Chromophores rhodopsins

The visual chromophores. Rhodopsin has been an object of scientific interest for over 100 years.462 Wald and associates469 470 established that rhodopsin contains 11-ds-retinal bound to the opsin in Schiff base linkage (Eq. 23-36). When native rhodopsin is treated with sodium borohydride, little reduction is observed. However, after the protein is bleached by light, reduction of the Schiff base linkage becomes rapid, and the retinal is incorporated into a secondary amine, which was identified as arising from Lys 296. 

Photoisomerization.—Birge and Hubbard analyse the molecular dynamics of cis-trans isomerization in the visual pigment rhodopsin using INDO-CISD molecular orbital theory and semiempirical molecular dynamic theory. The analysis predicts that the excited-state species is trapped during isomerization in an activated complex that has a lifetime of 0.5ps. This activated species oscillates between two components which preferentially decay to form isomerized product (bathorhodopsin) or unisomerized 11-cw-chromophore (rhodopsin) within 1.9—2.3ps. The authors further conclude that the chromophore in bathorhodopsin has a distorted all-rraw-geometry and is the most realistic model for the first intermediate in the bleaching cycle of rhodopsin. 

After the cis-trans photoisomerization of the retinal chromophore, rhodopsin changes its protein conformation to a G-protein-activatmg state. Because the conformational change of the protein needs chemical-free energy, hght energy absorbed by the chromophore should be converted to chemical energy. 

Rozanowska, M and Sarna, T, 2005. Light-induced damage to the retina Role of rhodopsin chromophore revisited. Photochem Photobiol 81, 1305-1330. 

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]. 

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

The chromophore, 11-ris-retinal, is linked to the a-amino group of lysine-296 in helix 7 via a protonated Schiff base. Protonated Schiff bases usually absorb light maximally at around 440 nm, but the of rhodopsin is near 500 nm. 

M. Koyanagi, A. Terakita, K. Kubokawa and Y. Shichida, Amphioxus homologs of Go-coupled rhodopsin and peropsin having 11 -cis- and all-/ram-rctinals as their chromophores. FEBS Lett. 531 (2002) 525-528. 

The general strategy for establishing the sites of protein-chromophore interactions in rhodopsin involves introduction of selective 13 C labels at each position along the length of the retinal chromophore. 

Differences in the 13C chemical shifts between rhodopsin and retinal PSB model compounds reveal the regions of the chromophore where changes occur in the retinal s structure or environment. Figure 39 presents several solid-state 13C NMR spectra of rhodopsin that illustrate the resolution and sensitivity which can be obtained by using MAS methods. 

The study of the mechanism of vision in vertebrates 23>24) has progressed to the point where the first consequence of photon absorption has been described as an activation of the isomerization of the 11 -cis retinal chromophore of rhodopsin to all-trans. That triggers a complex sequence of reactions leading to the mysterious inside of the brain. Brrr, I had better get back — it looks dark in there. But the brain can generate sensations of light. Maybe, one day, we will be able to see enough to understand, but we ll go back just the same to a safer subject. 

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... 

Weit Phyllis Robinson, I believe, reported reconstitution of melanopsin and showed by roughly the same criteria used for rhodopsin that it binds a chromophore. 

In such a way we were able to conclude that the illumination of suspensions of photoreceptor outer segments by 450 nm light at 77°K, which was known to result in the rhodopsin— prelumirhodopsin transition (corresponding to 11-cis-retinal— transretinal photoisomerization of chromophore), leads also to the appearance of some reduction centers and to the conformational change of membrane. 

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. 

Answer. The n system chromophore of rhodopsin consists of 12 atoms in an unbranched arrangement, with a dicoordinated N atom at one end. Remember that SHMO theory cannot distinguish between isomers. A good model for the chromophore is a 12-atom n system, l-aza-l,3,5,7,9,ll-dodecahexaene (Figure B5.3). 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

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