Rhodopsin is a light absorbing pigment; it efficiently absorbs light in the middle of the visual spectrum with a maximum at 500nm. Rhodopsin is the major protein of the rod outer segment (ROS), a cellular domain that captures light and initiates the visual response. Rhodopsin is made in the rod inner segment (RIS) and is transported from there to the ROS on small membrane spheres called vesicles.
The protein Rhodopsin contains the protonated retinal-Schiff’s base complex, which naturally lies in the inter-membrane pocket formed by the seven trans-membrane a-helical receptors. There are many flat discs of rhodopsin within the outer segment of a rod cell, which upon light detection undergo a photo-isomeric change from Rhodopsin (11-cis) to all-trans retinal.
Rhodopsin consists of the protein opsin linked to 11-cis retinal a prosthetic group. Retinal is the light absorbing pigment molecule and is a derivative of vitamin A. Opsin is a member of the 7TM receptor family. Rhodopsin is a protein in the membrane of the rod photoreceptor cell in the retina of the eye. It catalyses the only light sensitive step in vision. The 11-cis-retinal chromophore lies in a pocket of the protein and is isomerised to all-trans retinal when light is absorbed. The isomerisation of retinal leads to a change of the shape of rhodopsin, which triggers a sequence of reactions, which lead to a nerve impulse. This is transmitted to the brain via the optical nerve.
Rhodopsin (4th of the 5 intermediates) triggers an enzymatic cascade process resulting in the hydrolysis of GMP. This in turn closes cation-specific channels within the rod cell membrane, which are naturally open to influx of Na + in the dark, and due to the effect of hyperpolarisation, the inner synaptic body sends a nerve signal to other neurons in the Retina. Finally the light-induced lowering of calcium levels aids recovery of excited neurons to a passive, “dark” state and the cycle starts again upon detection of light. The photoreceptors of cone cells are also seven a-helical receptors with 11-cis-retinal as their chromophore.
Humans cannot make Rhodopsin, instead they use and external source, b -carotene, that is found in food in order to synthesis it. Light falling on the retina brings about certain chemical changes in the rhodopsin and other substances present in the rods and cones. These changes occur very rapidly, but large quantities of vitamin A are needed to bring this about. If there is any marked deficiency of vitamin A, night blindness may occur.
When light energy is absorbed by rhodopsin, the rhodopsin begins within trillionths of a second to decompose. The cause of this is photo activation of electrons in the retinal portions of the rhodopsin which leads to an instantaneous change of the cis form of retinal into an all trans form, which still has the same chemical structure as the cis form but has a different physical structure- a straight molecule rather than an angulated molecule. It is the metarrhodopsin II also called activated rhodopsin that excites electrical changes in the rods that then transmit the visual image into the central nervous system.
The first stage in the reformation of rhodopsin is to reconvert the all trans retinal into 11-cis retinal. This process requires metabolic energy and is catalyzed by the enzyme retinal isomerase. Once the 11-cis retinal is formed it automatically recombines with the scotopsin to reform rhodopsin, which then remains stable until its decomposition is again triggered by absorption of light energy.
There is second chemical route by which all trans retinal can be converted into 11-cis retinal. This is by conversion of the all trans retinal first into all trans retinal, which is one form of vitamin A. then, the all trans retinal is converted into 11-cis retinol under the influence of the enzyme isomerase. And finally the 11-cis retinol is converted into 11-cis retinal that combines with scotopsin to form rhodopsin.
When the rhodopsin in the outer segment of the rod is exposed to light and begins to decompose, this decreases the outer segment conductance of sodium to the interior of the rod, even though the sodium ions continue to be pumped out of the inner segment.
When exposed to light rhodopsin is split up into the yellow compound retinene and colourless protein opsin. The retinene is actually retinene 1. Under the influence of light, rhodopsin is converted to orange red compound lumirhodopsin, which becomes changed to metarhodopsin, ultimately forming a yellow mixture of trans-retinene and opsin.
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