Chromatium cytochrome

Some metalloflavoproteins contain heme groups. The previously mentioned flavocytochrome b2 of yeast is a 230-kDa tetramer, one domain of which carries riboflavin phosphate and another heme. A flavocytochrome from the photosynthetic sulfur bacterium Chromatium (cytochrome c-552)279 is a complex of a 21-kDa cytochrome c and a 46-kDa flavoprotein containing 8a-(S-cysteinyl)-FAD. The 670-kDa sulfite reductase of E. coli has an a8P4 subunit structure. The eight a chains bind four molecules of FAD and four of riboflavin phosphate, while the P chains bind three or four molecules of siroheme (Fig. 16-6) and also contain Fe4S4 clusters.280 281 Many nitrate and some nitrite reductases are flavoproteins which also contain Mo or... 

Succinate dehydrogenase is the only enzyme of the citric acid cycle which is bound to the inner membrane of mitochondria. It is also one of three flavoproteins known in which flavin is covalently linked to the protein. The other two are monoamine oxidase of the outer membrane of liver mitochondria (138) and Chromatium cytochrome c-552 (139). 

CD measurements in the range 205—500 nm were reported for cytochromes from Rhodospirillum rubrum and Rhodopseudomonas palustris under various conditions, and the relatively high helix content of about 63% was estimated by standard methods for both ferricytochromes (215) [see, however, Ref. (216)]. ORD measurements on Chromatium cytochrome c552, which contains two mesohemes and one flavin per molecule, showed complex behavior in the Soret region, indicating possible interaction between the hemes (217). 

The initial suggestion of quantum mechanical tunneling came not from respiratory cytochrome c, but from the photosynthetic c of the bacterium Chromatium (10,261-268). DeVault, Chance, and co-workers studied the oxidation of Chromatium cytochrome c, which transfers electrons to the... 

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. 

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. 

The temperature dependence of ET rates between cytochrome-c and the reaction center in Chromatium (Figure 1), fitted to eqs 3-5 (8), demonstrated that, unlike many redox reactions in... 

Components of the electron transport chain in bacteria have been shown to include b- and c-type cytochromes, ubiquinone (fat-soluble substitute quinone, also found in mitochondria), ferredox (an enzyme containing nonheme iron, bound to sulfide, and having the lowest potential of any known electron-canying enzyme) and one or more flavin enzymes. Of these a cytochrome (in some bacteria, with absorption maximum at 423.5 micrometers, probably Cj) has been shown to be closely associated with the initial photoact. Some investigators were able to demonstrate, in chromatium, the oxidation of the cytochrome at liquid nitrogen temperatures, due to illumination of the chlorophyll. At the very least this implies that the two are bound very closely and no collisions are needed for electron transfers to occur. 

Fig. 3. Temperature dependence [14] of the characteristic time, t1/2, of electron transfer from cytochrome c to the oxidized form of chlorophyll for Chromatium bacteria [14].

The appearance of similar absorption bands has also been observed upon the formation of a complex between the reduced form of cytochrome c and the simple inorganic acceptor Fe(III)(CN)6[106]. The tunneling distance evaluated from the intensity of this band amounts to 7—10 A. However, more recent experiments have failed to detect such a band [107]. The situation is more favourable in the system [cytochrome c/P870] of the Chromatium reaction centre, where the intensity of the charge transfer band centred at 200 nm could be correlated with the data obtained in kinetic experiments [108]. 

For Chromatium vinosum three possible electron acceptors, a mobile cytochrome c-SSl located in the periplasmic space, a membrane-bound... 

Elemental sulfur was also formed during sulfide oxidation by a cytochrome c-flavocytochrome c-552 complex in Chromatium vinosum (42). Flavocytochromes of different phototrophic bacteria act as sulfide cytochrome c reductases and there was one report that a flavocytochrome possessed even elemental sulfur reductase activity (see 4.9V All flavocytochromes examined so far are heat-labile and are reduced by sulfide forming thiosulfate under strictly anaerobic conditions (4.9V The small acidic cytochromes c-551 of Ectothiorhodospira halochloris and Ectothiorhodospira abdelmalekii. both located on the outside of the cell membrane, stimulated the velocity of sulfide... 

APS-reductase does not occur in Rhodospirillaceae, Ectothiorhodospiraceae, and was not found in two species of the Chromatiaceae (see Tables I-IV). In Chromatium vinosum. APS-reductase activity is very high at the end of the exponential growth phase (48). and in Thiocapsa roseopersidna the spedfic activity of this enzyme increases, when sulfide is completely consumed by the cells (U. Fischer, unpublished). Normally, ferriqranide is used as an artificial electron acceptor for the enzyme, but Ulbricht (25) could show for Chromatium grarile that the cell s own c-type cytochrome could replace ferricyanide as electron acceptor. 

