Superoxide reduction by Nanoarchaeum equitans neelaredoxin, an enzyme lacking the highly conserved glutamate iron ligand.

Superoxide reductases (SORs) are antioxidant enzymes present in many prokaryotes, either anaerobes or microaerophiles, which detoxify superoxide by reducing it to hydrogen peroxide. The reaction mechanism involves the diffusion-limited encounter of superoxide with the reduced iron site and concomitant formation of an Fe(3+)-(hydro)peroxo adduct that, upon protonation, leads to the formation of hydrogen peroxide. By the end of this process, a glutamate residue coordinates the ferric ion, acting as a sixth ligand. Although this residue is able to shuttle protons to the intermediate at low pH, it seems to have a minor relevance to the overall reduction mechanism. Nevertheless, this ligand is conserved in most SORs known thus far, with the notable exception of neelaredoxin from Nanoarchaeum equitans. The protein of this organism was cloned and overexpressed, and its spectroscopic characterization revealed distinct pH-equilibrium properties in comparison with those of glutamate-containing SORs. A three-dimensional model of this protein was generated in an effort to identify structural properties that could explain these distinct features. Pulse radiolysis measurements showed that the efficiency of this enzyme in reducing superoxide is comparable to that of glutamate-containing SORs, thus definitely ruling out the requirement for such a ligand in the reduction mechanism.