title: Retinal β-ionone ring-salinixanthin interactions in xanthorhodopsin: a study using artificial pigments.
abstract: Xanthorhodopsin (xR) is a retinal protein that contains, in addition to the retinal chromophore, a carotenoid (salinixanthin) that functions as a light-harvesting antenna [Balashov, S. P., et al. (2005) Science 309, 2061-2064]. The center-center distance between the two polyene chains is 12-13 Å, but the distance between the two rings of retinal and salinixanthin is surprisingly small (~5 Å) with an angle of ~45° [Luecke, H., et al. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 16561-16565]. We aimed to clarify the role of the β-ionone ring in the binding of retinal to apo-xR, as well as a possible role that the β-ionone ring plays in fixation of the salinixanthin 4-keto ring. The binding of native retinal and series of synthetic retinal analogues modified in the β-ionone ring to apo-xR was monitored by absorption and circular dichroism (CD) spectroscopies. The results indicate that the β-ionone ring modification significantly affected formation of the retinal-protein covalent bond as well as the pigment absorption and CD spectra. It was observed that several retinal analogues, modified in the retinal β-ionone ring, did not bind to apo-xR and did not form the pigment. Also, none of these analogues induced the fixation of the salinixanthin 4-keto ring. In addition, we show that the native retinal within its binding site adopts exclusively the 6-s-trans ring-chain conformation.
authors: Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
authors: Elena
authors: E
authors: Smolensky Koganov
authors: Amiram
authors: A
authors: Hirshfeld
authors: Mordechai
authors: M
authors: Sheves
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002338:Carotenoids / Q000737:chemistry* / Q000378:metabolism; D002942:Circular Dichroism; D053138:Cyclohexenes / Q000737:chemistry; D006027:Glycosides / Q000737:chemistry* / Q000378:metabolism; D008968:Molecular Conformation; D045792:Norisoprenoids / Q000737:chemistry / Q000378:metabolism; D012172:Retinaldehyde / Q000737:chemistry* / Q000378:metabolism; D025604:Rhodopsins, Microbial / Q000737:chemistry* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D053138:Cyclohexenes; D006027:Glycosides; D045792:Norisoprenoids; D025604:Rhodopsins, Microbial; C470489:salinixanthin; C519152:xanthorhodopsin, Salinibacter; C052568:cyclohexene; D002338:Carotenoids; C008157:beta-ionone; D012172:Retinaldehyde