Bacteria reaction centres

We have seen that, in photosynthetic bacteria, visible light is harvested by the antenna complexes, from which the collected energy is funnelled into the special pair in the reaction centre. A series of electron-transfer steps occurs, producing a charge-separated state across the photosynthetic membrane with a quantum efficiency approaching 100%. The nano-sized structure of this solar energy-conversion system has led researchers over the past two decades to try to imitate the effects that occur in nature. 

Figure 2 Molecular structures and IUPAC numbering scheme of organic cofactors occurring in photosynthetic reaction centres (bRC, PS I, PS II). (Bac-teriolpheophytin is the free base of (bacterio)chlorophyll plastoquinone (PQ) is found in PS If phylloquinone or vitamin K, ( VK,) in PS I many bacteria contain ubiquinone (UQ). Shown is also the amino acid tyrosine (Tyr, Y) that is redox active in PS II.

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

The photosynthetic apparatus is found in and on membrane structures, which, in plant cells and algae, are located in chloroplasts and are called thylakoids. In bacteria the photosynthetic membrane is derived by complex invagination of the cytoplasmic membrane. The photosynthetic apparatus is made up of antennae, which contain light-harvesting pigment molecules (usually chlorophylls or bacteriochlorophylls) and photochemical reaction centres, which also contain pigments, together with the necessary enzymes and coenzymes. 

The reaction centres from the photosynthetic purple bacteria are amongst the best characterized. Each reaction centre contains four bacteriochlorophyll molecules, two bacteriophenophytin... 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

The reaction centre (RC) of purple bacteria seems to be a suitable object for elucidating the mechanisms providing extremely high efficiency of electron transfer in photosynthetic systems. 

Kinetics of Electron Transfer in the Reaction Centre Proteins from Photosynthetic Bacteria... 

M. E. Michel-Beyerle, ed, The Reaction Centre of Photosynthetic Bacteria (Springer-Verlag, Berlin, 1996). 

The reaction centre found in many purple non-sulphur bacteria is a simple example of a group of proteins that are natureis solar batteries. The reaction centre uses the energy of sunlight to generate positive and negative charges on opposite sides of the bacterial cytoplasmic membrane. This potential difference drives a circuit of electron transfer reactions that are linked to proton translocation across this membrane. 

The larger part of research on light-driven eleetron transfer in purple photosynthetic bacteria has involved three speeies, Rhodopseudomonas (Rps.) viridis, Rhodobacter (Rb.) sphaeroides and Rb. capsulatus. The bulk of this article is written in reference to the Rb. sphaeroides reaetion eentre, the subject of the majority of spectroscopic and mutagenesis work earried out to date. However, much of the research described below has involved the reaction centre from Rb. capsulatus or Rps. viridis, or reaetion eentres from other species of purple bacteria. 

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