Comparing the Electronic Properties of the Low-Spin Cyano−Ferric [Fe(N4)(Cys)] Active Sites of Superoxide Reductase and P450cam Using ENDOR Spectroscopy and DFT Calculations

• Published in: Journal of the American Chemical Society Vol. 128; no. 51; pp. 16566 - 16578
• Main Authors: Yang, Tran-Chin, McNaughton, Rebecca L, Clay, Michael D, Jenney, Francis E, Krishnan, Rangan, Kurtz, Donald M, Adams, Michael W. W, Johnson, Michael K, Hoffman, Brian M
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 27.12.2006
• Subjects: Binding Sites; Camphor 5-Monooxygenase - chemistry; Chemistry; Crystallography, X-Ray; Electron Spin Resonance Spectroscopy - methods; Electrons; Exact sciences and technology; Ferric Compounds - chemistry; General and physical chemistry; Hydrogen Bonding; Kinetics and mechanisms; Models, Chemical; Models, Molecular; Molecular Conformation; Organic chemistry; Oxidoreductases - chemistry; Reactivity and mechanisms; Sensitivity and Specificity; Stereoisomerism; Cysteine; Nitrile; Nitrogen heterocycle; Ligand; Active site; Thiolate; Molecular interaction; Spin density; Iron; Organic sulfide; Heme; Reaction mechanism; Semiempirical method; Carbon 13; Electronic properties; Catalytic reaction; Enzyme; ENDOR spectrometry; Theoretical study; Transition metal; Electron delocalization; Sulfur containing aminoacid; Hyperoxides; Density functional method; Oxidoreductases; Hydrogen bond; Reductase
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja064656p

Superoxide reductase (SOR) and P450 enzymes contain similar [Fe(N)4(SCys)] active sites and, although they catalyze very different reactions, are proposed to involve analogous low-spin (hydro)peroxo-Fe(III) intermediates in their respective mechanisms that can be modeled by cyanide binding. The equatorial FeN4 ligation by four histidine ligands in CN-SOR and the heme in CN-P450cam is directly compared by 14N ENDOR, while the axial Fe−CN and Fe−S bonding is probed by 13C ENDOR of the cyanide ligand and 1Hβ ENDOR measurements to determine the spin density delocalization onto the cysteine sulfur. There are small, but notable, differences in the bonding between Fe(III) and its ligands in the two enzymes. The ENDOR measurements are complemented by DFT computations that support the semiempirical equation used to compute spin densities on metal-coordinated cysteinyl and shed light on bonding changes as the Fe−C−N linkage bends. They further indicate that H bonds to the cysteinyl thiolate sulfur ligand reduce the spin density on the sulfur in both active sites to a degree that exceeds the difference induced by the alternative sets of “in-plane” nitrogen ligands.