Superoxide reductase activity

About: Superoxide reductase activity is a research topic. Over the lifetime, 12 publications have been published within this topic receiving 708 citations.

Oxygen detoxification in the strict anaerobic archaeon Archaeoglobus fulgidus: superoxide scavenging by neelaredoxin.

Abstract: Archaeoglobus fulgidus is a hyperthermophilic sulphate-reducing archaeon. It has an optimum growth temperature of 83°C and is described as a strict anaerobe. Its genome lacks any homologue of canonical superoxide (O2·−) dismutases. In this work, we show that neelaredoxin (Nlr) is the main O2·− scavenger in A. fulgidus, by studying both the wild-type and recombinant proteins. Nlr is a 125-amino-acid blue-coloured protein containing a single iron atom/molecule, which in the oxidized state is high spin ferric. This iron centre has a reduction potential of +230 mV at pH 7.0. Nitroblue tetrazolium-stained gel assays of cell-soluble extracts show that Nlr is the main protein from A. fulgidus which is reactive towards O2·−. Furthermore, it is shown that Nlr is able to both reduce and dismutate O2·−, thus having a bifunctional reactivity towards O2·−. Kinetic and spectroscopic studies indicate that Nlr's superoxide reductase activity may allow the cell to eliminate O2·− quickly in a NAD(P)H-dependent pathway. On the other hand, Nlr's superoxide dismutation activity will allow the cell to detoxify O2·− independently of the cell redox status. Its superoxide dismutase activity was estimated to be 59 U mg−1 by the xanthine/xanthine oxidase assay at 25°C. Pulse radiolysis studies with the isolated and reduced Nlr proved unambiguously that it has superoxide dismutase activity; at pH 7.1 and 83°C, the rate constant is 5 × 106 M−1 s−1. Besides the superoxide dismutase activity, soluble cell extracts of A. fulgidus also exhibit catalase and NAD(P)H/oxygen oxidoreductase activities. By putting these findings together with the entire genomic data available, a possible oxygen detoxification mechanism in A. fulgidus is discussed.

Microbial Links between Sulfate Reduction and Metal Retention in Uranium- and Heavy Metal-Contaminated Soil

Abstract: Dissimilatory sulfate-reducing bacteria (SRB) play an important role in the sulfur cycle and the mineralization of organic matter in anoxic marine and freshwater environments (53). In addition, sulfate reduction can occur in oxygenated habitats where anoxic niches (8) and the expression of superoxide reductase activity (34) provide protection for SRB against oxygen toxicity. The rate-limiting step of sulfate reduction is catalyzed by the dissimilatory (bi-)sulfite reductase (encoded by the dsrAB gene). Phylogenetic investigations have shown that this key enzyme for sulfate and sulfite respiration was present in early ancestors of modern Bacteria and Archaea (66). Dissimilatory sulfate reduction has been shown to be a terminal-electron-accepting process (TEAP) in acid mine drainage (AMD)-impacted and radionuclide- and metal-contaminated environments. Sulfate-reducing activities as well as SRB abundances show a wide range in these habitats (24, 29, 69). SRB are able to reductively transform metal ions, e.g., uranium and chromium, into insoluble and chemically inert forms via direct enzymatic reduction (41, 42). Sulfide, the end product of microbial sulfate reduction, may further contribute to metal attenuation through reduction of metal oxycations and oxyanions, such as those of uranium and chromium (4, 19), or through precipitation of metal cations as sulfides (15, 20). In addition, SRB have the potential to enhance metal retention via extracellular binding, cellular uptake and accumulation of metals, oxidation/reduction processes, and surface-mediated mineral precipitation (20, 52). Metal stress for SRB in uranium-contaminated sediments (48, 63) and biofilms from Pb-Zn deposits (39) can be reduced by the formation of uraninite and metal sulfides. Previous work in uranium-contaminated environments has emphasized the role of biostimulated SRB in mediating uranium and/or technetium reduction (3, 46, 63), although other metal contaminants are present (55, 63). The long-term stability of immobilized, reduced contaminants is a concern due to the potential for remobilization after carbon addition is stopped. Therefore, it is important to understand alternative remediation processes, such as those involved in natural attenuation. In the former uranium mining district of Ronneburg (Thuringia, Germany), leaching of low-grade black shale by acid mine drainage and sulfuric acid and pyrite oxidation resulted in serious large-scale contamination with heavy metals and radionuclides (28). Metal- and sulfate-enriched seepage waters and surface runoff infiltrated adjacent soils and surface waters, leading to elevated concentrations of sulfate, nickel, copper, cadmium, zinc, arsenic, and uranium in creek bank soils (9). At the Ronneburg site, the presence of high levels of mixed contaminants provides a unique environment to look at complex processes involved in natural attenuation of contaminants. It is hypothesized that resident SRB contribute to natural uranium and heavy metal attenuation at the Ronneburg site. Thus, the objective of this study was to resolve the potential importance of SRB in contaminated creek bank soils both in situ and in biostimulated soil microcosms using stable isotope probing (SIP).

Desulfoferrodoxin: a modular protein.

Abstract: The gene encoding the non-heme iron-containing desulfoferrodoxin from Desulfovibrio vulgaris was cloned in two fragments in order to obtain polypeptides corresponding to the N- and C-terminal domains observed in the tertiary structure. These fragments were expressed in Escherichia coli, purified to homogeneity and biochemically and spectroscopically characterized. Both recombinant fragments behaved as independent metal-binding domains. The N-terminal fragment exhibited properties similar to desulforedoxin, as expected by the presence of a Fe(S-Cys)4 metal binding motif. The C-terminal fragment, which accommodates a Fe(Ne-His)3(Nδ-His)(S-Cys) center, was shown to have properties similar to neelaredoxin, except for the reaction with superoxide. The activities of desulfoferrodoxin and of the expressed C-terminal fragment were tested with superoxide in the presence and absence of cytochrome c. The results are consistent with superoxide reductase activity and a possible explanation for the low superoxide consumption in the superoxide dismutase activity assays is proposed.