Crystal Structure of Escherichia coli SsuE: Defining a General Catalytic Cycle for FMN Reductases of the Flavodoxin-like Superfamily Public Deposited

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  • The Escherichia coli sulfur starvation utilization (ssu) operon includes a two-component monooxygenase system made up of an NADPH-dependent FMN reductase, SsuE, and a monooxygenase, SsuD. SsuE is part of the flavodoxin-like superfamily, and we report here the crystal structures of its apo, FMN-bound and FMNH₂-bound forms at ~2 Å resolution. In the crystals, SsuE forms a tetramer that is a dimer of dimers similar to those of seen for homologous FMN-reductases, quinone reductases, and the WrbA family of enzymes. A π-helix present at the tetramer building interface is unique to the reductases from two component monooxygenase systems. Analytical ultracentrifugation studies of SsuE confirm a dimer-tetramer equilibrium exists in solution with FMN binding favoring the dimer. As the active site includes residues from both subunits, at least a dimeric association is required for the function of SsuE. The structures show that one FMN binds tightly in a deeply held site which makes available a second binding site, in which either a second FMN or the nicotinamide of NADPH can bind. The FMNH₂-bound structure shows subtle changes consistent with its weaker binding compared to FMN. Combining this information with published kinetics studies, we propose a general catalytic cycle for two-component reductases of the flavodoxin-like superfamily, by which the enzyme can potentially provide FMNH₂ to its partner monooxygenase by different routes depending on the FMN concentration and the presence of a partner monooxygenase.
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  • Driggers, C. M., Dayal, P. V., Ellis, H. R., & Karplus, P. A. (2014). Crystal structure of Escherichia coli SsuE: Defining a General Catalytic Cycle for FMN reductases of the Flavodoxin-like Superfamily. Biochemistry. 53(21), 3509-3519. doi:10.1021/bi500314f
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  • 53
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  • 21
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  • This project was supported in part by Grant MCB-0545048 to H.R.E. from the National Science Foundation.We thank Drs. Babak Andi and Allen Orville for collecting coordinated spectroscopic and diffraction data on reduced and oxidized FMN-bound crystals at beamline X26-C at the National Synchrotron Light Source (NSLS), supported by contracts DE-AC02-05CH11231 from the Office of Basic Energy Sciences of the U.S. Department of Energy. Synchrotron data were collected at the Advanced Light Source, supported by contract DE-AC02-98CH10886 from the Office of Basic Energy Sciences of the U.S. Department of Energy.
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  • This is an author's peer-reviewed final manuscript, as accepted by the publisher.



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