Rhodopsin light response

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. 

Because one molecule of activated cGMP phosphodiesterase can hydrolyze more than 105 molecules of cGMP per second the light response is highly amplified. There is also an earlier stage of amplification. Each molecule of light-activated rhodopsin (R ) is able to catalyze the exchange of GTP for GDP on hundreds of molecules of... 

Figure 2.9 (a) Light-responsive rod cell from the human retina. The photoresponsive pigment rhodopsin... 

Figure 2.9 (a) Light-responsive rod cell from the human retina. The photoresponsive pigment rhodopsin is located in the outer lamellae. The foot makes contact with the optic nerve, (b) Operation of the rod cell. 

Receptors that remain in the activated state even in the absence of ligand are often known as constitutively active mutants (CAMs). The resulting disruptions in rhodopsin signaling also often result in alterations in the phosphorylation of rhodopsin by rhodopsin kinase (GRKl), the specialized GRK enzyme expressed in the retina that is largely responsible for rapidly desensitizing the receptor when it is exposed to light. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

Rod and cone cells are the light sensitive receptor cells in the retina of the human eye. About three million rod cells are responsible for our vision in dim light, whereas the hundred million cone cells are responsible for our vision in the bright light and for the perception of bright colours. In the rod cells, ll-cw-retinal is converted to rhodopsin. 

The chromophore (light-absorbing part of the molecule) of rhodopsin, the protein responsible for vision, is a polyunsaturated imine with one of the double bonds in the less stable (Z) configuration. 

S ATP + rhodopsin <1-16> (<1> phosphorylation of rhodopsin may control passive permeability to certain ions in rod outer segments, so mediating the responsiveness to a light impulse [1] <1,3> enzyme in vivo is probably inactive in the dark, but is almost fully activated in the light... 

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