Reaction center pigment arrangement

The reaction center of . aurantiacus contains the following components the primary donor, P865, is a BChl a dimer while two additional BChl a molecules are present, which are responsible for the absorption bands around 800 nm. Three molecules of BPheo are present in contrast to two in the reaction center of purple bacteria. The one with absorption band at 540 nm at 4 K probably acts as an early electron acceptor. Menaquinone (vitamin K2) is a secondary electron acceptor. The reaction center pigment arrangement and the organization of the electron acceptor chain show clear similarities between the reaction center of C. aurantiacus and those of purple bacteria. 

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. 

Figure 12.15 Schematic arrangement of the photosynthetic pigments in the reaction center of Rhodopseudomonas viridis. The twofold symmetry axis that relates the L and the M subunits is aligned vertically in the plane of the paper. Electron transfer proceeds preferentially along the branch to the right. The periplasmic side of the membrane is near the top, and the cytoplasmic side is near the bottom of the structure. (From B. Furugren, courtesy of the Royal Swedish Academy of Science.)...

Spectroscopic measurements show that the reaction center and LHl are tightly associated and therefore it is assumed that the ring of pigments in LHl surrounds the reaction center. Careful model building indicates that the hole in the middle of LHl is large enough to accommodate the whole reaction center molecule. We do not know exactly how the LH2 complexes are arranged in the membrane around the LHl-reaction center complex, but at least some of them should be in contact with the outer rim of LHl for efficient... 

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

Fig, 1. Arrangement of the pigments in the reaction center of Rhodopseudomonas viridis, based on the crystallographic data in Ref. 16. Upon excitation an electron is transferred from the special pair P to the menaquinone Q. (From Ref. 18). 

With sufficiently large crystals of the bacterial reaction centers available, the determination of crystal structure by X-ray diffraction became possible and in 1983, Deisenhofer, Epp, Miki, Huber and Michel determined tbe crystal structure of the Rp. viridis reaction centers at 3 A resolution [later refined to 2.3 A ]. From the electron-density map, the spatial arrangement of the polypeptide subunits, the pigment molecules and the electron carriers in the reaction center was determined. For this work, Deisenhofer, Michel and Huber were awarded the Nobel Prize in 1988. 

Fig. 9. Stereo view of the three-dimensional arrangement of the pigment moiecules and cofactors in the Rp. viridis reaction center without the background protein structures. He=heme. Figure constructed as a composite from Deisenhofer, Michel and Huber (1985) The structural basis of photosynthetic light reactions in bacteria. Trends Biochem Sci, 10 245 and Deisenhofer and Michel (1993) Three-dimensional structure of the reaction center of Rhodopseudomonas viridis. In J Deisenhofer and JR Norris (eds) The Photosynthetic Reaction Center, Vol. II, p 348. Acad Press.

VA Shuvalov and AA Asadov (1979) Arrangement and interactions of pigment molecules in reaction centers of Rhodopseudomonas viridis. Photodichroism and circular dichroism of reaction centers at 100 K. Biochim Biophys... 

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

Fig. 3. (A) Arrangement of pigment molecules and electron-transfer cofactors in the PS-1 reaction center, viewed along the membrane plane. Numerical values are distances in A. (B) stereo view ofthe same pigment and cofactor molecules as in (A). Both figures adapted from Schubert, Klukas, KrauB, Saenger, Fromme and Witt (A) (1995) Present state of the crystal structure analysis of photosystem I at 4.5 A resolution. In P Mathis (ed) Photosynthesis From Light to Biosphere, II 5. Kluwer (B) (1997) Photosystem I of Synechococcus elongatus at 4 A resolution comprehensive structure analysis. J Mol Biol 272 p 756. Also see Color Plate 10 for a color rendition ofthe electron-density map of (A).

Figure 4. Left-. Structural model of the photosynthetic reaction center of Rps. viridis from crystal structure data. Right. Arrangement of the special pair (dark gray), accessory bacterio-chlorophyll (black), and the bacteriopheophytin (light gray) pigments.

Figure 15. Top Arrangement of pigments in the reaction center Rb. Sphaeroides. Bottom. Plot of the absorption spectrum of this RC, with absorption features attributed to the pigments H, B, and P indicated.

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