title: Sulfur K-edge XAS and DFT calculations on nitrile hydratase: geometric and electronic structure of the non-heme iron active site.
abstract: The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS(-))-, sulfenate (RSO(-))-, and sulfinate (RSO(2)(-))-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO(-) species changes upon protonation as the S-O bond is elongated (by approximately 0.1 A). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe(III) in the active site of NHase as CysS(-), CysSOH, and CysSO(2)(-) both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z(eff) of the Fe and reveals that the Fe in [FeNO](6) NHase species has a Z(eff) very similar to that of its photolyzed Fe(III) counterpart. DFT calculations reveal that this results from the strong pi back-bonding into the pi antibonding orbital of NO, which shifts significant charge from the formally t(2)(6) low-spin metal to the coordinated NO.
authors.affiliation: Department of Chemistry, Stanford University, California 94305, USA.
authors.forename: Abhishek
authors.initials: A
authors.lastname: Dey
authors.forename: Marina
authors.initials: M
authors.lastname: Chow
authors.forename: Kayoko
authors.initials: K
authors.lastname: Taniguchi
authors.forename: Priscilla
authors.initials: P
authors.lastname: Lugo-Mas
authors.forename: Steven
authors.initials: S
authors.lastname: Davin
authors.forename: Mizuo
authors.initials: M
authors.lastname: Maeda
authors.forename: Julie A
authors.initials: JA
authors.lastname: Kovacs
authors.forename: Masafumi
authors.initials: M
authors.lastname: Odaka
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
mesh_terms: D001665:Binding Sites; D006836:Hydro-Lyases / Q000737:chemistry* / Q000378:metabolism; D008956:Models, Chemical; D008958:Models, Molecular; D019159:Nonheme Iron Proteins / Q000737:chemistry* / Q000378:metabolism; D013052:Spectrometry, X-Ray Emission
chemical_list: D019159:Nonheme Iron Proteins; D006836:Hydro-Lyases; C048831:nitrile hydratase