Bacterial photosynthetic reaction centers crystal structures

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. ... 

The Bacterial Photosynthetic Reaction Center Chemical Composition and Crystal Structure... 

Roth, M., et al. (1989). Detergent Structure in Crystals of a Bacterial Photosynthetic Reaction Center, Nature 340 659-662. 

The nature of the excited states of the special pair (SP) bacteriochlorophylls (BChl) in the bacterial photosynthetic reaction center (RC) has been a subject of considerable controversy over the past decade. The crystal structure of the RC, in conjunction with numerous spectroscopic experiments," has provided a substantial basis for investigating this problem. However, the existence of complicated interactions between multiple electronic surfaces and the difficulty of computing the underlying electronic wavefunctions from a reliable quantum chemical method have rendered a definitive solution difficult to achieve. 

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 acceptor side of the PS II reaction center is structurally and functionally homologous to the reducing side of reaction centers from a number of photosynthetic bacteria, including Rhodopseudomonas viridis. Rhodobacter sphaeroides and capsulatus. and Chloroflexus aurantiacus. The reaction center complexes of viridis and sphaeroides have been crystallized, and the three-dimensional structure of these has been determined at high resolution [3-7]. With the exception of (a) the His residues in the bacterial reaction center that serve as ligands to the Mg of the accessory bacteriochlorophylls, and (b) the Glu residue that serves as a ligand to the non-heme iron between and Q0, all of the amino acid residues that function as important... 

As mentioned earlier, a structural model that many workers over the years had suggested for the bacterial photosynthetic apparatus consisted of the reaction center inside a cylinder of the core antenna. Based on the cylindrical structures ofLH2 ofRp. acidophila and LHl ofRs. rubrum, Kiihlbrandt presented a model in 19% for the bacterial photosynthetic unit as shown in Fig. 12 (A). It is of interest to note that in 1997 Walz and Ghosh prepared two-dimensional crystals ofthe LH1 RC complex from Rs. rubrum and obtained electron micrographs which confirmed that the RC is located inside the LHl cylinder, as LHl in the undissociated RC LHl complex has the same ring diameter as that for the reconstituted LHl reported by Karrasch et al . ... 

The function of photosynthetic bacterial reaction centers (RCs) is closely related to their structure. In the last 15 years a wealth of structural data has been accumulated on bacterial RCs, mainly through X-ray structure analysis of three-dimensional RC crystals. In this chapter, the arrangement of protein subunits and cofactors in the RC complexes ofthe non-sulfur purple bucienn Rhodobacter (Rb.) sphaeraides mARhodopseudomonas (Rp.) viridis are delineated. A prominent feature ofthe bacterial RCs is their location in the photosynthetic membrane. Inside the RC complex, a finely tuned arrangement of amino acid residues and cofactors maintains a highly ordered system. The positions and likely functions of hydrogen bonds are described, since they play a key role in protein-cofactor interactions. Special emphasis is placed on the symmetry relations in the RC and on the functional asymmetry of electron and proton transfer that contradicts the observed pseudo two-fold structural symmetry. 

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