11-cA-retinal

When light strikes the rod cells, isomerization of the C-ll/C-12 double bond takes place, and tra 5 -rhodopsin (metarhodopsin II) is formed. This cis-trans isomerization is accompanied by an alteration in molecular geometry, which generates a nerve impulse to be sent to the brain, resulting in the perception of vision. Metarhodopsin II is recycled back to rhodopsin by a multi-step sequence that involves the cleavage to all-trani-retinal and cis-trans isomerization back to 11-cA-retinal. 

Cone cells, like rod cells, contain visual pigments. Like rhodopsin, these photoreceptor proteins are members of the 7TM receptor family and utilize 11-cA-retinal as their chromophore. In human cone cells, there are three distinct photoreceptor proteins with absorption maxima at 426, 530, and 560 nm (Figure 32.26). These absorbances correspond to (in fact, define) the blue, green, and red regions of the spectrum. Recall that the absorption maximum for rhodopsin is 500 nm. 

The eyes of arthropods, mollusks, and vertebrates use the cis-trans isomerization reaction to detect light. When light enters the eye, it is absorbed by an imine of 11-cA-retinal, which isomerizes to the lower energy n -lrans-retinal imine. The isomerization is detected by various enzymes that initiate an electrical impulse that enters the brain via the optic nerve. Meanwhile, the all-frans-retinal is transported to the liver ( ), where the enzyme retinal iso-merase uses acid catalysis and ATP to convert it back to the higher energy 11 -cis form. The 11 -r/.v-retinal is then sent back to the eye, ready to receive the next photon. 

Figure 29-3. Chemical structures of important vitamin A species and the provitamin A carotenoid i-carotene. All-fra/w-fi-carolene (T) is the most important provitamin A carotenoid, which can be converted to all-fraws-retinal and then all-tram-retinol (If), which by definition is vitamin A. All-tram-retinol can be esterified with long-chain fatty acids to form retinyl ester (III), the storage form of vitaminA in the body.The active form of vitamin A in vision is 11-cts-retinal (TV).The transcriptionally active forms of vitaminA are all-tram-retinoic acid (V) and 9-cts-retinoic acid (VI). 13-cA-Retinoic acid (VII) has poor transcriptional regulatory activity but is used clinically as isotretinoin to treat skin diseases.

While 11 -cis-retinal absorbs at 380 nm and a model protonated Schiff base of retinal with n-butylamine in methanol [210,211] absorbs at 440 nm, the absorption maxima of visual pigments based on 11 -cis-retinal span a wide range of values, from 430 to ca. 600 nm. Therefore, the problem consists of determining the nature of the... 

Absorption of light by the retinal portion of the complex isomerizes the cA-bond in 11-cis retinal to a trans-bond, forming an dX -trans compound called bathorhodopsin. 

In these spectra, the protein has been regenerated with retinal specifically 13 C labeled at positions 11,12 and 13, and in each case the retinal resonance exhibits a sharp centerband at the isotropic chemical shift and is flanked by rotational sidebands. Other lines in the spectrum are the natural-abundance 13C resonances of the protein carbonyls ca 175 ppm) and aliphatic carbons (0-100 ppm). Contributions from the Ammonyx-LO detergent in these spectra are seen in the different intensities in the 0-100 ppm region. Ammonyx-LO does not exhibit NMR resonances above 100 ppm. Spectra of the 9-cis pigment isorhodopsin are similar. Table 38 summarizes the isotropic chemical shifts from the solid-state NMR spectra of rhodopsin regenerated with retinal13 C labeled at each position along... 

Roussat B, Ruellan YM. Traitement du decollement de reine par vitrectomie et injection d huile de silicone. Resultats a long terme et complications dans 105 cas. [Treatment of retinal detachment by vitrectomy and injection of silicone oil. Long-term results and complications in 105 cases.] J Fr Ophtalmol 1984 7(1) 11-18. 

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