Photosynthetic pigments arrangement

The structurally similar L and M subunits are related by a pseudo-twofold symmetry axis through the core, between the helices of the four-helix bundle motif. The photosynthetic pigments are bound to these subunits, most of them to the transmembrane helices, and they are also related by the same twofold symmetry axis (Figure 12.15). The pigments are arranged so that they form two possible pathways for electron transfer across the membrane, one on each side of the symmetry axis. 

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.)...

Chlorophylls are also composed of four pyrroles, but these are arranged in a ring, called a porphyrin (Figure 22.25c). The chlorophylls are green photosynthetic pigments foimd in plants and some bacteria (Figure 22.25d). Another important class of... 

In photosynthetic eukaryotic cells, both the light-de-pendent and the carbon-assimilation reactions take place in the chloroplasts (Fig. 19-38), membrane-bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes, an outer membrane that is permeable to small molecules and ions, and an inner membrane that encloses the internal compartment. This compartment contains many flattened, membrane-surrounded vesicles or sacs, the thylakoids, usually arranged in stacks called grana (Fig. 19-38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the aqueous phase enclosed by the inner membrane) contains most of the enzymes required for the carbon-assimilation reactions. 

The primary classification of algae is based on cellular properties the nature of the photosynthetic pigments, the chemical composition of the cell wall, nature of reserve materials and the nature and arrangement of the flagella. In terms of these characters, the major groups of algae are arranged in Table 4.1. 

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. 

Fig. 1. The structure of the LHCII monomer as derived from electron crystallography [51], A proposed topography of the polypeptide in the photosynthetic membrane. Letters A, B and C indicate the three hydrophobic ix-helices spanning the membrane. Chlorophyll molecules are arranged into two rings roughly parallel to the membrane plane. B Approximate position of the chlorophyll in the upper level (left) and lower level (right) on the membrane plane. Dashed lines outline a-helices A, B and C. Chlorophyll molecules are oriented perpendicular to the membrane plane and are thus represented as black bars. Chlorophylls numbered as 6,7 and 8 are closer to those belonging to the lower layer than the other pigment molecules...

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. 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.

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