Chromatium function

The functions of the heme is uncertain. The soluble mammalian succinate dehydrogenase resembles closely that of E. coli and contains three Fe-S centers binuclear SI of E° 0 V, and tetranuclear S2 and S3 of -0.25 to -0.40 and + 0.065 V, respectively. Center S3 appears to operate between the -2 and -1 states of Eq. 16-17 just as does the cluster in the Chromatium high potential iron protein. The function of the very low potential S2 is not certain, but the following sequence of electron transport involving SI and S3 and the bound ubiquinone QD-S66 has been proposed (Eq. 18-4). 

Racker and Schroeder (85) questioned the importance of the alkaline FDPase in photosynthesis because of its lack of activity at neutral pH, its apparent cytoplasmic localization, and the presence of a second enzyme or enzymes which appeared to be associated with the chloroplasts and which hydrolyzed both FDP and SDP. Later work, however, has clearly established the function of this enzyme in the photosynthetic carbon cycle. Smillie has shown that the alkaline FDPase is associated with photosynthetic tissues in higher plants and Euglena (101, 102). The enzyme was also shown to be localized in the chloroplasts and to be absent in nonphotosynthetic tissue or bleached algae. It was the only FDPase detected in the photosynthetic bacterium Chromatium grown under autotrophic conditions (102). Preiss et al. (103) have pointed... 

NADH dehydrogenase and, 189 ubiquinone reductase and, 178,182,183 Cholesterol, side chain cleavage, 83, 84-85 Choline, oxidation to betaine, 260 Choline dehydrogenase electron transport system and, 261-263 properties, 260-201 Chromatium sulfate reduction by, 281 transhydrogenase of, 54 function of, 80 molecular properties, 58, 69 purification, 55, 56 Chromium... 

Sinskey, A.)., and Stubbe,). (2000) Lipases provide a new mechanistic model for polyhydroxybutyrate (PHB) synthases characterization of the functional residues in Chromatium vinosum PHB synthase. Biochemistry, 39, 3927-3936. 

The most disappointing loose ends in the Chromatium cytochrome story are the lack of clear-cut roles for either cytochrome cc or flavin-c562. For the latter we can only offer the proposal of Kennel and co-workers (381) that flavin-C662 enhances the rate of reoxidation of the primary photoreductant X when readded to chromatophores depleted of their flavin-C552, and thus may function somewhere in the chain between X and the C662/C668 complex. 

Other bacteria are able to use as electron donors compounds that constitute the reduced components of redox systems with E g values more negative than that of the quinone/quinol redox system (E g - OV) which binds at the Qg site of BPS [Fig. 3(c)]. Examples of such electron donors are H2S (E g of S/ HgS = -0.23V), which is used by some purple sulfur bacteria (including the versatile Chromatium), and malic acid (E of oxaloacetate/malate = -0.17V), which is used by some purple- and green non-sufur bacteria. Organisms using such electron donors have an enzyme that catalyses the generalized reaction shown in eq. 2 this enzyme is the functional equivalent of the succinate UQ oxidoreductase of complex II of the mitochondrial electron transport chain. 

A thiohemiacetal function is possibly contained in a related enzyme, flavocytochrome C553 from Chlorobium thiosulfatophilum (2), where in fact a flavocoenzyme can be released from the protein under condition similar to those employed for the Chromatium cytochrome C552, and has properties very similar to those of the flavin from Chromatium cytochromes C552. 

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