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

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 (O 2 · − ) dismutases. In this work, we show that neelaredoxin (Nlr) is the main O 2 · − 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 O 2 · − . Furthermore, it is shown that Nlr is able to both reduce and dismutate O 2 · − , thus having a bifunctional reactivity towards O 2 · − . Kinetic and spectroscopic studies indicate that Nlr's superoxide reductase activity may allow the cell to eliminate O 2 · − quickly in a NAD(P)H‐dependent pathway. On the other hand, Nlr's superoxide dismutation activity will allow the cell to detoxify O 2 · − 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 × 10 6  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.