| Abstract
| - 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 equatorialFeN4 ligation by four histidine ligands in CN-SOR and the heme in CN-P450cam is directly compared by14N ENDOR, while the axial Fe−CN and Fe−S bonding is probed by 13C ENDOR of the cyanide ligandand 1Hβ ENDOR measurements to determine the spin density delocalization onto the cysteine sulfur. Thereare small, but notable, differences in the bonding between Fe(III) and its ligands in the two enzymes. TheENDOR measurements are complemented by DFT computations that support the semiempirical equationused to compute spin densities on metal-coordinated cysteinyl and shed light on bonding changes as theFe−C−N linkage bends. They further indicate that H bonds to the cysteinyl thiolate sulfur ligand reducethe spin density on the sulfur in both active sites to a degree that exceeds the difference induced by thealternative sets of “in-plane” nitrogen ligands.
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