Purple nonsulfur bacteria, photosynthetic

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]. 

Anoxygenic photosynthetic bacteria. Green sulfur bacteria. Chlorobium, Prosthecochloris purple nonsulfur bacteria Rhodopseudomonas, Rhodospirillum purple sulfur bacteria Chromatium, Thiospirillum... 

CO2 fixation is also found in many bacteria, both photosynthetic and non-photosynthetic. The purple sulfur and purple nonsulfur bacteria employ the RPP cycle as do plants. The photosynthetic green bacteria, however, use a group of ferre-doxin-linked carboxylases in a pathway known as the reductive carboxylic acid cycle [ ] ... 

Rhodospririllum rubrum is the most exhaustively studied of the purple nonsulfur bacteria. The photosynthetic apparatus is located in extensive infolded membrane vesicles called chromatophores. Cytochromes C2, cd, and 1)557.5 have all been found to be associated with the chromatophores (371), with C2 being the major component. Several lines of evidence, including fast kinetics following the excitation of the baeteriochlorophyll photocenter by laser flash, have suggested the scheme shown in Fig. 29, where cytochrome Cj is the immediate source of electrons to the electron-... 

This is a remarkable state of affairs. We tend to think of photosynthesis and respiration as having had a long independent history, yet in regions of shared structure, respiratory c from mammals and respiratory C560 from Micrococcus are no different from the photosynthetic Cg of purple nonsulfur bacteria than the individual Ca s are among themselves. Part of our impression that man and Micrococcus are farther apart than R. capsulata and R. spheroides may lie in the eye of the beholder, but we would have expected more distinction between Ca and the other proteins than is observed. 

It could be that the break between respiration and photosynthesis in these bacteria is more recent than we think. Cytochrome Ca has been suggested to have a respiratory as well as a photosynthetic role in R. spheroides (S72) and R. capsulata (372a-c) and no alternative respiratory chain has yet been identified in any of the Athiorhodaceae. In some of these organisms a situation may exist as in Fig. 46 with electrons flowing to both from light-excited bacteriochlorophyll and from external donors, and then from c either to an electron-depleted bacteriochlorophyll or to an oxidase molecule. This would account for the observed control mechanism in the purple nonsulfur bacteria. Under aerobic conditions in the dark, bacteriochlorophyll would not be electron-defi.cient, whereas the oxidase would be in its oxidized state and capable of accepting electrons from c. Under anaerobic conditions, electrons would reduce the oxidase, and further electron transfer down that path would be blocked. Light then would promote electrons away from bacteriochlorophyll and set cyclic photophosphorylation in motion. 

Fic. 46. Possible combination of photosynthetic and respiratory electron pathways in purple nonsulfur bacteria with c, being shared by both paths. See text for discussion. 

One of the most striking features of the structure of the photosynthetic reaction center from purple nonsulfur bacteria is the local C-2 symmetry present in both the arrangement of the chromophores and the polypeptides (Deisenhofer et al.,... 

Aside from C. vinosum, type III-PHA synthases have so far been detected exclusively in the phototrophic purple sulfur bacteria such as Thiocystis viol-acea [51] and Thiocapsa pfennigii [26, 57] and in cyanobacteria such as Synechocystis sp. PCC6803 [49] or Synechococcus sp. MAI 9 [58]. In contrast, the photosynthetic nonsulfur purple bacteria possess type I-PHA synthases. 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

Woodbury, N. W., and Allen, J. P., 1995, The pathway, kinetics and thermodynamics of electron transfer in wild type and mutant hacterial reaction centers of purple nonsulfur hacteria. In Anoxygenic Photosynthetic Bacteria, (R. E. Blankenship, M. T. Madigan, and C. E. Bauer, eds.) pp. 5279557, Kluwer Academic Publishers, Dordrecht, The Netherlands. 

The membrane-bound PPase does not only hydrolyze PPj for the maintenance of a pool of high energy, it can also form PPj at the expense of the energy liberated in the electron transport chain [13,14]. This enzyme has been found in both purple nonsulfur [13-17] and sulfur photosynthetic bacteria [18], and in mitochondria of lower [19] and higher [20] heterotrophic organisms, and also in chloroplasts from algae and higher plants [21]. 

Decoloring. Much attention is now given to proteins from photo-synthetic origin (88), especially those from blue-green algae (89), nonsulfur purple bacteria (90), and green leaves (91), as possible food protein sources. These proteins contain large amounts of photosynthetic... 

NapB. NapB is a subunit of the heterodimeric periplasmic nitrate reductase (NapAB) and transfers electrons to the catalytic NapA molybdoprotein. Nap systems are found in a number of bacteria, including enterobacteria, aerobic denitrifiers, and nonsulfur purple photosynthetic bacteria. Their physiological function is different in these groups of bacteria and includes redox balancing using nitrate as an electron sink to dispose of excess reductant, aerobic denitrification, and nitrate scavenging in nitrate-limited environments. 

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