Sulfur K-edge XAS and DFT calculations on nitrile hydratase: geometric and electronic structure of the non-heme iron active site.
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.
Dey
Abhishek
A
Department of Chemistry, Stanford University, California 94305, USA.
Chow
Marina
M
Taniguchi
Kayoko
K
Lugo-Mas
Priscilla
P
Davin
Steven
S
Maeda
Mizuo
M
Kovacs
Julie A
JA
Odaka
Masafumi
M
Hodgson
Keith O
KO
Hedman
Britt
B
Solomon
Edward I
EI
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
D019159:Nonheme Iron Proteins; D006836:Hydro-Lyases; C048831:nitrile hydratase