Pigment configuration in the light-harvesting protein of the xanthophyte alga Xanthonema debile
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The soil chromophyte alga Xanthonema (X.) debile contains only non-carbonyl carotenoids and Chl-a. X. debile has an antenna system denoted Xanthophyte light-harvesting complex (XLH) that contains the carotenoids diadinoxanthin, heteroxanthin, and vaucheriaxanthin. The XLH pigment stoichiometry was calculated by chromatographic techniques and the pigment-binding structure studied by resonance Raman spectroscopy. The pigment ratio obtained by HPLC was found to be close to 8:1:2:1 Chl-a:heteroxanthin:diadinoxanthin:vaucheriaxanthin. The resonance Raman spectra suggest the presence of 8–10 Chl-a, all of which are 5-coordinated to the central Mg, with 1–3 Chl-a possessing a macrocycle distorted from the relaxed conformation. The three populations of carotenoids are in the all-trans configuration. Vaucheriaxanthin absorbs around 500–530 nm, diadinoxanthin at 494 nm and heteroxanthin at 487 nm at 4.5 K. The effective conjugation length of heteroxanthin and diadinoxanthin has been determined as 9.4 in both cases; the environment polarizability of the heteroxanthin and diadinoxanthin binding pockets is 0.270 and 0.305, respectively.
KeywordsLight-harvesting complex Algae Resonance Raman Chl-a Carotenoids Diadinoxanthin Heteroxanthin
This work was supported by the ERC funding agency (PHOTPROT project), the French Infrastructure for Integrated Structural Biology (FRISBI) ANR-10-INBS-05, and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant agreement No 675006 (SE2B). The research in the Czech Republic was supported by the Czech Science Foundation Grant P501/12/G055, European Regional Development Fund (No. CZ.02.1.01/0.0/0.0/15_003/0000441), and by institutional funding RVO:60077344.
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