Purification, crystallization and X-ray crystallographic analysis of Archaeoglobus fulgidus neelaredoxin

Neelaredoxins are a type of superoxide reductase (SOR), which are blue 14 kDa metalloproteins with a catalytic nonhaem iron centre coordinated by four histidines and one cysteine in the ferrous form. Anaerobic organisms such as Archaeoglobus fulgidus, a hyperthermophilic sulfate-reducing archaeon, have developed defence mechanisms against toxic oxygen species in which superoxide reductases play a key role. SOR is responsible for scavenging toxic superoxide anion radicals (O₂^·-), catalysing the one-electron reduction of superoxide to hydrogen peroxide. Crystals of recombinant A. fulgidus neela­redoxin in the oxidized form (13.7 kDa, 125 residues) were obtained using polyethylene glycol and ammonium sulfate. These crystals diffracted to 1.9 Å resolution and belonged to the tetragonal space group P4₁2₁2, with unit-cell parameters a = b = 75.72, c = 185.44 Å. Cell-content analysis indicated the presence of a tetramer in the asymmetric unit, with a Matthews coefficient (V_M) of 2.36 Å³ Da^-1 and an estimated solvent content of 48%. The three-dimensional structure was determined by the MAD method and is currently under refinement.