Desulfoarculus baarsii

Two types of SORs have been firstly described by Lombard et al. [44] and Jenney et al. [45]. The first one is a small protein called desulfoferrodoxin (Dfx) found in anaerobic sulfate-reducing bacteria Desulfoarculus baarsii containing two protein domains iron center I and iron center II [44]. Iron center II is supposed to be responsible for the superoxide reducing activity. Another SOR has been isolated from anerobic archaea, Pyrococcus furiosus, which has a unique mononuclear iron center [45], Lombard et al. [46] and Jovanovic et al. [47] also demonstrated that the Treponema pallidum protein of T. pallidum belongs to a new class of SORs. 

Rbo, which has also been named desulfoferrodoxin (Moura et al. 1990), is the product of the rbo gene in several sulfate-reducing bacteria (Brumlik and Voordouw 1989). Rbo from the sulfate-reducing bacterium Desulfoarculus baarsii was shown to complement an SOD deficiency in E. coli (Pianzzola et al. 1996), even though this Rbo showed almost no detectable SOD activity. In fact, Rbo has no detectable amino acid sequence homologies to known... 

Liochev SI, Fridovich 1.1997. A mechanism for complementation of the sodA sodB defect in Escherichia coli by overproduction of the rbo gene product (desulfo-ferrodoxin) from Desulfoarculus baarsii. J Biol Chem 272 25573-5. 

Lombard M, Fontecave M,Touati D, Niviere V. 2000. Reaction of the desulfoferro-doxin from Desulfoarculus baarsii with snperoxide anion. Evidence for a superoxide reductase activity. J Biol Chem 275 115-21. 

Another important contribution of pulse radiolysis is in the evaluation of redox processes in native SODs and development of SOD mimics. SOD is an endogenous antioxidant enzyme which catalyzes the conversion of Oj radicals to H2O2. Different types of SODs are present in cells such as Mn-SOD in mitochondria and Cu, Zn-SOD in the cytosol and in extracellular surfaces. Reactions of O " radicals with the active site of native SODs from bacterial and animal sources have been examined. In one recent study involving superoxide reductase (SOR) from Desulfoarculus baarsii, the precise step responsible for the catalytic action was examined. Its active site contains an unusual mononuclear ferrous center. Since protonation processes are essential for the catalytic action, the pH dependence of the redox properties of the active site, both in the absence and in the presence of O radicals, was studied using pulse radiolysis. The results confirmed that the reaction of SOR with O2" radicals involves two reaction intermediates, an iron(III)-peroxo species and an iron(III)-hydroperoxo species. The protonation takes place in the second step, and therefore responsible for its catalytic activity. 

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