Photochemical reaction center, bacterial

The photooxidation of chlorophyll indicated in Eq. 23-31 is accompanied by bleaching in the principal light absorption band. However, since there is so much light-gathering chlorophyll for each reaction center, the effect is small. The study of the process has been aided greatly by preparation of isolated bacterial photochemical reaction centers. 

MG Rockley, MW Windsor, RJ Cogdell and WW Parson (1975) Picosecond detection of an Intermediate In the photochemical reaction of bacterial photosynthesis. Proc Nat Acad Sci, USA 72 2251-2255 J Fajer, DC Brune, MS Davis, A Forman and LD Spaulding (1975) Primary charge separation In bacterial photosynthesis Oxidized chlorophylls and reduced pheophytin. Proc Nat Acad Sci, USA 72 4956 960 PL Dutton, KJ Kaufmann, B Chance and PM Rentzepis (1975) Picosecond kinetics of the 1250 nm band of the Rps. sphaeroldes reaction center. The nature of the primary photochemical Intermediary state. FEES Lett 60 275-280... 

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).

Oxygenic photosynthesis takes place in two photochemical reaction centers Photosystem I (PSI) and Photosystem II (PSII). While PSn is believed to be closely related to the reaction center of purple bacteria (1), PSI spears to be unique to oxygenic photosynthetic organisms. It is clear that this photosynthetic complex has no structural or functional similarities to the bacterial reaction center, nevertheless it plays a major role in the oxygenic photosynthetic process. Its function in the reducing site of the electron transfer chain enables the reduction of ferredoxin, and eventually the reduction of one of the energy components formed in the process, NADPH. 

Bacterial photosynthesis. What is the relationship of the Z scheme of Fig. 23-17 to bacterial photosyntheses In photoheterotrophs, such as the purple Rhodospirillum, organic compounds, e.g., succinate, serve as electron donors in Eq. 23-30. Because they can utilize organic compounds for growth, these bacteria have a relatively low requirement for NADPH or other photochemically generated reductants and a larger need for ATP. Their photosynthetic reaction centers receive electrons via cytochrome c from succinate (E° ... 

Woodbury, N. W., M. Becker, D. Middendorf, and W. W. Parson, Picosecond kinetics of the initial photochemical electron transfer reaction in bacterial photosynthetic reaction centers. Biochem. 24 7516, 1985. Fast spectrophotometric techniques are used to follow the initial steps in reaction centers purified from photosynthetic bacteria. 

This review highlights recent studies of synthetic, covalently linked multicomponent molecular devices which mimic aspects of photosynthetic electron transfer. After an introduction to the topic, some of the salient features of natural bacterial photosynthetic reaction centers are described. Elementary electron transfer theory is briefly discussed in order to provide a framework for the discussion which follows. Early work with covalently linked photosynthetic models is then mentioned, with references to recent reviews. The bulk of the discussion concerns current progress with various triad (three-part) molecules. Finally, some even more complex multicomponent molecules are examined. The discussion will endeavor to point out aspects of photoinitiated electron transfer which are unique to the multicomponent species, and some of the considerations important to the design, synthesis and photochemical study of such molecules. 

Warshel, A., Chu, Z. T., and Parson, W. W., 1994, On the energetics of the primary electron-transfer process in bacterial reaction centers. J. Photochem. Photobiol., 82 123nl28. 

BPh are BChl b and BPh b in most of the other species that have been characterized, they are BChl a and BPh a. (BChl b differs from BChl a in having a vinyl group on ring II in place of an ethyl group. Thiocapsa pfennigii, another bacterial species that contains BChl b, resembles Rp.viridis in its photochemical activities [119,171].) Reaction centers isolated from Cf. aurantiacus are unusual in having three molecules of BChl a and three of BPh a, instead of four BChls and two BPhs [46,93]. 

RK Clayton, WR Sistrom and WS Zaugg (1965) The role of reaction centers in photochemical activities of bacterial chromatophores. Biochim Biophys Acta 102 341-348... 

The primary photochemical charge-separation process, i.e., P870-t-A -> P870 +A in purple photosynthetic bacteria requiresthat there is a reaction partner to accept the electron released by the primary donor. Again, using D-[P-A] to represent the core composition of the bacterial reaction center, we can write the following sequence of events ... 

