Stoichiometrically Bound β-Carotene in the CytochromebfComplex of Oxygenic Photosynthesis Protects against Oxygen Damage

Abstract

The cytochrome b 6 f complex of oxygenic photosynthesis carries out "dark reactions" of electron transfer that link the light-driven reactions of the reaction centers, and coupled proton transfer that generates part of the electrochemical potential utilized for ATP synthesis. In contrast to the bc 1 complex of the respiratory chain, with which there are many structural and functional homologies, the b 6 f complex contains bound pigment molecules. Along with the specifically bound chlorophyll a previously found to be bound stoichiometrically in the dimeric b 6 f complex, it was found in the present study that ␤-carotene is also present in the b 6 f complex at stoichiometric levels or nearly so. Chlorophyll and carotenoid pigments were quantitatively extracted from b 6 f complex purified from (i) the thermophilic cyanobacterium, Mastigocladus laminosus, (ii) spinach chloroplasts, and (iii) the green alga, Chlamydomonas reinhardtii. Visible and mass spectra showed the carotenoid to be a ␤-carotene of molecular weight ‫؍‬ 536, with a stoichiometry of 1.0:1 relative to cytochrome f in the highly active M. laminosus complex but somewhat lower stoichiometries, 0.77 and 0.55, in the b 6 f complex obtained from spinach chloroplasts and C. reinhardtii. A photoprotective function for the ␤-carotene was inferred from the findings that the rate of photobleaching of the chlorophyll a bound in the complex was found to vary inversely with ␤-carotene content and to decrease markedly in the presence of ambient N 2 instead of air. The presence of ␤-carotene in the b 6 f complex, and not in the related bc 1 complexes of the mitochondrial respiratory chain and photosynthetic bacteria, suggests that an additional function is to protect the protein complexes in oxygenic photosynthetic membranes against toxic effects of intramembrane singlet O 2 .

A defined protein–detergent–lipid complex for crystallization of integral membrane proteins: The cytochrome b 6 f complex of oxygenic photosynthesis

“…Crystals of the cyt b 6 f complex were dissolved in a buffer containing 30 mM Tris⅐HCl (pH 7.5), 50 mM NaCl, and 0.05% undecyl-maltoside. Visible spectra of the cytochromes were measured on Cary 3 UV-visible spectrophotometer (25). Ferricyanide, ascorbate, and dithionite were used as oxidant and reductant, respectively.…”

Structure and Function of CytochromebcComplexes

“…Attempts to remove these "contaminants" led to the conclusion that there is one mole each of specifically bound chlorophyll a and carotenoid (63,186,187) associated with the complex. The carotenoid has been suggested to protect against free radicals formed at the Q o site (188).…”

The Biosynthetic Pathway for Synechoxanthin, an Aromatic Carotenoid Synthesized by the Euryhaline, Unicellular Cyanobacterium Synechococcus sp. Strain PCC 7002

“…␤-Carotene is the primary carotenoid associated with the photosystems in most cyanobacteria (3,17), and the cyanobacterial cytochrome b 6 f complex also contains one carotenoid molecule (4,47,50). Carotenoids are also found in the water-soluble orange carotenoid protein (20), which binds the xanthophyll 3Ј-hydroxy-echinenone in Synechocystis sp.…”