Stoichiometrically bound beta-carotene in the cytochrome b(6)f complex of oxygenic photosynthesis protects against oxygen damage

The cytochrome b(6)f complex of oxygenic photosynthesis carries out "dark r eactions" of electron transfer that link are light-driven reactions of the reaction centers, and coupled proton transfer that generates part of the el ectrochemical potential utilized for ATP synthesis. In contrast to the bc(1 ) complex of the respiratory chain, with which there are many structural an d functional homologies, the b(6)f complex contains bound pigment molecules . Along with the specifically bound chlorophyll a previously found to be bo und stoichiometrically in the dimeric b(6)f complex, it was found in the pr esent study that beta-carotene is also present in the b(6)f complex at stoi chiometric levels or nearly so, Chlorophyll and carotenoid pigments were qu antitatively extracted from b(6)f complex purified from (i) the thermophili c cyanobacterium, Mastigocladus laminosus, (ii) spinach chloroplasts, and ( iii) the green alga, Chlamydomonas reinhardtii, Visible and mass spectra sh owed the carotenoid to be a beta-carotene of molecular weight = 536, with a stoichiometry of 1.0:1 relative to cytochrome f in the highly active M. la minosus complex but somewhat lower stoichiometries, 0.77 and 0.55, in the b (6)f complex obtained from spinach chloroplasts and C, reinhardtii, A photo protective function for the beta-carotene was inferred from the findings th at the rate of photobleaching of tho chlorophyll a bound in the complex was found to vary inversely with beta-carotene content and to decrease markedl y in the presence of ambient N-2 instead of air. The presence of beta-carot ene in the b(6)f complex, and not in the related be, complexes of the mitoc hondrial respiratory chain and photosynthetic bacteria, suggests that an ad ditional function is to protect the protein complexes in oxygenic photosynt hetic membranes against toxic effects of intramembrane singlet O-2.