Abstract
Nitrogenases catalyze the reduction of dinitrogen (N2) gas to ammonium at a complex heterometallic cofactor. This most commonly occurs at the FeMo cofactor (FeMoco), a [Mo–7Fe–9S–C] cluster whose exact reactivity and substrate-binding mode remain unknown. Alternative nitrogenases replace molybdenum with either vanadium or iron and differ in reactivity, most prominently in the ability of vanadium nitrogenase to reduce CO to hydrocarbons. Here we report the 1.35-Å structure of vanadium nitrogenase from Azotobacter vinelandii. The 240-kDa protein contains an additional α-helical subunit that is not present in molybdenum nitrogenase. The FeV cofactor (FeVco) is a [V–7Fe–8S–C] cluster with a homocitrate ligand to vanadium. Unexpectedly, it lacks one sulfide ion compared to FeMoco, which is replaced by a bridging ligand, likely a μ-1,3-carbonate. The anion fits into a pocket within the protein that is obstructed in molybdenum nitrogenase, and its different chemical character helps to rationalize the altered chemical properties of this unique N2- and CO-fixing enzyme.
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Acknowledgements
This work was supported by the European Research Council (grant no. 310656 to O.E.) and Deutsche Forschungsgemeinschaft (RTG 1976 and PP 1927 to O.E.) and the BIOSS Centre for Biological Signaling Studies at Albert-Ludwigs-Universität Freiburg (to O.E.). We thank the beamline staff at the Swiss Light Source, Villigen, Switzerland, G. Fritz and A. Brausemann for their excellent assistance with data collection, and S. Andrade for critical reading of the manuscript and helpful discussions.
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D.S. performed the experiments and built and refined the structural model; O.E. designed the experiments, built and refined the structural model and wrote the manuscript.
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Sippel, D., Einsle, O. The structure of vanadium nitrogenase reveals an unusual bridging ligand. Nat Chem Biol 13, 956–960 (2017). https://doi.org/10.1038/nchembio.2428
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DOI: https://doi.org/10.1038/nchembio.2428
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