Photosynthetic bacteria reaction-center complex

The chemical composition of the highly purified reaction-center complex from several photosynthetic bacteria is now well established. In addition to protein subunits designated as L (light), M (medium) and H (heavy), some reaction centers also contain a c-type cytochrome (C) subunit. The chemical composition of reaction-center complexes of several of these bacteria is shown in Table 1 ... 

Fig. 3. Arrangement of bacteriochlorophyll c in the rod elements of chlorosomes of Chloroflexus aurantiacus. (A) Chlorosome-reaction center complex (B) Molecular arrangement of BChl c in the rod elements (C) Molecular structure of BChl c (D) and (E) show two different spatial organizations proposed for self-aggregation of BChl c. (B) from Matsuura, Hirota, Shimada and Mimuro (1993) Spectral forms and orientation of bacteriochlorophylls c and a in chlorosomes of the green photosynthetic bacterium Chloroflexus aurantiacus. Photochem Photobiol 57, 96 figures (D) and (E) from Mimuro, Nozawa, Tamai, Shimada, Yamazaki, Lin, Knox, Wittmershaus, Brune and Blankenship (1989) Excitation energy flow in chiorosome antennas of green photosynthetic bacteria. J Phys Chem. 93 7504.

In DR Ort and CF Yocum (eds) Oxygenic Photosynthesis. The Light Reactions, pp 213-247. Kluwer R2. M Seibert (1993) Biochemical, biophysical, and structural characterization of the isolated photosystem II reaction center complex. In J Deisenhofer and JR Norris (eds) The photosynthetic Reaction Center, vol 1 319-356 R3. WW Parson and B Ke (1982) Primary photochemical reactions. In Govindjee (ed) Photosynthesis Energy Conversion by Plants and Bacteria, Vol 1, pp 331-385. Acad Press R4. VV Klimov and AA Krasnovsky (1981) Pheophytin as the primary electron acceptor in photosystem 2 reaction centres. Photosynthetica 15 592-609... 

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. 

Reaction Center Complexes in Photosynthetic Bacteria (Figure 17.19)... 

The biochemical architecture of photosynthetic bacteria is not as complex as that of green plants. For example, photosynthetic bacteria have only one photosystem, while green plants have two. The reaction center protein from several species of photosynthetic bacteria can be isolated from the photosynthetic membrane. Reaction centers from the species Rhodopseudomonas sphaeroides have been extensively studied. Although minor details will change from one species to another, the important features are nearly identical. The reaction center protein has a molecular weight of about 70,000 daltons. Within the reaction center protein extracted from the carotenoidless mutant strain R26 of the species R. sphaeroides, are found four molecules of bacteriochlorophyll a, two molecules of bacteriopheophytin a, one atom of nonheme iron, and, depending on the isolation procedure used, one or two molecules of ubiquinone. The absorption spectrum of the isolated reaction center has been well characterized. It is shown in Fig. 4. Based on in vitro absorption spectra, the bands at 870, 800, and 600 nm have been assigned to the bacteriochlorophyll a molecule. Bands at 760 and 530 nm have been attributed to the bacteriopheophytin a. 

When a chromophore is excited, the absorption spectrum of a nearby chromophore may also be influenced owing to an interaction with the excited molecule, which is observed, for instance, in the photosynthetic bacterial reaction center of purple bacteria. In the same complex the... 

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... 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

The photosynthetic reaction center (RC) of purple nonsulfur bacteria is the core molecular assembly, located in a membrane of the bacteria, that initiates a series of electron transfer reactions subsequent to energy transfer events. The bacterial photosynthetic RCs have been characterized in more detail, both structurally and functionally, than have other transmembrane protein complexes [1-52]. 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

Wavepacket motion is now routinely observed in systems ranging from the very simple to the very complex. In the latter category, we note that coherent vibrational motion on functionally significant time scales has been observed in the photosynthetic reaction center [15], bacteriorhodopsin [16], rhodopsin [17], and light-harvesting antenna of purple bacteria (LH1) [18-20]. Particularly striking are the results of Zadoyan et al. [21] on the... 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

In contrast, the reaction centers of green sulfur bacteria resemble PSI of chloroplasts. Their reaction centers also receive electrons from a reduced quinone via a cytochrome be complex.245 However, the reduced form of the reaction center bacteriochlorophyll donates electrons to iron-sulfur proteins as in PSI (Fig. 23-17). The latter can reduce a quinone to provide cyclic photophosphorylation. Cyanobacteria have a photosynthetic apparatus very similar to that of green algae and higher plants. 

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