Purple photosynthetic bacteria electron flow

Those photosynthetic eubacteria with RC-2 centers (filamentous and purple bacteria) reduce NAD" for CO2 fixation by reverse electron flow from the quinone pool, whereas the green sulfur bacteria (RC-1 center) reduce ferredoxin and NAD directly from the secondary acceptor (Fe-S center) of the RC. In both cases an external reductant such as H2S is required. The mechanism of NAD reduction in the gram-positive line has not yet been investigated, but H. chlorum is a het-erotroph rather than an autotroph, and may not need to fix CO2. 

The presence of the two new chlorophyll molecules ( A and A ) is significant in that it points to the similarity between the photosystem-1 and the purple-bacterial reaction centers with regard to the electron-transport pathway and the kinds of pigment molecules involved, as well as their locations. While the involvement of an intermediary chlorophyll in electron transport in photosynthetic bacteria has gradually become clear (see Chapter 7), a similar involvement of an intermediary chlorophyll in photosystem 1 can only be surmised at present. With regard to the various cofactors involved, it is not known yet which ofthe two branches, primed or unprimed in Fig. 3, constitutes the photoactive electron-transport pathway. In any event, a unidirectional electron flow along a P700->(A[Chl] )->Ao->-A ->-FeS-X- FeS-(A/B) pathway is clearly indicated. 

The photochemically oxidized reaction-center chlorophyll of PSII, Peso, is the strongest biological oxidant known. The reduction potential of Peso is more positive than that of water, and thus it can oxidize water to generate Q2 and H ions. Photosynthetic bacteria cannot oxidize water because the excited chlorophyll a in the bacterial reaction center is not a sufficiently strong oxidant. (As noted earlier, purple bacteria use H2S and H2 as electron donors to reduce chlorophyll in linear electron flow.)... 

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. 

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