The availability of a photochemically active bacterial reaction-center complex has proved to be extremely useful for extraction and reconstitution experiments, the results of which can provide more conclusive evidence as to the identity of the stable primary electron acceptor in Rb. sphaeroides. 

F/g. 5. Relationship between photochemical activity (measured by AA due to P870 photooxidation and by production of EPR signai at g=2.0026) and the number of ubiquinone molecuies per reaction center in Rb. sphaeroides R-26. See text for other details. Figure modified from Okamura, Isaacson and Feher (1975) Primary acceptor in bacterial photosynthesis Obligatory role of ubiquinone in photoactive reaction centers of Rhodopseudomonas sphaeroides. Proo Nat Acad Sci, USA 72 3494,... 

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.

At this point, a brief mention will be made regarding the redox potential of the [Brb/BO"] couple. Being an early electron acceptor in bacterial reaction centers, its redox potential is an important property for the understanding of the energetics ofthe photochemical reactions. Klimov, Shuvalov, Krakhmaleva, Klevanik and Krasnovsky determined the redox potential of the [BO/BO"] couple by redox potentiometry. The approach taken was to gradually decrease in a straightforward manner the ambient... 

Fig. 3. Top row formulation of the reaction sequence involved In the photochemical charge separation of the photosynthetic bacterial reaction center. (D is the excitation and charge separation. the electron transfer to the (secondary) acceptors, and the electron donation by a secondary donor, the cytochrome, to the photooxidized primary donor P. Figure adapted from RK Clayton (1980) Photosynthesis. Physical Mechanism and Chemical Patterns, p 91. Cambridge Univ Press.

As seen earlier in Chapter 2 on bacterial reaction centers, crystallization of the reaction-center protein of the photosynthetic h iCttn xm Rhodopseudomonas viridis by Michel in 1982 and subsequent determination ofthe three-dimensional structure ofthe reaction center by Deisenhofer, Epp, Miki, Huber and Michel in 1984 led to tremendous advances in the understanding ofthe structure-function relationship in bacterial photosynthesis. Furthermore, because of certain similarities between the photochemical behavior of the components of some photosynthetic bacteria and that of photosystem II, research in photosystem-II was greatly stimulated to its benefit by these advances. In this way, it became obvious that the ability to prepare crystals from the reaction-center complexes of photosystems I and II would be of great importance. However, it was also recognized that, compared with the bacterial reaction center, the PS-I reaction center is more complex, consisting of many more protein subunits and electron carriers, not to mention the greater number of core-antenna chlorophyll molecules. 

The photochemically oxidized reaction-center chlorophyll of PSII, Peso, is the strongest biological oxidant known. The reduction potential of Peso is more positive than that of water, and thus it can oxidize water to generate Q2 and H ions. Photosynthetic bacteria cannot oxidize water because the excited chlorophyll a in the bacterial reaction center is not a sufficiently strong oxidant. (As noted earlier, purple bacteria use H2S and H2 as electron donors to reduce chlorophyll in linear electron flow.)... 

Frank HA (1992) Electron paramagnetic resonance studies of carotenoids. Meth Enzymol 213 305-312 Erank HA (1993) Carotenoids in photosynthetic bacterial reaction centers Structure, spectroscopy, and photochemistry. In Deisenhofer J andNorris JR(eds) The Photosynthetic Reaction Center, Vol II, pp 221-237. Academic Press, San Diego Prank HA and CogdeU RJ (1996) Carotenoids in Photosynthesis. Photochem Photobiol 63 257-264... 

Frank HA and Violette CA (1989) MonomericbacteriochlorophyU is required for the triplet energy transfer between the primary donor and the carotenoid in photosynthetic bacterial reaction centers. Biochim Biophys Acta 976 222-232 Frank HA, Bolt JD, de B. Costa SM and Sauer K (1980) Electron paramagnetic resonance detection of carotenoid triplet states. J Am Chem Soc 102 4893 898 Frank HA, Machniki J and Felber M (1982a) Carotenoid triplet states in photosynthetic bacteria. Photochem Photobiol 35 713-718... 

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