Rhodopseudomonas viridis photosystems

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

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. 

JE Mullet, JJ Burke and CJ Arntzen (1980) Chlorophyll proteins of photosystem I. Plant Physiol 65 814-822 E Lam, W Ortiz, S Mayfield and R Malkin (1984) Isolation and characterization of a light-harvesting chlorophyll a/b protein complex associated with photosystem I. Plant Physiol 74 650-655 E Lam, W Ortiz and R Malkin (1984) Chlorophyll alb proteins of photosystem I. FEBS Lett 168 10-14 H Michel (1982) 3-dimensional crystals of a membrane protein complex. The photosynthetic reaction center from Rhodopseudomonas viridis. J Mol Biol 158 567-572... 

Michel H and Deisenhofer J (1988) Relevance of the photosynthetic reaction center from purple bacteria to the structure of Photosystem II. Biochemistry 27 1-7 Michel H, Weyer KA, Gruenberg K, Dunger I, Oesterhelt D and Lottspeich F (1986a) The Tight and medium subunits of the photosynthetic reaction centre from Rhodopseudomonas viridis Isolation of the genes, nucleotide and amino acid sequence. EMBOJ5 1149-1158... 

In all of the hgures mentioned, down corresponds to inside the membrane, and up is outside. In other words, in Rhodopseudomonas viridis the special pair of chlorophylls is near the cytoplasmic side of the membrane (inside the cell), whereas in both photosystems I and II, the special pairs are near the stroma and away from the thy-lakoid lumen, that is, facing outward. This is interesting because other structures in the thylakoid membrane are also upside down compared to bacterial and mitochondrial systems. 

M. Losche, G. Feher, and M.Y. Okamura, The Stark Effect in Reaction Centers from Rhodobacter Sphaeroides R-26, Rhodopseudomonas Viridis and the D1D2 Complex of Photosystem II from Spinach, in "The Photosynthetic Bacterial Reaction Center", J. Breton and A. Vermeglio, eds.. Plenum Publishing Corp., San Diego (1988). 

The photochemically active pigments of photosystem II (PSII) are housed in an apoprotein environment provided by the D1 and D2 polypeptides which also contain binding sites for the acceptor quinones (Q and Q ). The organisation of the polypeptides and chromophores has been inferred (1-3) from various similarities and homologies between PSII reaction centres and the photosynthetic reaction centres of purple bacteria, such as Rhodopseudomonas viridis, which have been structurally resolved in considerable detail (e.g. 4). By analogy with the L and M polypeptides of bacterial reaction centres, D1 and D2 each contain 5 transmembrane helical spans. There is strong sequence homology... 

Recently, the structures of the reaction centers from Rhodopseudomonas viridis and Rhodobacter sphaeroides have been solved at the atomic level (1,2). In both organisms the reaction center "core" polypeptides consist of the L and M subunits (responsible for charge separation), a cytochrome, and the H subunit (possibly required for stable assembly). It has become apparent that the bacterial reaction center, particularly the L and M subunits, can be directly compared to the reaction center polypeptides, D1 and D2, of Photosystem II (3). 

The crystallization of RCs in view of structure determination by x-ray crj taUography requires special methods that have worked successfully for two kinds of purple bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides) and for Photosystems I and II of thermophilic cyanobacteria. 

The bacterial photosystem functions without dioxygen production which simplifies the task at hand. Namely, electrons are obtained from more easily oxidized terminal electron donors such as H2S instead of water. Nonetheless, the basic design needed to transform solar energy into stored chemical energy is present. The protein subunits and cofactors that comprise the photosystem in purple bacteria, such as Rhodobacter (Rb.) sphaeroides and Rhodopseudomonas (Rps.) viridis,33 are shown schematically in Fig. 1 which is based on a crystal structure of this assembly.34... 

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