Green sulfur bacteria reaction-center complexes

The Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

In contrast, the reaction centers of green sulfur bacteria resemble PSI of chloroplasts. Their reaction centers also receive electrons from a reduced quinone via a cytochrome be complex.245 However, the reduced form of the reaction center bacteriochlorophyll donates electrons to iron-sulfur proteins as in PSI (Fig. 23-17). The latter can reduce a quinone to provide cyclic photophosphorylation. Cyanobacteria have a photosynthetic apparatus very similar to that of green algae and higher plants. 

The Fenna-Matthews-Olson (FMO) protein is an unusual, water-soluble chlorophyll protein found only in green sulfur bacteria. [18] It is believed to be located between the chlorosome and the cytoplasmatic membrane and functions as an excitation transfer link between the chlorosome and the reaction center. Each subunit contains 7 BChl a molecules embedded in a primarily /3 sheet structured protein. The protein has a trimeric quaternary structure, with a three-fold axis of symmetry in the center of the complex. [55] The green nonsulfur bacteria do not contain the FMO protein. In these organisms the chlorosome transfers energy directly to the integral membrane core antenna B808-865, and then to the reaction center. 

Schmidt, K.A., Neerken, S., Permentier, H.P., Hager-Braun, C., and Amesz, J. (2000) Electron transfer in reaction center core complex from green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum, Biochemistry 39, 7212-7220. 

Another antenna complex where high-resolution structural information is available is the bacteriochlorophyll a binding protein (also known as the Fenna-Matthews-Olson or FMO protein) from green sulfur bacteria. This complex serves as the bridge between the peripheral chlorosome complex and the membrane-bound reaction center complexes. In this... 

A. Preparation and Properties of Reaction-Center Complexes of Green Sulfur Bacteria.160... 

Attempts to prepare pure reaction-center complexes from green sulfur bacteria have proven quite difficult in the past, and usually resulted in severely diminishing the photochemical activity of the preparation. However, the new preparative procedure of Francke et al has yielded RC-complexes that are photochemically active and reasonably pure. They resemble the core complex of photosystem 1, but only have 16 BChls a and 4 molecules of the Chl-a isomer, as subsequently determined by Griesbeck, Hager-Braun, Rogl and Hauska ° for the P840-reaction center from Chlorobium tepidum. 

A similar conclusion was reached by Feiler, Albouy, Pourcet, Mattioli, Lutz and Roberts from reso-nance-Raman studies of active reaction-center complexes of Chlorobium. They further showed a similarity in the binding of this Chl-a isomer in green-bacterial reaction centers to that of the Chl-type acceptor molecules in other bacteria and green-plant reaction centers. Finally, one may note that the identification of BChl 663 as an isomer of Chi a has placed more emphasis on the similarities between the reaction centers of green sulfur bacteria and the green-plant photosystem I, where the primary electron acceptor is Chi a, and the recently discovered heliobacteria, where the primary electron acceptor is an 8 -hydroxy-Chl a. 

Figure 3. A comparison of energy diagrams for a photosynthesizing ZnS nanoparticle (left panel, the picture is taken from the accompanying article [97] and is based on references [98,103,122]) and a bacterial photochemical reaction center (right panel, a primitive, sulfide-oxidizing reaction center complex of green sulfur bacteria [276,277] is shown schematically as an example).

FIGURE 1 Schematic models of chlorosomes from the green gliding bacteria, e.g. Chloroflexus aurantiacus (A) and the green sulfur bacteria, e.g. Chlorobium vibrioforme (B). The numbers refer to wavelength maxima of antenna bacteriochlorophyll complexes. The reaction centers are of the "quinone type in A, with a QA Qb acceptor complex and the Fe-S type in B, with Fe-S proteins as early electron acceptors. The rod elements probably contain oligomers of bacteriochlorophyll c, d or e, as discussed in Ae text. 

Steady-state and time-resolved spectroscopy has been used to follow the flow of excitations from the BChl c,dor e in the interior of the chlorosome, through the baseplate and into the membrane, where the energy is trapped by the reaction center (7-11). The results are consistent with a sequential energy transfer pathway, from the BChl c/d ore in the chlorosome interior to the BChl a 795 pigment in the chlorosome baseplate, to membrane-bound BChl a antenna complexes and finally to the reaction center (Fig. 1). The green sulfur bacteria (Fig. IB) also contain an additional antenna species, the trimeric water-soluble BChl a protein the three dimensional structure of which was determined by Fenna and Matthews (12). 

The structure of the membrane of green sulfur bacteria appears to be fundamentally different. Most of the BChl a is contained in a water soluble BChl a protein which is attached to, rather than imbedded in the membrane (13). About one-fourth of the BChl a, however, forms part of a core complex (Fig. 3), which contains approximately 20 BCls a per reaction center (14). The complex also contains about 15 molecules of BChl c or a closely related pigment (15). Flash spectroscopic evidence indicates that one of these molecules acts as electron acceptor in the primary charge separation (16,17). The peptide composition of the core complex may suggest a structural relationship to the core of photosystem I of plants (18). 

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.

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