will ligand to a metal atom. Interactions of the chromophore of the structure determines the wavelength of the maximum absorption and changes in these interactions among rhodopsins facilitate color discrimination (green-blue light vs. other light).
Scotopsin
is an opsin which is a protein that forms as part of the visual pigment rhodopsin and is released by the action of light. Scotopsin is a light-sensitive G- protein coupled receptor (GPCR) that embeds in the lipid bilayer of cell membranes using seven protein transmembrane domains. These domains form a pocket where the photoreactive chromophore, retinal, lies horizontally to the cell membrane, linked to a lysine 296 residue in the seventh transmembrane domain of the protein.
