Photosynthetic bacteria reaction-center associated

The second kind of reaction center, as represented by that of Chromatium vinosum or Rhodopseudo-monas viridis, has a tightly bound c-type cytochrome [see Fig. 2, right]. This so-called reaction center-associated cytochrome is a tetraheme of molecular mass of 40 kDa and structurally quite different from the other known, c-type cytochromes. One of the hemes in this RC-associated, c-type cytochrome also serves as the immediate electron donor to the photooxidized primary donor of the photosynthetic bacteria (either P870 in C. vinosum or P960 in Rp. viridis). The oxidized cytochrome in the tetraheme is in turn reduced by the soluble cytochrome C2. The RC-associated cytochromes are not easily dissociated from the RC, even at high ionic strength. 

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

The nonheme iron enzymes discussed so far in this section either utilize oxygen as a substrate or form it as a product. Other nonheme iron sites that do not bind O2 as part of their catalytic function have similar ligand environments. An example of such a system is the QFe site associated with the reaction centers of photosynthetic bacteria and with photosystem II of chloroplasts (Feher et al., 1989). 

We now summarize in Fig. 11 the reaction-center structure and the known electron-transport reactions in purple bacteria. A simplified representation of the reaction-center and the light-harvesting complexes contained in the bacterial membrane is shown in Fig. 11 (A), followed by a column model and a cofactor model in Fig. 11 (B). The cofactor model is used to illustrate the various electron-transport steps with the associated rate constants in Fig. 11 (C), where the cofactors in the starting state (oxidized or reduced) are shown in solid black. When a cofactor first becomes reduced or oxidized, it is shown as an open symbol. We will also use this cofactor model and reaction sequence as a framework for introducing the remaining chapters throughout the section on photosynthetic bacteria. 

Fig. 6. Photochemical cycles showing coupling of electron transfer to proton transfer, cytochrome oxidation and quinone exchange in (A) native reaction centers where two Cyt c are oxidized in the cycie, (B) reaction centers where uptake of the first proton is inhibited, and (C) reaction centers where uptake ofthe second proton is inhibited (shading indicates the quinone pool). Figure source (A) Paddock, Rongey, McPherson, Juth, Feher and Okamura (1994) Pathway of proton transfer in bacterial reaction centers role of aspartate-L21Z in proton transfers associated with reduction of quinone to dihydroquinone. Biochemistry 33 734 (B) Okamura and Feher (1992) Proton transfer in reaction centers from photosynthetic bacteria. Annu Rev Biochemistry. 61 868 (C) Feher, Paddock, Rongey and Okamura (1992) Proton transfer pathways in photosynthetic reaction centers studied by site-directed mutagenesis. In A Pullman, J Jortner and B Pullman (eds) Membrane Proteins Structures, Interactions and Models, p 485. Kluwer.

Fig. 7. A frame of reference for the Rp. viridis RC-associated cytochrome complex (A) and a more detailed view of the cytochrome subunit with the four hemes shown (B). See text for the various nomenclatures used. P represents the [BChllj (the primary donor). The table also includes the redox-potential values of the hemes, and the wavelength of the a-band of the hemes both at room and cryogenic temperatures. Figure (A) the same as Fig. 7 in Chapter 2. (B) is taken from CRD Lancaster, U Ermler and H Michel (1995) The structure of photosynthetic reaction centers from purple bacteria as revealed by X-ray crystallography. In RE Blankenship, MT Madigan and CE Bauer (eds) Anoxygenic Photosysnthetic Bacteria, p 511. Kluwer.

Light reactions taking place in photosynthetic reaction center depend on a supply of light energy harvested by a number of antenna chlorophyll-protein complexes. The arrangement of these complexes in photosystem II is similar to that in the photosynthetic purple bacteria discussed earlier in Chapter 3. Basically, there is a core antenna complex closely associated with the reaction center, plus some slightly more distant, peripheral antenna complexes. 

The photophysical and electron transfer properties of bacteriochlorophylls (Bchl) and bacteriopheophytins (Bpheo) found in the reaction centers of photosynthetic bacteria have been directly associated with the mechanism of charge separation which underlies photosynthesis [1]. The appearance of the Bpheo anion (Bpheo ) within 3-5 ps after excitation of the special pair of Bchl (P) is well documented from transient absorption spectroscopy [2-4]. The 200 ps lifetime of Bpheo which is primarily determined by the electron transfer process to a quinone also has been established by picosecond changes in absorption [5,6], Thus, the general kinetic time scale for the primary processes in bacterial photosynthesis has been determined by the transient differences in electronic state properties. 

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