Photoreceptors rhodopsin generation

We have already encountered the protein rhodopsin, the photoreceptor that generates and transmits nerve impulses in retinal cells (Real Life 18-2). Other proteins serve for transport and storage. Thus, hemoglobin carries oxygen iron is transported in the blood by transferrin and stored in the liver by ferritin. Proteins play a crucial role in coordinated motion, such as muscle contraction. They give mechanical support to skin and bone they are the antibodies responsible for our immune protection and they control growth and differentiation—that is, which part of the information stored in DNA is to be used at any given time. 

In mammals, (1IZ)-Retinal, generated from retinol in the retina, is the photo-reactive chromophore, which forms a Schiff-base to a lysine residue of opsin, a G-protein-coupled receptor (GPCR) protein, to give rhodopsin. This visual purple is concentrated in the outer parts of the rod and cone photoreceptors. Upon light absorption, the chromophore converts photons into a chemical signal by isomerisation to (all )-retinal, which causes a conformational change of... 

The retinal pigment epithelium cells (RPE) of the retina form an epithelial cell layer that takes up retinol from choroid capillaries and stores it as retinyl esters, to be used as substrate for the generation of 11-ds-retinal. In the layer of rod and cone photoreceptor cells adjacent to the RPE, 11-ds-retinal combines covalently with the protein opsin to generate the visual pigment rhodopsin in rods and, similarly, iodopsin in cones. Each rod outer segment is densely packed with some 10 molecules of rhodopsin per cell. The small quantity of vitamin A stored in the retina would be inadequate to maintain vision were it not for the visual cycle, a process in which 11-ds-retinal is regenerated after... 

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