The Bleach and Recycle Pathway
When a photon of light is absorbed, 11-cis retinal is transformed to all-trans retinal, and it moves to the exit site of rhodopsin. It will not leave the opsin protein until another fresh chromophore comes to replace it, except for in the ABCR pathway. Whilst still bound to the opsin, all-trans retinal is transformed into all-trans retinol by all-trans Retinol Dehydrogenase. It then proceeds to the cell membrane of the rod, where it is chaperoned to the Retinal Pigment Epithelium (RPE) by Interphotoreceptor Retinoid Binding Protein (IRBP). It then enters the RPE cells, and is transferred to the Cellular Retinol Binding Protein (CRBP) chaperone.
When inside the RPE cell, bound to CRBP, the all-trans retinol is esterified by Lecithin Retinol Acyltransferase (LRAT) to form a retinyl ester. The retinyl esters of the RPE are chaperoned by a protein known as RPE65. It is in this form that the RPE stores most of its retinoids, as the RPE stores 2-3 times more retinoids than the neural retina itself. When further chromophore is required, the retinyl esters are acted on by isomerohydrolase to produce 11-cis retinol, which is transferred to the Cellular Retinaldehyde Binding Protein (CRALBP). 11-cis retinol is transformed into 11-cis retinal by 11-cis Retinol Dehydrogenase, then it is shipped back to the rod photoreceptors via IRBP. There, it replaces the spent chromophore in opsin molecules, allowing the opsin to function again as a light receptor.
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