Superoxide reductase: fact or fiction?

It was recently proposed that anaerobic microorganisms contain a new pathway for detoxification of reactive oxygen species. This is centered around a novel mononuclear iron-containing enzyme, superoxide reductase (SOR), which catalyzes the reduction, rather than the dismutation, of superoxide to hydrogen peroxide. A surprisingly large amount of relevant data has accumulated in the two years or so since the proposal was made. Herein we address the questions: to what extent has the pathway been validated, and what fundamental issues have yet to be answered in considering the response of anaerobes to reactive oxygen species? The evidence for superoxide reduction by SOR is now overwhelming and comes from a variety of anaerobic and microaerophilic species. Moreover, the available spectroscopic and structural information provide a convincing case that the catalytic Fe site of SOR is structurally and electronically tuned to mediate superoxide reduction rather than oxidation. Kinetic analyses also support the original proposal of NAD(P)H, via rubredoxin and NAD(P)H:rubredoxin oxidoreductase, as the source of reductant. What is still to be determined is the fate of the peroxide generated by the SOR reaction. In particular, the role of otherwise well-characterized proteins like rubrerythrin, NADH peroxidase, and rubredoxin:oxygen oxidoreductase in "anaerobic" oxygen metabolism has yet to be established.