Light, rhodopsin

The eye of higher animals is remarkably sensitive to low light intensities, but saturation can occur relatively easily at high light intensities and the recovery is then slow. After the absorption of light rhodopsin (which has a red colour) is bleached when the retinal leaves the opsin, and recovery goes through a sequence of enzyme-catalysed reactions which takes several seconds. 

Light (Rhodopsin) cGMP-PDE stimulation Tr(G,.r) retinal rod cells (night)... 

The distribution of rods and cones is shown in Figure 3b centered about the fovea, the area of the retina that has the highest concentration of cones with essentially no rods and also has the best resolving capabiUty, with a resolution about one minute of arc. The fovea is nominally taken as a 5° zone, with its central 1° zone designated the foveola. There are about 40 R and 20 G cones for each B cone in the eye as a whole, whereas in the fovea there are almost no B cones. A result of this is that color perception depends on the angle of the cone of light received by the eye. The extremely complex chemistry involved in the stimulation of opsin molecules, such as the rhodopsin of the rods, and the neural connections in the retinal pathway are well covered in Reference 21. 

Given the difficulty of obtaining three-dimensional crystals of membrane proteins, it is not surprising that the electron microscope technique is now widely used to study large membrane-bound complexes such as the acetylcholine receptor, rhodopsin, ion pumps, gap junctions, water channels and light-harvesting complexes, which crystallize in two dimensions. 

FIGURE 18.36 The incorporation of retinal into the light-sensitive protein rhodopsin involves several steps. All- ram-retinol is oxidized by retinol dehydrogenase and then iso-merized to ll-cis-retinal, which forms a Schiff base linkage with opsin to form light-sensitive rhodopsin. 

Members of the first group of NCS are recoverin and visinin. Recoverin is a 23 kDa myristoylated protein found under normal conditions only in photoreceptor cells (rods and cones). The main function of recoverin is to bind to and inhibit rhodopsin kinase, thereby prolonging the light response. 

Farrens DL, Altenbach C, Yang K, Hubbell WL, Khorana HG. Requirement of rigid-body motion of transmembrane helices for light activation of rhodopsin. Science 1996 274(5288) 768-770. 

The prototype family A receptor, rhodopsin itself, clearly functions as a monomer despite its occurrence in very high density in the light-sensitive membranes however, it could be argued that rhodopsin should not be used as an example for receptors in general as it has very special requirements in respect to signaling due to its function as an ultra-rapid light sensor. 

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