Superoxide reductases

SODs. Nevertheless, the D. baarsii Rbo was reported to reduce the steady-state level of intracellular superoxide in the SOD-deficient E. coli strain (Liochev and Fridovich 1997). This Rbo in vitro showed evidence for superoxide reductase (SOR) activity (Lombard et al. 2000), i.e., reduction of O2, presumably to FI2O2, without dismutation (Eq. 10.3). 

Some aspects of the proposed Rbr/Rbo oxidative stress defense system in D. vulgaris resemble those recently suggested for oxidative stress protection in the anaerobic hyperthermophilic archaeon Pyrococcus furiosus (Jenney et al. 1999). Pyrococcus furiosus contains an Nlr-like protein with superoxide reductase activity as well as an Rbr, the genes for which are tandemly located. The microorganismic segregation of SOD/catalase between aerobes and anaerobes appears to be less distinct than for Rbo/Rbr, which, as noted above, have so far been found only in air-sensitive microbes (Kirschvink et al. 2000). The latter segregation suggests that the Rbo/Rbr oxidative stress protection system is well suited to protection of anaerobic life in an aerobic world. 

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. 

The Moura group propose the formation of a stable cyano-bridged diiron derivative following ferricyanide oxidation of the superoxide reductases (SORs) from Treponema pallidum and from Desulfovibrio vulgaris.10 Spectroscopic evidence for a ferrocyanide adduct with the bridge (SOR)Fe-NsC-Fen(CN)5 came from mass and IR measurements, where the Fe co-ordination is made up of four equatorial histidine N atoms and an axial cysteinyl S atom, and it is proposed... 

Superoxide Reductase (see Iron Proteins with Mononuclear Active Sites). Detoxification of reactive oxygen species in anaerobic microorganisms has recently been shown to center around SOR, a novel mononuclear iron enzyme that reduces superoxide to hydrogen peroxide (see equation 4), rather than dismuting superoxide to oxygen and hydrogen peroxide as is the case for the superoxide dismutases found in aerobic organisms. 

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. 

Niviere V, Asso M, Weill CO, Lombard M, GuigKarelK B, Favaudon V, Houee-Levin C. (2004) Superoxide Reductase from Desulfoareulus baarsii Identification of protonation steps in the enzymatic mechanism. Bioehemistry 43 808-818. 

Molina-Heredia FP, Houee-Levin C, Berthomieu C, Touati D, Tremey E, Favaudon V, Adam V, Niviere V. (2006) Detoxification of superoxide without production of HjOj Antioxidant activity of superoxide reductase complexed with ferrocyanide. Proo NatlAoad Sei USA 103 14750-14755. 

The second system was discovered in prokaryotic cells a few years ago [22]. It is the recently characterized non-heme iron superoxide reductase (SOR) that catalyzes the reduction of 02 into H2O2 by an intracellular reductant ... 

Some anaerobes that lack SOD contain superoxide reductase (SOR EC 1.15.1.2) which catalyzes only the reduction of superoxide to hydrogen peroxide ... 

The oxidized form of superoxide reductase formed in this reaction is reduced back by rubredoxin, dependent ultimately on reduced pyridine nucleotides via intermediate electron carriers [65]. The reaction of SOR with superoxide is also very fast, the reaction rate constant being of an order of 10 M s. It has been demonstrated that CuZnSOD can also function as superoxide reductase reducing superoxide at the expense of oxidation of ferrocyanide, or as superoxide oxidase, oxidizing superoxide at the expense of reducing ferricyanide [66]. Both ferri- and ferrocyanide are unphysiological substrates but the enzyme can also act as superoxide reductase with nitroxyl anion oxidizing it to nitric oxide [67]. 

Clay M, Johnson MK, Barden CJ, Schaefer HF. Characterization of P. furiosa superoxide reductase (manuscript in preparation). 

Shearer, J., R.C. Scarrow, and J.A. Kovacs (2002). Synthetic models for the cysteinate-ligated nonheme iron enzyme superoxide reductase Observation and structural characterization by XAS of an Fe(IIl)-OOH intermediate. J. Am. Chem. Soc. 117, 11709-11717. 

Spectroscopic studies of Pyrococcus furiosus superoxide reductase Implications for active-site structures and the catalytic mechanism. J. Am. Chem. Soc. 124, 788-805. 

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