Photosynthetic reaction center mechanism

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

J. Hasegawa and H. Nakatsuji, Mechanism and unidirectionality of the electron transfer in the photosynthetic reaction center of Rhodopseudomonas Viridis SAC-CI theoretical study, J. Phys. Chem. B, 102 (1998) 10420-10430. 

When the donors and acceptors lack spherical symmetry, there will also be an orientation dependence. In cases such as those to be discussed below, where the donor and acceptor moieties are linked by covalent bonds, there is considerable evidence that in certain situations the electron transfer occurs through the linkage bonds [22]. Although such linkages are not present in photosynthetic reaction centers, it has been proposed that the accessory Bchl or other intervening material may still take part in electron transfer through a superexchange mechanism [8, 26]. The distance dependence of photoinitiated electron transfer has recently been reviewed [13]. 

Now upon establishing the theoretical framework for the description of molecular-wire behavior, we should analyze the transport phenomena as a function of molecular-wire properties. Firstly, we will discuss the superexchange mechanism. The latter is considered as the main mechanism for efficient electron-transfer within the photosynthetic reaction center [37, 38] and has been studied in various biomimetic systems [39, 40]. 

Three main tendencies have been underlined in recent studies of structure and action mechanism ofbacterial photosynthetic reaction centers. The crystallographic structure of the reaction centers from Rps. viridis and Rb. spheroids was initially determined to be 2.8 and 3 A resolutions (Michel and Deisenhofer et al., 1985 Allen et al., 1986). Resolution and refinement of these structures have been subsequently extended to 2.2, 2.3 and 2.6 A. (Rees et al., 1989 Stowell et al., 1997, Fyfe and Johns, 2000 Rutherford and Faller, 2001). Investigations of the electronic structure of donor and acceptor centers in the ground and exited states by modern physical methods with a combination ofpico-and femtosecond kinetic techniques have become more precise and elaborate. Extensive experimental and theoretical investigations on the role of orbital overlap and protein dynamics in the processes of electron and proton transfer have been done. All the above-mentioned research directions are accompanied by extensive use of methods of sit-directed mutagenesis and substitution of native pigments for artificial compounds of different redox potential. 

Stowell, M.H.B., McPhillips, T.M., Rees, D.C., Solitis, S.M., Abresch, E. and Feher, G. (1997) Light-induced structural changes in photosynthetic reaction center implications for mechanism of electron-proton transfer, Science (Washington, D. C.) 276, 812-816. 

Phytochrome, Photosynthetic Reaction Center, Electron Transfer, Color-tuning Mechanism, Retinal Protein, Green Fluorescent Protein, Firefly Luciferase... 

Bixon, M., Jortner, J., Michel-Beyerle, M. E., and Ogrodnik, A., 1989, A superexchange mechanism for the primary charge separation in photosynthetic reaction centers. Biochim. Biophys. Acta, 977 2739286. 

Bacterial photosynthetic reaction centers (PRC) have been among the most actively studied ET proteins since DeVault and Chance first measured C. vinosum tunneling rates in the early 1960s. In many cases, measurements of ET kinetics preceded determination of the three-dimensional structure of the membrane-bound protein assembly. It was not until the X-ray crystal-stracture determinations of the Rhodopseudomonas (Rps.) viridus and Rhodobacter (Rb.) sphaeroides PRCs that distances could be assigned to specific rate constants. The recent crystal structures of photosystems l and from cyanobacteria promise to clarify critical aspects of the ET mechanisms in oxygenic PRC. ... 

Reconstitution of membranes from a small number of molecular components provides simplified structures to study. Thus, cytochrome oxidase or photosynthetic reaction centers, both electron transfer proteins, may be extracted from their native membranes, purified, and reincorporated at relatively high concentration into a simple well defined lipid bilayer. Diffraction investigation then provides information about the distribution and structure of the protein in the membrane. Understanding the mechanism for electron transport in these proteins will require considerable additional information. One key element of structural informations is the location of the redox centres in the membrane profile. 

Deisenhofer was independent, too. This project was not simple crystallography, not at all. It was a most complex structure determination. Even the methods of measuring the intensities were not automated at the time. We had developed instruments. X-ray cameras and methods for that purpose also. We had a small workshop at that time, with a mechanic and an electronics person. One day the mechanic had a stroke. A week later the electronics person had a heart attack. They were very important in servicing the instruments. I was the only one in my department able to service the instruments. It was at a critical time for the photosynthetic reaction center work, around 1983, and I spent much time each day taking care of the instruments. It is a side issue, not even a scientific one, but it shows you that things may look different from the perspective of today s possibilities than they actually were. The work at that time required a background also concerning the availability of the samples for isomorphous replacement and methods to apply them. I have made many of these samples and I built up an enormous collection. 

The lack of energy transfer in 24 is in marked contrast to the results for a variety of other bichromophoric molecules where singlet energy transfer occurs over many tens of angstroms via the Forster dipole-dipole mechanism. Since the effidency of Forster energy transfer depends upon the fluorescence quantum yield of the donor, we postulated that the lack of energy transfer in 24 was due to the very low fluorescence quantum yield of the carotenoid, and further concluded that energy transfer from carotenoid polyenes to chlorophyll in photosynthetic reaction centers could therefore not occur by the dipole-dipole mechanism [72]. 

VA Shuvalov and FF Litvin (1969) Mechanism of prolonged afterluminescence of plant leaves and energy storage in the photosynthetic reaction centers. Mol Biol (Kiev) 3 59-73... 

CC Moser JM Keske K Warncke RS arid and PL Dutton (1993) Electron-transfer mechanisms in reaction centers Engineering guidelines. In J Deisenhofer and JR Norris (eds) The Photosynthetic Reaction Center Vol II 1-22... 

Fig. 15. Plot ofthe extent of absorbance change due to P7007P430" recombination measured at 695 nm vs. the redox potential of 14 quinones and 7 non-quinone carbonyl compounds, the fluorenones (individual compounds are identified below the plot). The solid curve is the theoretical, one-electron Nernst curve centered near the redox potential of FeS-X in vivo. Data adapted from Itoh and Iwaki (1992) Exchange ofthe acceptor phylloquinone by artificial quinones and fluorenones in green plant photosystem I photosynthetic reaction center. In N Malaga, T Okada and H Masuhara (eds) Dynamics and Mechanism of Photoinduced Transfer and Related Phenomena. p.533. Elsevier,...

Boxer, 8. G, 1990, Mechanisms of long-distance electron translei in proteins Lessons from photosynthetic reaction centers Anna Rev. Biophys. Bivphys. Chem. 19 267 299. 

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