Sulfite cytochrome c reductase and

Fig. 46. Comparison of the absorption spectra of wild-type and mutant (cys G-439 and cys 1-68) sulfite reductases from Salmonella typhimurium. Spectra of S. typhi-murium sulfite reductase, cys G-439 NADPH-cytochrome c reductase, and cys 1-68 NADPH-cytochrome c reductase, each dissolved in 0.05 M potassium phosphate buffer, pH 7.7, containing 0.1 mM EDTA, were read against a blank containing only buffer. The spectrum of each enzyme is presented in terms of its millimolar extinction coeflScients, assuming 8 moles of flavin per mole of enzyme. Light broken line, calculated difference spectrum between those of wild-type and cys G enzymes when both enzyme solutions contain equal concentrations of flavin. From Siegel et al. (.394).

Lyric RM, Suzuki I (1970c) Kinetic studies of sulfite cytochrome c oxidoreductase, thiosulfate-oxidizing enzyme, and adenosine-5 -phosphosulfate reductase from Thiobacillus thioparus. Can J Microbiol 48 594-603... 

The original demonstration of APS reductase in T. thioparus used the methylviologen-dependent assay of APS cleavage to AMP and sulfite (Peck 1960), but in the oxidation of thiosulfate the reaction proceeds in the oxidative direction (Eq. 15.4), forming APS. This is the thermodynamically favorable direction of the reaction. Later work showed that APS formation by the reductase could be coupled to the reduction of ferricyanide or to cytochrome c (Peck et al. 1965 Lyric and Suzuki 1970), thereby showing the thermodynamic feasibility of APS as an intermediate in the oxidation pathways for sulhte and thiosulfate. 

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... 

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