Opsin-retinal

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. 

Data is also shown for bovine rhodopsin, 63, for comparison. The hydroretinals 66-68 presumably assume 9-cis or 11 -cis like conformations when bound to opsin. Retinals 67 and 68 form non-bleachable pigments, i.e., no change in their A, occurs upon exposure to room light irradiation by UV light leads to decomposition products instead of separation of the chromophore from opsin. a In MeOH. In case of split chromophores the absorption maxima of the enal moieties are given. b Protonated Schiff base with n-butylamine in MeOH. ... 

The conversion of a cis isomer to the trms isomer (or vice versa) is a process called cis-trans isomerization. Such conversions are crucial in the vision process. The molecules in the retina that respond to light are rhodopsin, which has two components called 11-cw-retinal and opsin. Retinal is the light-sensitive component, and opsin is a protein molecule. Upon receiving a photon in the visible region, 11-cw-retinal isomer-izes to all-trans retinal by breaking a carbon-carbon tt bond. With the TT bond broken, the atoms connected by the carbon-... 

Isralewitz et eil., 1997] Isralewitz, B., Izrailev, S., and Schulten, K. Binding pathway of retinal to bacterio-opsin A prediction by molecular dynamics simulations. Biophys. J. 73 (1997) 2972-2979... 

FIGURE 17 11 Imine formation between the aldehyde function of 11 as retinal and an ammo group of a protein (opsin) is involved in the chemistry of vision The numbering scheme in retinal is specifically developed for carotenes and related compounds... 

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. 

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. 

Bleaching is reversed in the dark and the red-purple color of rhodopsin returns. This is thought to occur by the reduction of all-Pms-retinal to vitamin Ai (retinal), which diffuses from the rod into the pigment epithelium, where it is converted enzymatically to the 1 l-c isomer of vitamin At. The enzymatic isomerization is followed by diffusion back into the rod, oxidation to 11 -rfr-retinal, and combination with opsin to form rhodopsin. This process is shown schematically in Figure 12.5.[Pg.289]

The visual pigment present in rods has been termed rhodopsin and consists of 11-m-retinal, a derivative of vitamin A1( and a lipoprotein called opsin. Recent evidence(43) suggests that in native rhodopsin the retinal chromo-phore is covalently bonded to a phosphatidylethanolamine residue of the lipid portion of opsin. The structure of 1 l-cis-retinal is as follows ... 

The bleaching of rhodopsin has been found to lead to the all-trans form of retinal through several intermediate steps.(llb,45) These steps are temperature dependent consequently low temperatures must be used to observe the intermediate products. A solution of rhodopsin is subjected to a flash of light at liquid nitrogen temperature and intermediates are detected by changes in the absorption spectrum. The first intermediate, with an absorption maximum at 543 nm, believed to be an all-fra/w-retinal bound to opsin, has been termed prelumirhodopsin. Warming the sample to a temperature greater than... 

Schadel, SA, Heck, M, Maretzki, D, Filipek, S, Teller, DC, Palczewski, K, and Hofmann, KP, 2003. Ligand channeling within a G-protein-coupled receptor—The entry and exit of retinals in native opsin. J Biol Chem 278, 24896-24903. 

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]. 

Rhodopsin is a transmembrane protein linked to 1 l as-retinal, which, on photoabsorption, decomposes to opsin and all-trans-retinal 809... 

Rhodopsin is a transmembrane protein linked to 11-c/s-retinal, which, on photoabsorption, decomposes to opsin and all-f/a/75-retinal. Rhodopsin has a molecular weight of about 40,000. Its C-terminus is exposed on the cytoplasmic surface of the disk, and its sugar-containing... 

The photochemistry of vision provides us with an example of host-guest supramolecular photochemistry where the smaller 11-cis-retinal guest molecule is held within the internal cavity of the much larger protein host molecule (opsin) as a result of noncovalent bonding. 

The light-absorbing part of a rod cell contains the pigment rhodopsin, which consists of the opsin attached to the 11-cis-retinal molecule (1) (Figure 12.3). Free 11-cis-retinal absorbs in the ultraviolet, but when attached to opsin the absorption is in the visible region. 

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