Photosynthetic bacteria iron-sulfur type

The RCs in photosynthetic organisms have been classified into two groups, i.e., quinone-type and iron sulfur-type. The quinone-type RCs are present in purple non-sulfur bacteria as well as in the PS II of cyanobacteria and chloroplasts (of algae and higher plants), whereas the iron sulfur-type RCs are present in green and purple sulfur bacteria as well as in the PS I of cyanobacteria and chloroplasts (Blankenship, 1992 HauskaetaL, 1995). 

Fig. 17.15-C(s-carotenoids identified so far in the RCs of various photosynthetic organisms (a) neurosporene, (b) spheroidene and (c) spirilloxanthin in the quinone-type RC of puiple non-sulfur bacteria, Rb. sphaeroides QIC, 2.4.1 and Rs. rubrum SI (d) y-carotene and (e) chlorobactene in the iron sulfiir-type RC of green sulfur bacterium, Cb. tepidum-, and (f) Carotene in the quinone-type PS II RC of spinach chioropiasts as well as in the iron sulfur-type PS I RC of a cyanobacterium. Sc. vulcanus, and spinach chioropiasts.

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... 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

The occurrence of SOD in photosynthetic and nonphotosynthetic and anaerobic and aerobic organisms can again provide a useful criterion for evolution studies. Figure 12 shows a number of hypotheses which can be postulated for such an evolution (24, 27). The second scheme has been favored (24). As with the iron-sulfur proteins, there seems to be an important transition between the anaerobic and the aerobic phase of life it is possible that the red non-sulfur bacteria provide key transition-type organisms, probably in parallel, with the sulfate and nitrate... 

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. 

Many cytochromes in energy-transducing organelles are membrane-bound proteins. Present in mitochondria and in purple photosynthetic bacteria, the cyt bc complex (also called ubiquinol-cytochrome c oxidoreductase, or complex III) catalyzes the electron transfer from ubiquinol to ferricyt c and pumps protons from the matrix to the cytosol.The catalytic core of the cyt bci complex comprises three redox-active subunits they are a cyt b with two ft-type hemes bn and bf), a cyt Cl, and a Reiske iron sulfur protein. While this catalytic core has enzymatic activity in some a proteobacteria like Paracoccus, Rhodospirillum rubrum, and Rb. capsulatus, mitochondrial cyt bci complexes have an additional seven or eight subunits." ... 

High potential iron-sulfur protein (HiPIP) is a special type of Fd which has been isolated from some photosynthetic bacteria and detected by ESR spectroscopy in other bacteria. HiPIP also contains a single 4Fe-4S cluster, but it differs from the other Fd in having a positive standard potential of about-h 350 mV (most Fd have standard potentials in the range of the hydrogen electrode, about -420 mV). Furthermore, the HiPIP from Chromatium is paramagnetic in the oxidized state. 

Jagannathan B, Golbeck JH Unifying principles in homodimeric type I photosynthetic reaction centers Properties of PscB and the F-A, F-B and F-X iron-sulfin clusters in green sulfur bacteria. Biochim Biophys Acta 2008, 1777(12) 1535-1544. 

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