Chromophores of rhodopsin and bacteriorhodopsin

UC Chemical Shift-Conformation Relationship in the Chromophores of Rhodopsin and Bacteriorhodopsin... 

The chromophores of rhodopsin and bacteriorhodopsin are 11 -cis- and all-trans-retinal Schiff bases, respectively. Upon binding to the proteins, their unsaturated carbons show anomalous 13C chemical shifts compared with those of corresponding model compounds. This indicates the occurrence of interactions between the chromophore and its surrounding protein matrix. Ab inito shielding calculation reveals that the major part of such anomalous shifts originates in the conformational change of the chromophore. 

Raman spectroscopy has been particularly useful in studies of rhodopsin and bacteriorhodopsin. As discussed in Chap. 4, excitatirai of rhodopsin or bacteriorho-dopsin by light causes isomerization of the retinyl chromophore. In rhodopsin, the chromophore changes from W-cis to all-trans in bacteriorhodopsin, it goes from dA -trans to 13-cA. Resonance Raman measurements showed that the isomerization is essentially complete in metastable intermediate states that form within a few ps [32-38]. The conformations of these states were ascertained by comparisons of the resonance Raman spectra with those of model compounds. 

Infrared spectroscopy (IR) has not been extensively used in retinoid analysis (17,18). However, the newer techniques of Resonance Raman and infrared-difference spectroscopy have been applied to retinal proteins m rhodopsin and bacteriorhodopsin. By use of these techniques, it is possible to determine the structures of the chromophores m the visual pigments and in the intermediates of their photoreactions Also, it is possible to study the interactions between the chromophores and the protein, and the structural changes evoked in the protein by the photoreaction (1,19). 

Photoreceptor Pigments. There have been several reviews on the structures, photochemistry, and functioning of the retinal-protein photoreceptor pigments involved in the processes of visionand in the purple membrane of Halobacteria (bacteriorhodopsin). ° ° In addition to the papers quoted earlier on the spectroscopy of these pigments, many other reports have appeareddealing with rhodopsin and intermediates in its photocycle, especially photochemistry, chromophore-protein conformation and binding, and reaction kinetics. Similar studies on bacteriorhodopsin have also been described." "-"" ... 

Furthermore, many carotenoid metabolites exist that have distinct functions. One such example shown in Fig. 3 is retinal, the chromophore of visual pigments (rhodopsins) and the light-driven proton pump, bacteriorhodopsin. Other examples are the plant hormone, abscisic acid, or volatile compounds that contribute to the fragrance of roses, for example. 

Figure 4.3 shows photochemical reactions in visual (Fig. 4.3A) and archaeal (Fig. 4.3B) rhodopsins. In visual rhodopsins, the 11-as-retinal is isomerized into the a -trans form. The selectivity is 100%, and the quantum yield is 0.67 for bovine rhodopsin [20]. In archaeal rhodopsins, the all-trans-retinal is isomerized into the 13-cis form. The selectivity is 100%, and the quantum yield is 0.64 for bacteriorhodopsin [21]. Squid and octopus possess a photoisomerase called retino-chrome, which supplies the 11-ris-retinal for their rhodopsins through the specific photoreaction. Retinochrome possesses all-trans-retinal as the chromophore, and the all-trans-retinal is isomerized into the 11-cis form with a selectivity of 100% [22]. Thus, the photoproduct is different between archaeal rhodopsins and retinochrome, the aU-trans form being converted into the 13-cis and 11-cis forms, respectively. This fact implies that protein environment determines the reaction pathways of photoisomerization in their excited states. 

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