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Scavenging of superoxide by a membrane-bound superoxide oxidase


Superoxide is a reactive oxygen species produced during aerobic metabolism in mitochondria and prokaryotes. It causes damage to lipids, proteins and DNA and is implicated in cancer, cardiovascular disease, neurodegenerative disorders and aging. As protection, cells express soluble superoxide dismutases, disproportionating superoxide to oxygen and hydrogen peroxide. Here, we describe a membrane-bound enzyme that directly oxidizes superoxide and funnels the sequestered electrons to ubiquinone in a diffusion-limited reaction. Experiments in proteoliposomes and inverted membranes show that the protein is capable of efficiently quenching superoxide generated at the membrane in vitro. The 2.0 Å crystal structure shows an integral membrane di-heme cytochrome b poised for electron transfer from the P-side and proton uptake from the N-side. This suggests that the reaction is electrogenic and contributes to the membrane potential while also conserving energy by reducing the quinone pool. Based on this enzymatic activity, we propose that the enzyme family be denoted superoxide oxidase (SOO).

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This work was supported by grants from the Knut and Alice Wallenberg foundation Wallenberg Academy Fellows (2012.0233), the Swedish Foundation For Strategic Research (FFL09-0008) and the Swedish Research Council (2017-04018) to M.H., and the Swiss National Science Foundation to C.v.B. (153351). The help of H. Luidalepp with RNA extraction from E. coli and A. Eberle (University of Bern) with RT–qPCR and data analysis is greatly acknowledged. The authors wish to thank the staff of beamline X06SA at the Swiss Light Source for their support with data collection.

Author information

D.S. and M.H. conceived the study. C.A.K.L., D.S., M.B. and M.H. solved the structure. C.A.K.L., D.S., O.B., P.B., C.v.B. and M.H. designed functional experiments and analyzed data. O.B., C.A.K.L., A.-L.J. and C.v.B. performed functional experiments. A.R. and J.C. performed and analyzed docking experiments. C.v.B. and M.H. wrote the manuscript with input from all authors.

Competing interests

The authors declare no competing interests.

Correspondence to Dan Sjöstrand or Christoph von Ballmoos or Martin Högbom.

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Fig. 1: CybB reacts with superoxide and ubiquinol.
Fig. 2: CybB exhibits superoxide-oxidase activity in vitro.
Fig. 3: High-resolution structure of CybB.
Fig. 4: Hypothetical functional context of SOO.