Retinal cycle

Figure 23-44 Reactions of retinol and the retinal cycle of mammalian rod cells. After Palczewski et al.5i3...

FIGURE 10.22 The reaction cycle of bacteriorhodopsin. The intermediate states are indicated by letters, with subscripts to indicate the absorption maxima of the states. Also indicated for each state is the configuration of the retinal chromophore (all-tram or 13-cas) and the protonation state of the Schiff base (C=N or C=N H). 

Lamb, TD and Pugh, EN, 2004. Dark adaptation and the retinoid cycle of vision. Prog Retin Eye Res 23, 307-380. 

Takahashi, Y., G. Moiseyev et al. (2005). Identification of conserved histidines and glutamic acid as key residues for isomerohydrolase activity of RPE65, an enzyme of the visual cycle in the retinal pigment epithelium. FEBSLett. 579(24) 5414-5418. 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

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).

In lipid metabolism, ds-trans isomerism is particularly important. For example, double bonds in natural fatty acids (see p.48) usually have a as configuration. By contrast, unsaturated intermediates of p oxidation have a trans configuration. This makes the breakdown of unsaturated fatty acids more complicated (see p. 166). Light-induced cis-trans isomerization of retinal is of central importance in the visual cycle (see p.358). 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

An isomerase [3] transfers all-trans -retinal to the ll-cis -form, in which it is available for the next cycle. A dehydrogenase [4] can also allow retinal to be supplied from vitamin A (retinol). 

Foster Simplistically, in entrainment the rods and cones provide transitional information at the sharp junction between a LD cycle, and the inner retinal opsins provide irradiance information. What was striking about that rodless-coneless mouse under dim LD cycles is that it was phase advanced and the onsets were bouncing all over the place. Something about the precision of entrainment is lost with the rods and cones under relatively low light conditions. 

Menaker There is old work by Kavanau in the 1960s (Kavanau 1962a,b) which has recently been repeated in hamster (Boulos et al 2002). This clearly shows that natural transitions in light/dark cycles increase the range of entrainment dramatically. The clock will entrain to T cycles much longer or shorter with natural transitions than it will with abrupt transitions. This has to mean something. It isn t clear what it means, but the complexity of the retinal input to the SCN must be functional at some level. Perhaps the function is in getting information about the rate of transition. 

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