Photoreceptors opsins

Post-ingestion from a-, (3- y-carotene other carotenes from plant leaves a wide variety of fruit, root seed sources e.g. Daucus carota (carrot) (Apiaceae) [root] Retinal covalently linked to opsins (— light receptor Rhodopsins in vision) colour blind John Dalton (atomic theory, 1766-1844) bequeathed his eyes to science 2 centuries on molecular biology confirmed the absence of the gene for the green photoreceptor opsin... 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

In photoreceptor cells, the rods and cones of the human retina, the retinal is linked to a specific protein termed opsin. The resulting pigment is known as rhodopsin. When a photon of light of the proper wavelength hits a molecule of rhodopsin, two chemical events take place. First, the ll-c -retinal is converted to the all-trans form and, secondly, the all-trani-retinal is released from the rhodopsin ... 

The photoreceptor complex comprises cw-retinal and opsin (R-O) for which the regulatory signal is light. This interacts with the regulatory component of the complex R, which then dissociates (in the form ciT-R) from the O component. The free O is now catalytically active and catalyses the initial reaction in the visual cascade sequence which leads to activation of the target system (i.e. produces an action potential in the optic nerve). 

Figure 15.11 The biochemical reactions that result in the conversion of trans-retinal to ds-retinal, to continue the detection of light To continue the process, trans-retinal must be converted back to c/s-retinal. This is achieved in three reactions a dehydrogenase converts trans-retinal to trans-retinol an isomerase converts the trans-retinol to c/s-retinol and another dehydrogenase converts c/s-retinol to c/s-retinal. To ensure the process proceeds in a clockwise direction (i.e. the process does not reverse) the two dehydrogenases are separated. The trans-retinal dehydrogenase is present in the photoreceptor cell where it catalyses the conversion of trans-retinal to trans-retinol which is released into the interstitial space, from where it is taken up by an epithelial cell. Here it is isomerised to c/s-retinol and the same dehydrogenase catalyses its conversion back to c/s-retinal. This is released by the epithelial cell into the interstitial space from where it is taken up by the photoreceptor cell. This c/s-retinal then associates with the protein opsin to produce the light-sensitive rhodopsin to initiate another cycle. The division of labour between the two cells may be necessary to provide different NADH/NAD concentration ratios in the two cells. A high ratio is necessary in the photoreceptor cell to favour reduction of retinal and a low ration in the epithelial cell for the oxidation reaction (Appendix 9.7).

Vitamin A is essential for proper functioning of the retina, for the integrity of epithelial tissue, for growth and bone development and for reproduction. For vision the active vitamin appears to be retinal as the chromophore of both rods and cones is 11-cis-retinal which, in combination with the protein opsin, forms the photoreceptor rhodopsin. Retinoic acid is the active form associated with growth, differentiation, and transformation. Both all-trans and 9-cis retinoic acid act as a steroid hormone to affect cellular differentiation, especially for morphogenesis, reproduction and for immune responses. At... 

H hstract. Until recently, all ocular photoreception was attributed to the rods and cones of the retina. However, studies on mice lacking rod and cone photoreceptors (rdjrdcl), has shown that these mice can still use their eyes to detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity and modulate pupil size. In addition, action spectra for some of these responses have characterized a novel opsin/vitamin A-based photopigment with a 480nm. Electrophysiological... 

The rd rd cl mouse has provided a powerful model to characterize the ocular non-rod, non-cone photoreceptors of mammals using action spectrum techniques. The first completed action spectrum was for the pupillary light reflex PLR (Lucas et al 2001). The results demonstrated that the PLR in rd rdcl mice is driven by a single opsin/vitamin A-based photopigment with a 479 nm. The... 

The opsins probably perform a variety of different tasks, but their known roles are as photosensors or photoisomerases (Foster Bellingham 2002). Photosensory opsins such as the rod and cone opsins, P opsin and VA opsin use light to activate a phototransduction cascade that ultimately results in a change in membrane potential of the photoreceptor cell. By contrast, photoisomerases are involved in photopigment regeneration. The best described photoisomerase is... 

Studies on teleost fish and mammals have demonstrated the existence of non-rod, non-cone ocular photoreceptors. In the case of VA opsin in the roach, electrophysiological evidence suggests that one function of this photosensory photopigment is to modulate the activity of retinal horizontal cells. To what end remains unclear, but this fits with the general role of horizontal cells in the teleost... 

