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Visual cycle

The visual cycle occurs via G-protein coupled receptors called retinylidene proteins which consists of a visual opsin and a chromophore 11- cis -retinal. The 11- cis -retinal is covalently linked to the opsin receptor via Schiff base. When it absorbs 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 cyclic GMP-gated cation channel, and hyperpolarization of the photoreceptor cell. Following photoisomerization, all- trans -retinal is released from the opsin protein and reduced to all- trans - retinol, which travels 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. The isomerase activity of RPE65 has been shown; it is uncertain whether it also acts as the hydrolase. Finally, it is oxidized to 11- cis -retinal before traveling back to the photoreceptor cell outer segment where it is again conjugated to an opsin to form new, functional visual pigment (retinylidene protein), namely photopsin or rhodopsin.