Yang, Zhi-YongJimenez-Vicente, EmilioKallas, HaydenLukoyanov, Dmitriy A.Yang, HaoDel Campo, Julia S. MartinDean, Dennis R.Hoffman, Brian M.Seefeldt, Lance C.2021-08-182021-08-182021-03-292041-6520d0sc06561g (PII)http://hdl.handle.net/10919/104671The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V3+, 3Fe3+, 4Fe2+]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined 51V (I = 7/2) hyperfine splitting, aiso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V3+, 4Fe3+, 3Fe2+]. Our findings suggest that the V3+ does not change valency throughout the catalytic cycle.Pages 6913-692210 page(s)application/pdfenCreative Commons Attribution 4.0 InternationalPhysical SciencesChemistry, MultidisciplinaryChemistry03 Chemical SciencesArticle - Refereed2021-08-18https://doi.org/10.1039/d0sc06561g1220Dean, Dennis [0000-0001-8960-6196]34123320 (pubmed)2041-6539