Reaction of the Desulfoferrodoxin from Desulfoarculus baarsii with Superoxide Anion

Abstract

Desulfoferrodoxin is a small protein found in sulfate-reducing bacteria that contains two independent mononuclear iron centers, one ferric and one ferrous. Expression of desulfoferrodoxin from <i>Desulfoarculus baarsii</i> has been reported to functionally complement a superoxide dismutase deficient<i>Escherichia coli</i> strain. To elucidate by which mechanism desulfoferrodoxin could substitute for superoxide dismutase in <i>E. coli</i>, we have purified the recombinant protein and studied its reactivity toward O⨪₂. Desulfoferrodoxin exhibited only a weak superoxide dismutase activity (20 units mg⁻¹) that could hardly account for its antioxidant properties. UV-visible and electron paramagnetic resonance spectroscopy studies revealed that the ferrous center of desulfoferrodoxin could specifically and efficiently reduce O⨪₂, with a rate constant of 6–7 × 10⁸m⁻¹ s⁻¹. In addition, we showed that membrane and cytoplasmic <i>E. coli</i> protein extracts, using NADH and NADPH as electron donors, could reduce the O⨪₂oxidized form of desulfoferrodoxin. Taken together, these results strongly suggest that desulfoferrodoxin behaves as a superoxide reductase enzyme and thus provide new insights into the biological mechanisms designed for protection from oxidative stresses.