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Key Role of Proline L209 in Connecting the Distant Quinone Pockets in the Reaction Center of Rhodobacter sphaeroides
J. Tandori, P. Maroti, E. Alexov, P. Sebban and L. Baciou
Proceedings of the National Academy of Sciences of the United States of America
Vol. 99, No. 10 (May 14, 2002), pp. 6702-6706
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/3058729
Page Count: 5
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Photosynthetic bacterial reaction centers convert light excitation into chemical free energy. The initial electron transfer leads to the consecutive semireductions of the primary (QA) and secondary (QB) quinone acceptors. The QA - and QB - formations induce proton uptake from the bulk. Their magnitudes (H+/QA - and H+/QB -, respectively) probe the electrostatic interactions within the complex. The pH dependence of H+/QA - and H+/QB - were studied in five single mutants modified at the L209 site (L209P→F,Y,W,E,T). This residue is situated at the border of a continuous chain of water molecules connecting QB to the bulk. In the wild type (WT), a proton uptake band is present at high pH in the H+/QA - and H+/QB - curves and is commonly attributed to a cluster of acidic groups situated nearby QB. In the H+/QA - curves of the L209 variants, this band is systematically absent but remains in the H+/QB - curves. Moreover, notable increase of H+/QB - is observed in the L209 mutants at neutral pH as compared with the WT. The large effects observed in all L209 mutants are not associated with significant structural changes (Kuglstatter, A., Ermler, U., Michel, H., Baciou, L. & Fritzsch, G. Biochemistry (2001) 40, 4253-4260). Our data suggest that, in the L209 mutants, the QB cluster does not respond to the QA - formation as observed in the WT. We propose that, in the mutants, removal of the rigid proline L209 breaks a necessary hydrogen bonding connection between the quinone sites. These findings suggest an important role for structural rigidity in ensuring a functional interaction between quinone binding sites.
Proceedings of the National Academy of Sciences of the United States of America © 2002 National Academy of Sciences