Foster RG, Bellingham J 2002 Opsins and melanopsins. Curr Biol 12 R543 Foster RG, Follett BK, Lythgoe JN 1985 Rhodopsin-like sensitivity of extra-retinal photoreceptors mediating the photoperiodic response in quad. Nature 313 50-52 Foster RG, Schalken J J, Timmers AM, De Grip WJ 1989 A comparison of some photoreceptor characteristics in the pineal and retina I. The Japanese quail Coturnix coturnix). J Comp Physiol A 165 553-563... 

Minamoto T, Shimizu I 2002 A novel isoform of vertebrate ancient opsin in a smelt fish, Plemglossus altivelis. Biochem Biophys Res Commun 290 280—286 Miyashita Y, Moriya T, Yamada K et al 2001 The photoreceptor molecules in Xenopus tadpole tail fin, in which melanophores exist. Zoolog Sci 18 671—674 Moriya T, Miyashita Y, Arai J, Kusunoki S, Abe M, Asami K 1996 Light-sensitive response in melanophores of Xenopus laevis I. Spectral characteristics of melanophore response in isolated tail fin of Xenopus tadpole. J Exp Zool 276 11-18 Moutsaki P, Bellingham J, Soni BG, David-Gray ZK, Foster RG 2000 Sequence, genomic structure, and tissue expression of carp (Cyprinus carpio L.) vertebrate ancient (VA) opsin. FEES Lett 473 316-322... 

Soni BG, Foster RG 1997 A novel and ancient vertebrate opsin. FEBS Lett 406 279—283 Soni BG, Philp AR, Knox BE, Foster RG 1998 Novel retinal photoreceptors. Nature 394 27—28 Starace DM, Knox BE 1997 Activation of transducin by a Xenopus short wavelength visual pigment. J Biol Chem 272 1095—1100... 

Cahill We did an action spectrum for suppression of melatonin in IKenopus retina. We tried to deal with this variability issue. We got some estimates from our all-fit program of 95% confidence intervals which we did plot on the action spectrum. When we did this we were fairly disappointed with what we were able to exclude as possible photoreceptors. We were able to eliminate the 650 nm cones, but anything from 500 nm down we couldn t really throw out. When we plotted the actual points they fell very nicely on the green opsin. The peak could have been over a range of 40 nm. 

There are a large number of reasons to revert to an alternate baseline. The most important is the fact that the chromophores of vision are not formed within the photoreceptor cell. There is an extensive database on how they are formed and stored within the RPE cells and subsequently transported to the Outer Segments for coating of the protein substrate known as opsin. 

Typical The coating of the protein substrate, Opsin, by the chromophore Rhodonine as it is released from the fluid filling the cavity between the RPE and the layer of photoreceptor Inner Segments. 

Where visible absorption peaks have been found in natural photoreceptor material, no NMR or X-ray crystallographic data has been put forward to demonstrate the presence of a retinene in the material (See Chapter 7). It is proposed here that any such tests will demonstrate the presence of a Rhodonine instead. Rhodonine contains two polar groups separated by a conjugated carbon chain. It is further proposed that opsin is only present as a completely passive substrate. Its association with the liquid crystalline Rhodonine is via a weak hydrogen bond that does not change the electronic configuration of the Rhodonines. 

Some of these inferential efforts have found it necessary to define four genetic paths, representing proteins associated with three chromatic paths and one achromatic path. Unfortunately, there is no achromatic photoreceptor type and there are actually four variants of the visual chromophores, instead of the commonly recognized three. The fourth is in the ultraviolet. Further, these chromophores are not proteins. As shown in this work, the protein opsin may come in many varieties but such a variety would have no effect on the chromophore coating it. 

The work of Papermaster and others have clearly shown that the protein used in the substrates (disks) of the outer segment of photoreceptor cells is formed within those cells. The material (opsin + retinal) is secreted by those cells and formed into disks within a cavity associated with the inner segment of the photoreceptor cells. The work of Bridges, et. al. shows clearly that the retinoids used in the functional part of the visual process, the Rhodonines, do not pass through the photoreceptor cells. They are delivered to the outer segments via the RPE. 

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