The visible absorption spectrum of a partially purified enzyme of Chromatium vinosum indicates a high-spin cytochrome c (36.49). Whether cytochrome c itself can be regarded as the real enzyme or whether c is only attached to the enzyme during the purification procedure, still remains unclear. On the other hand, Ulbricht (35) could clearty demonstrate that the enzymes of Chromatium gracile and Chromatium minutissimum have no cytochrome character. 

Early works on electron transfer in RC from Chromatium vinosum [204] and Rhodopseudomonas sphaeroides [205] demonstrated that these could occur down to 1 K. Then authors of [9] recognized that the oxidation of cytochrome c in Chromatium vinosum at low temperatures occurs by a quantum mechanical tunneling mechanism, providing one of the first demonstrations of this phenomenon (see above and Refs. [232-237] for reviews). Figure 25 which represents the rates of the reactions at ambient temperatures and below 100 K, shows the remarkable capability of reaction centres to support nearly temperature-independent electron transfer at low temperatures. Indeed, for electron transfer from BPh to Qa and from QA to (BChl)2 the rates have been shown to become independent of temperature below 100 K [238-243], But it is necessary to note that electron transfer can occur in a RC also by an alternative thermally activated route which becomes dominant at high enough temperatures [244-248],... 

Bogatyrenko, V.R., Sabo, Ya., Chamorovskii, S.K., Zakharova, N.I., Kononenko, A.A. and Kulikov A.V. (1991) Study of localization of bacteriochlorophyl dimer and cytochrome c in reaction centers from Chromatium minutissium by ESR, Biofizika 36, 289-290. 

Relaxation phenomena are equally evident at the donor side of the reaction center. The well studied, fast (< 1 //s) electron transfer reactions from bound c-type cytochromes of RCs from such species as Chromatium and Rps. viridis f requentlv show a progressive shutoff of electron transfer from the high potential heme, which is closest to P (Gao et al., 1990). It is suggested that this is due to a large (> 100 mV) increase in the midpoint potential of the heme, associated with the freezing out of solvent or hydration-related relaxation processes (Kaminskaya et al., 1990). This is consistent with a deuterium solvent isotope effect for this reaction, as reported by Kihara and McCray (1973). 

Devault, D., and Chance, B., 1966, Studies of photosynthesis using a pulsed laser 1. Temperature dependence of cytochrome oxidation rate in chromatium. Evidence for Tunneling Biophysical J. 6 825n847. 

FCSD is a periplasmic enzyme found in a number of phototrophic bacteria, as well as in Paracoccus denitrificans, that catalyzes the oxidation of sulfide to elemental sulfur (Cusanovich et al., 1991 Wodara et al., 1997). FCSD from Chromatium vinosum is a 67kDa heterodimer consisting of a 46kDa flavoprotein subunit and a 21 kDa diheme cytochrome. The secondary electron acceptor is probably a cytochrome (Gray and Knaff, 1982). The FAD is bound covalently to the flavoprotein subunit via an 8-a-methyl(S-cysteinyl) thioether linkage. 

Gray, G., and Knaff, D. B., 1982, The role of a cytochrome C552 -cytochrome c complex in the oxidation of sulfide in Chromatium vinosum. Biochem. Biophys. Acta. 680 290n296. 

Shuvalov, V. A., and Klimov, V. V., 1976, The primary photoreactions in the complex cytochrome P-890. P-760 (bacteriopheophytin 7gQ) of Chromatium minutissimum at low redox potentials. Biochim. Biophys. Acta, 440 587n599. 

Electron transport between cubane [Fe4S4] clusters, cytochrome c, or Cu + centers in blue copper proteins " and the periphery of the proteins has been examined by complexing ruthenium species to surface histidines. In the case of the iron sulfur cubane in Chromatium vinosum, four surface histidines served as points of ruthenium attachment. The rates of electron transport from the Fe4S4 core to ruthenium varied over two orders of magnitude and were used to diagnose the preferred channel for electron transport. Cysteine and lysine residues have also been used as binding sites in studies of cytochrome c and cytochrome P450 cam proteins. 

Figure 2. Electron transfer kinetics of cytochrome c oxidation in Chromatium vinosum [4] and Rhodopseudomonas viridis [16] display temperature independence at low temperature, a herald of tunneling. The early Chromatium data were analyzed as a single phase, while the Rp. viridis data were analyzed into three phases, dominated by very fast (VF) and fast (F) phases at high temperatures, and dominated by slow (S) phase at low temperatures.

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