Menaquinone as pool quinone in a purple bacterium

Purple bacteria have thus far been considered to operate light-driven cyclic electron transfer chains containing ubiquinone (UQ) as liposoluble electron and proton carrier. We show that in the purple γ-proteobacterium Halorhodospira halophila, menaquinone-8 (MK-8) is the dominant quinone component a...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2009-05, Vol.106 (21), p.8549-8554
Main Authors: Schoepp-Cothenet, Barbara, Lieutaud, Clément, Baymann, Frauke, Verméglio, André, Friedrich, Thorsten, Kramer, David M, Nitschke, Wolfgang
Format: Article
Language:English
Subjects:
Amino Acid Sequence
Bacteria
Bacteriology
Biological Sciences
Cognitive science
Cytochrome P-450 Enzyme System - metabolism
Cytochromes
Electrons
Environmental Sciences
Enzymes
Evolution
Genome, Bacterial - genetics
Genomics
Halophila
Humanities and Social Sciences
Life Sciences
Molecular Sequence Data
Phosphorus - metabolism
Photosynthesis
Photosynthetic Reaction Center Complex Proteins - metabolism
Pigments
Protein Binding
Proteins
Proteobacteria
Proteobacteria - genetics
Proteobacteria - metabolism
Proteobacteria - radiation effects
Protons
Quinones
Quinones - metabolism
Sequence Alignment
Titration
Vitamin K - metabolism
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Summary:Purple bacteria have thus far been considered to operate light-driven cyclic electron transfer chains containing ubiquinone (UQ) as liposoluble electron and proton carrier. We show that in the purple γ-proteobacterium Halorhodospira halophila, menaquinone-8 (MK-8) is the dominant quinone component and that it operates in the QB-site of the photosynthetic reaction center (RC). The redox potentials of the photooxidized pigment in the RC and of the Rieske center of the bc₁ complex are significantly lower (Em = +270 mV and +110 mV, respectively) than those determined in other purple bacteria but resemble those determined for species containing MK as pool quinone. These results demonstrate that the photosynthetic cycle in H. halophila is based on MK and not on UQ. This finding together with the unusual organization of genes coding for the bc₁ complex in H. halophila suggests a specific scenario for the evolutionary transition of bioenergetic chains from the low-potential menaquinones to higher-potential UQ in the proteobacterial phylum, most probably induced by rising levels of dioxygen 2.5 billion years ago. This transition appears to necessarily proceed through bioenergetic ambivalence of the respective organisms, that is, to work both on MK- and on UQ-pools. The establishment of the corresponding low- and high-potential chains was accompanied by duplication and redox optimization of the bc₁ complex or at least of its crucial subunit oxidizing quinols from the pool, the Rieske protein. Evolutionary driving forces rationalizing the empirically observed redox tuning of the chain to the quinone pool are discussed.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.0813173106