Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K‑Edge High Resolution X‑ray Absorption Spectroscopy

The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affor...

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Bibliographic Details
Published in:Journal of the American Chemical Society 2019-08, Vol.141 (34), p.13676-13688
Main Authors: Henthorn, Justin T, Arias, Renee J, Koroidov, Sergey, Kroll, Thomas, Sokaras, Dimosthenis, Bergmann, Uwe, Rees, Douglas C, DeBeer, Serena
Format: Article
Language:English
Subjects:
active sites
ammonia
asymmetry
Azotobacter vinelandii - chemistry
Bacterial Proteins - chemistry
chemical reduction
density functional theory
Electrons
fluorescence
geometry
INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
iron
Models, Molecular
Molybdoferredoxin - chemistry
nitrogen
nitrogenase
Protein Conformation
selenium
Selenium - chemistry
sulfur
X-ray absorption spectroscopy
X-Ray Absorption Spectroscopy - methods
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Summary:The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe–Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron–sulfur clusters. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco.
ISSN:0002-7863
1520-5126
1520-5126
DOI:10.1021/jacs.9b06988