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The visual cycle is the biological conversion of a photon into an electrical signal in the retina. This process occurs via G-protein coupled receptors called opsins which contain the chromophore 11-cis retinal. 11-cis retinal is covalently linked to the opsin receptor via a Schiff base forming a retinylidene protein. When struck by a photon, 11-cis retinal undergoes photoisomerization to all-trans retinal which changes the conformation of the opsin GPCR leading to signal transduction cascades which causes closure of a cyclic GMP-gated cation channel, and hyperpolarization of the photoreceptor cell.
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Following isomerization and release from the opsin protein, all-trans retinal is reduced to all-trans retinol and travels back to the retinal pigment epithelium to be "recharged". It is first esterified by lecithin-retinol acyltransferase (LRAT) and then converted to 11-cis retinol by the isomerohydrolase RPE65. Finally, it is oxidized to 11-cis retinal before traveling back to the rod outer segment where it can again be conjugated to an opsin to form a new, functional visual pigment (rhodopsin).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Visual_cycle". A list of authors is available in Wikipedia.|