Rhodopsin Vision

In connection with the problem of oscillations discussed by previous speakers and other types of dynamical behavior of membranes, it would probably be timely to mention here in some more detail the experiments with vision rhodopsin that were performed in our institute by using the Mossbauer spectroscopy method [G. R. Kalamkarov et al., Doklady Biophys., 219, 126 (1974)]. These experiments manifested the existence of reversible photo-induced conformational changes in the photoreceptor membrane even at such low temperature as 77°K. We have labeled various samples of solubilized rhodopsin and of photoreceptor membranes by iron ascorbate enriched with Fe57 and looked for the change of Mossbauer spectra caused by the illumination of our samples. 

Photochemical cis-trans isomerization is a major area of interest in modem photochemical research and is also studied as part of organic photochemistry. Photochemical cis-trans isomerization has a major role in many photobiological phenomena, such as vision (rhodopsin) [1], ATP synthesis (bacteriorhodopsin) [2], phototaxis (Chlamydomonas) [3], and other allied processes. It has practical application in industry [4-6], i.e., vitamin A and D processes. Furthermore, it is a likely candidate for many optoelectrical and optomechanical switching and storage devices [7]. In this chapter, mainly various aspects of cis-trans isomerization originating from the singlet excited state will be discussed. 

Rhodopsin is present in the eye and primarily responsible for vision. Rhodopsin is a polytopic integral membrane protein, spanning the membrane seven times,... 

Even though the number of biological processes amenable to study by time-resolved spectroscopy is extremely large and range from hemoglobin and vision to photosynthesis,this chapter shall be restricted to energy-transfer processes and specifically to the primary proton transfer in the visual pigments, the chromophore of vision, rhodopsin. 

C20H28O, Mr 284.44, orange-red cryst., mp. (all-trans-R.) 61-64°C, (11-cis-R.) 63.5-64.5°C. A diter-pene of the cyclophytane type. Soluble in all common organic solvents. All 16 possible stereoisomers, of which the all-trans-fonn is the most stable, are known. In the form of a Schiff base bound to opsins, R. constitutes the pigments of vision rhodopsin and iodop-sin as well as bacteriorhodopsin that has other func-... 

Two molecules of vitamin A are formed from one molecule of -carotene. Vitamin A crystallizes in pale yellow needles m.p. 64 C. It is optically inactive. It is unstable in solution when heated in air, but comparatively stable without aeration. Vitamin A is manufactured by extraction from fish-liver oils and by synthesis from / -ionone. The role of vitamin A in vision seems to be different from its systemic function. See also relincne and rhodopsin. 

Schoenlein R W, Peteanu LA, Mathies R A and Shank C V 1991 The first step in vision femtosecond isomerization of rhodopsin Sc/ence 254 412-15... 

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 analysis of CD and absorption data has been used since 1974, mainly by Nakanishi and his coworkers, to study the structure of the rhodopsins, pigments involved in the mechanism of vision, and of their chromophores50. 

Bleached rhodopsin must be regenerated to maintain normal vision 809... 

Bleached rhodopsin must be regenerated to maintain normal vision. Regeneration occurs by several mechanisms. The major pathway occurs in the retinal pigmented... 

Mutations in rhodopsin and other photoreceptor proteins are linked to retinitis pigmentosa. Retinitis pigmentosa (RP) is a group of inherited retinopathies that affects about 1 in 4,000 humans [26], RP maybe classified into four types autosomal dominant (19%), autosomal recessive (19%),X-linked (8%) and allied diseases (54%). RP is characterized by loss of night vision in the early stage, followed by loss of peripheral vision. Chromosomal loci for numerous RP genes have been mapped and mutations characterized [27]. 

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. 

A deficiency of vitamin A leads to vision defects, including a visual impairment at low light levels, termed night blindness. For the processes of vision, retinol needs to be converted first by oxidation into the aldehyde retinal, and then by enzymic isomerization to cw-retinal. c -Retinal is then bound to the protein opsin in the retina via an imine linkage (see Section 7.7.1) to give the red visual pigment rhodopsin. 

It triggers conformational changes in retinoid proteins, such as rhodopsin and bacteriorhodopsin, relevant to vision and ATP synthesis, respectively. 

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... 

Example 2—The first stage in this process of vision has been the excitation of rhodopsin. Rhodopsin partially gets deactivated forming an intermediate, prelumirhodopsin or bathorhodopsin. Picosecond spectroscopy reveals that prelumirhodopsin gets formed because of an intramolecular proton transfer—a jump of a proton from one position to another. 

An example of a transmembrane receptor that registers sensory signals is rhodopsin. Rhodopsin is a sensory receptor that plays a role in vision, by receiving light signals and converting them into intracellular signals. 

The best investigated is the desensitization of the adrenaline receptor type P2 and of rhodopsin. Rhodopsin has the function of a light receptor in the process of vision, ft receives light signals and conducts them to the relevant G-protein, transducin. The key reaction in desensitization of both systems is the phosphorylation of the receptor at the cytoplasmic side by specific protein kinases. 

The Gt- and Gg-proteins are also classed as Gi-proteins, based on sequence homologies. The Gt- and Gg-proteins are involved in transmitting sensory signals. Signal transmission in the vision process is mediated via G-proteins known as transducins (Gt). The Gt-proteins are activated by the photoreceptor rhodopsin and are located in the rods and cones of the retina. The sequential effector molecules of the Gt-proteins are cGMP-specific phosphdiesterases (see Fig. 17.9). 

Another regulatory Ca receptor is recoverin, which performs an important control function in the signal transduction cascade of the vision process, by inhibiting the activity of rhodopsin kinase (see Chapter 5.3.4). 

Rod and cone cells are the light sensitive receptor cells in the retina of the human eye. About three million rod cells are responsible for our vision in dim light, whereas the hundred million cone cells are responsible for our vision in the bright light and for the perception of bright colours. In the rod cells, ll-cw-retinal is converted to rhodopsin. 

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