Chapter 18: Anoxygenic Bacteria

Anoxygenic photosynthetic bacteria transform light energy into chemical free energy according to a light-induced cyclic electron transfer. This fast cyclic electron transfer is coupled to the translocation of protons and to the formation of an electrochemical potential across the inner membrane, ultimately used for ATP synthesis. This chapter presents a comprehensive overview of our present knowledge of this process with special emphasis on non-sulfur photosynthetic bacteria. Thanks to a multidisciplinary approach combining biophysics, biochemistry and molecular biology, a nearly complete picture of the thermodynamics and kinetic properties, structures and interactions of the different protein complexes involved in this process is now available. Three multimeric transmembrane protein complexes compose the photosynthetic apparatus: the light-harvesting complexes, the photochemical reaction center and the cytochrome bc₁ complex. These two last complexes are connected via electron carrier proteins in the periplasmic space and quinone molecules in the membrane. One important peculiarity of species of photosynthetic bacteria is the diversity of the biochemical nature of the immediate electron donor to the reaction center and of the shuttling electron carrier with the cytochrome bc₁ complex. The secondary electron donor to the reaction center could be either a tetra-, tri-, or mono-heme cytochrome c tightly bound to the reaction center or a soluble periplasmic monoheme cytochrome c. Different soluble electron carriers (cyt c₂, cyt c₈, HiPIP) connect the reaction center and the cytochrome bc₁ complex, depending upon the considered species. The rates of electron transfer between these different partners have been determined by flash spectroscopy. Site-directed mutagenesis experiments, based on the 3D structure of the various components of the photosynthetic apparatus, have underlined the importance of both nonpolar and electrostatic interactions in the docking process of these different partners. In some cases, a higher level of interaction between the complexes of the photosynthetic apparatus has been determined by electron and atomic force microscopy techniques, highlighting their supramolecular organization in native membranes.