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A model for dinitrogen binding in the E4 state of nitrogenase

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dc.contributor Háskóli Íslands
dc.contributor University of Iceland
dc.contributor.author Thorhallsson, Albert Th.
dc.contributor.author Benediktsson, Bardi
dc.contributor.author Bjornsson, Ragnar
dc.date.accessioned 2020-06-04T16:15:43Z
dc.date.available 2020-06-04T16:15:43Z
dc.date.issued 2019-10-15
dc.identifier.citation Thorhallsson, A. T., et al. (2019). "A model for dinitrogen binding in the E4 state of nitrogenase." Chemical Science 10(48): 11110-11124.
dc.identifier.issn 2041-6520
dc.identifier.issn 2041-6539 (eISSN)
dc.identifier.uri https://hdl.handle.net/20.500.11815/1878
dc.description Publisher's version (útgefin grein)
dc.description.abstract Molybdenum nitrogenase is one of the most intriguing metalloenzymes in nature, featuring an exotic iron-molybdenum-sulfur cofactor, FeMoco, whose mode of action remains elusive. In particular, the molecular and electronic structure of the N2-binding E4 state is not known. In this study we present theoretical QM/MM calculations of new structural models of the E4 state of molybdenum-dependent nitrogenase and compare to previously suggested models for this enigmatic redox state. We propose two models as possible candidates for the E4 state. Both models feature two hydrides on the FeMo cofactor, bridging atoms Fe2 and Fe6 with a terminal sulfhydryl group on either Fe2 or Fe6 (derived from the S2B bridge) and the change in coordination results in local lower-spin electronic structure at Fe2 and Fe6. These structures appear consistent with the bridging hydride proposal put forward from ENDOR studies and are calculated to be lower in energy than other proposed models for E4 at the TPSSh-QM/MM level of theory. We critically analyze the DFT method dependency in calculations of FeMoco that has resulted in strikingly different proposals for this state. Importantly, dinitrogen binds exothermically to either Fe2 or Fe6 in our models, contrary to others, an effect rationalized via the unique ligand field (from the hydrides) at the Fe with an empty coordination site. A low-spin Fe site is proposed as being important to N2 binding. Furthermore, the geometries of these states suggest a feasible reductive elimination step that could follow, as experiments indicate. Via this step, two electrons are released, reducing the cofactor to yield a distorted 4-coordinate Fe2 or Fe6 that partially activates N2. We speculate that stabilization of an N2-bound Fe(i) at Fe6 (not found for Fe2 model) via reductive elimination is a crucial part of N2 activation in nitrogenases, possibly aided by the apical heterometal ion (Mo or V). By using protons from the sulfhydryl group (to regenerate the sulfide bridge between Fe2 and Fe6) and the nearby homocitrate hydroxy group, we calculate a plausible route to yield a diazene intermediate. This is found to be more favorable with the Fe6-bound model than the Fe2-bound model; however, this protonation is uphill in energy, suggesting protonation of N2 might occur later in the catalytic cycle or via another mechanism.
dc.description.sponsorship RB acknowledges support from the Icelandic Research Fund, Grants No. 141218051 and 162880051 and the University of Iceland Research Fund. The Max Planck society is acknowledged for funding. Serena DeBeer is thanked for support. Some of the computations were performed on resources provided by the Icelandic High Performance Computing Centre at the University of Iceland.
dc.format.extent 11110-11124
dc.language.iso en
dc.publisher Royal Society of Chemistry (RSC)
dc.relation.ispartofseries Chemical Science;10(48)
dc.rights info:eu-repo/semantics/openAccess
dc.subject General Chemistry
dc.subject Nitrogenase
dc.subject Metalloenzymes
dc.subject Efnafræði
dc.subject Efnasambönd
dc.title A model for dinitrogen binding in the E4 state of nitrogenase
dc.type info:eu-repo/semantics/article
dcterms.license This article is Open Access. All publication charges for this article have been paid for by the Royal Society of Chemistry
dc.description.version Peer Reviewed
dc.identifier.journal Chemical Science
dc.identifier.doi 10.1039/c9sc03610e
dc.relation.url http://pubs.rsc.org/en/content/articlepdf/2019/SC/C9SC03610E
dc.contributor.department Raunvísindastofnun (HÍ)
dc.contributor.department Science Institute (UI)
dc.contributor.school Verkfræði- og náttúruvísindasvið (HÍ)
dc.contributor.school School of Engineering and Natural Sciences (UI)

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