Chlorobium

The most striking electrochemical feature of Rieske proteins is the pH dependence of the redox potential. This pH dependence has first been demonstrated for the Rieske protein from Chlorobium, where the redox potential shifted from +165 mV at pH 6.8 to +60 mV at... 

The RCC proceeds formally in exactly the opposite direction, from the C4 molecule to the C6 molecule via uptake of C02, but without the help of highly specialized enzymes. However, it is not a hypothetical archaic phenomenon, but has been observed in cells which exist today, for example in the photosynthetic bacterium Chlorobium thiosulfatophilum (Evans et al., 1966). 

Cork [283], Sublette [284], and others have identified a number of chemolithotrophic bacteria which oxidize elemental sulfur and use reduced or partially reduced sulfur compounds as an energy source, in the presence of various carbon sources (such as carbon dioxide or bicarbonate) and reduced nitrogen (e.g., ammonium ion). In the case of Cork et al. s work, the anaerobic photosynthetic bacterium Chlorobium thiosulfatophilum is used to convert sulfides to sulfate. The economics of this process was not favorable due to the requirement of light for the growth of the bacterium. 

Energy supplied by sunlight is necessary for photosynthesis (Fig. 2). The fixed organic carbon is then used to generate energy via respiration. Examples of microorganisms on/in solid phases which carry out photosynthesis are Rhodospirillum, Chromatium, and Chlorobium [36,41,42,46,49,50]. 

Purple bacteria (e.g., Rhodomicrobium) Green bacteria (e.g., Chlorobium) Fe(II) (donor) anaerobic Ferric oxide and hydroxide precipitation under anoxic conditions. 

Heising S, Richter L, Ludwig W, Schink B (1999) Chlorobium ferrooxidans sp. nov., a phototrophic green sulfur bacterium that oxidizes ferrous iron in coculture with a Geospirillum sp. strain. Arch Microbiol 172 116-124... 

Synecnocystis Chlorobium tepidum Methylococcus capsulatus Rhodobacter capsulatus Rhosdospirillum rubrum Chloroflexus aurantiacus Fibrobacter succinogenes Thermoanaerobacter tengcongensis... 

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

Figure 23-28 (A) Model of a light-harvesting chlorosome from green photosynthetic sulfur bacteria such as Chlorobium tepidum and species of Prosthecochloris. The chlorosome is attached to the cytoplasmic membrane via a baseplate, which contains the additional antenna bacteriochlorophylls (795 BChl a) and is adjacent to the trimeric BChl protein shown in (B) and near the reaction center. After Li et al.302 and Remigy et a/.304 (B) Alpha carbon diagram of the polypeptide backbone and seven bound BChl a molecules in one subunit of the trimeric protein from the green photosynthetic bacterium Prosthecochloris. For clarity, the magnesium atoms, the chlorophyll ring substituents, and the phytyl chains, except for the first bond, are omitted. The direction of view is from the three-fold axis, which is horizontal, toward the exterior of the molecule. From Fenna and Matthews.305 See also Li et al.302...

This cyclic pathway was first proposed for Chlorobium sp. that use an anoxygenic photosynthesis for energy supply [3]. It reverses the reactions of the oxidative citric acid cycle (Krebs cycle) and forms acetyl-CoA from two C02 (Figure 3.2). 

Figure 5. Anaerobic sulfide and thiosulfate oxidation by a growing culture of Chlorobium vibrioforme f. thiosulfatophilum (Reproduced with permission from Ref. 17. Copyright 1982 Springer-Verlag New York Inc.).

In Chlorobium limicola . thiosulfatophilum cytochrome c-SSl serves as electron acceptor of thiosulfate oxidation, before the electrons flow to oxidized bacteriochlorophyll via a soluble small cytochrome c-SSS and a membrane-bound cytochrome (2). The absence of cytochrome c-SSl in the non-thiosulfate-utilizing Chlorobiaceae might be one explanation why these organisms cannot use thiosulfate (4). 

Chlorobium liimcola . thiosulfatophilum (22) and ThiQgaBfia roseopersicina (461 do not contain siroheme sulfite reductases, although sulfite reductase activity was measured in the last organism. 

If phototrophic bacteria possess a dissimilatory ATP-sulfurylase, they convert APS with pyrophosphate directly to ATP and sulfate, without the help of an additional enzyme. Such an enzyme is necessary, if the organisms like Chlorobium vibrioforme f. thiosulfatophilum (Table IV) contain only the ADP-sulfurylase, because this enzyme liberates only ADP and sulfate from APS in the presence of inorganic phosphate. In this case, the organisms gain one ATP molecule from 2 molecules of ADP. This reaction is catalyzed by adenylate kinase which converts 2 ADP into 1 ATP and 1 AMP (38). 

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