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Retinal β-ionone ring-salinixanthin interactions in xanthorhodopsin: a study using artificial pigments
Elena Smolensky Koganov1, Amiram Hirshfeld, Mordechai Sheves
1Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
Biochemistry
|January 22, 2013
Summary
The retinal β-ionone ring is crucial for xanthorhodopsin (xR) pigment formation and salinixanthin binding. Modifications to this ring prevent retinal binding and light-harvesting complex assembly.
Area of Science:
- Biochemistry
- Spectroscopy
- Protein-ligand interactions
Background:
- Xanthorhodopsin (xR) is a unique retinal protein featuring a light-harvesting carotenoid, salinixanthin.
- The close proximity of retinal and salinixanthin suggests functional interplay within the xR binding site.
Purpose of the Study:
- To elucidate the role of the retinal β-ionone ring in retinal binding to apo-xR.
- To investigate the β-ionone ring's potential role in the fixation of salinixanthin's 4-keto ring.
Main Methods:
- Absorption spectroscopy was used to monitor retinal analogue binding.
- Circular dichroism (CD) spectroscopy assessed pigment formation and spectral properties.
- Synthetic retinal analogues with modified β-ionone rings were employed.
Main Results:
- Modification of the retinal β-ionone ring significantly impacted covalent bond formation with apo-xR.
- Altered retinal analogues affected pigment absorption and CD spectra.
- Several modified retinal analogues failed to bind apo-xR or induce salinixanthin 4-keto ring fixation.
- Native retinal adopts a specific 6-s-trans ring-chain conformation in the binding site.
Conclusions:
- The retinal β-ionone ring is essential for successful retinal binding to apo-xR.
- The β-ionone ring's integrity is critical for xanthorhodopsin pigment formation and light-harvesting complex stability.
- Native retinal exhibits a defined conformation within the xR binding pocket.
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