The superoxide reductase from the early diverging eukaryote Giardia intestinalis

Unlike superoxide dismutases (SODs), superoxide reductases (SORs) eliminate superoxide anion (O ₂ ^•-) not through its dismutation, but via reduction to hydrogen peroxide (H ₂O ₂) in the presence of an electron donor. The microaerobic protist Giardia intestinalis, responsible for a common intestinal disease in humans, though lacking SOD and other canonical reactive oxygen species-detoxifying systems, is among the very few eukaryotes encoding a SOR yet identified. In this study, the recombinant SOR from Giardia (SOR _Gi) was purified and characterized by pulse radiolysis and stopped-flow spectrophotometry. The protein, isolated in the reduced state, after oxidation by superoxide or hexachloroiridate(IV), yields a resting species (T _final) with Fe ³⁺ ligated to glutamate or hydroxide depending on pH (apparent pK _a = 8.7). Although showing negligible SOD activity, reduced SOR _Gi reacts with O ₂ ^•- with a pH-independent second-order rate constant k ₁ = 1.0 × 10 ⁹ M ^{- 1} s ^{- 1} and yields the ferric-(hydro)peroxo intermediate T ₁; this in turn rapidly decays to the T _final state with pH-dependent rates, without populating other detectable intermediates. Immunoblotting assays show that SOR _Gi is expressed in the disease-causing trophozoite of Giardia. We propose that the superoxide-scavenging activity of SOR in Giardia may promote the survival of this air-sensitive parasite in the fairly aerobic proximal human small intestine during infection.