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  • Retinal β-ionone ring-salinixanthin interactions in xanthorhodopsin: a study using artificial pigments.

Retinal β-ionone ring-salinixanthin interactions in xanthorhodopsin: a study using artificial pigments.

Biochemistry (2013-01-22)
Elena Smolensky Koganov, Amiram Hirshfeld, Mordechai Sheves
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.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
β-Ionone, natural, ≥95%, FG
Sigma-Aldrich
β-Ionone, 96%
Sigma-Aldrich
β-Ionone, predominantly trans, ≥97%, FCC, FG
Supelco
Cyclohexene, analytical standard, ≥99.5% (GC)
Sigma-Aldrich
Cyclohexene, contains 100 ppm BHT as inhibitor, ≥99.0%
Sigma-Aldrich
Cyclohexene, inhibitor-free, ReagentPlus®, 99%
Sigma-Aldrich
all trans-Retinal, powder, ≥98%