Green sulfur bacteria quinone-type” reaction center

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

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. 

Bacteria have a single reaction center in purple bacteria, it is of the pheophytin-quinone type, and in green sulfur bacteria, the Fe-S type. 

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.39 Green sulfur bacteria RCs are of the type I or the Fe-S-type (photosystem I). Here the electron acceptor is not the quinine instead, chlorophyll molecules (BChl 663, 81 -OII-Chi a, or Chi a) serve as primary electron acceptors, and three Fe4S4 centers (ferredoxins) serve as secondary acceptors. A quinone molecule may or may not serve as an intermediate carrier between the primary electron acceptor (Chi) and the secondary acceptor (Fe-S centers).40 The process sequence leading to the energy conversion in RCI is shown in Figure 21. 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

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. 

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