Local Electric Fields as a Natural Switch of Heme-Iron Protein Reactivity

Abstract

Heme-iron oxidoreductases operating through the high-valent Fe<sup>IV</sup>O intermediates perform crucial and complicated transformations, such as oxidations of unreactive saturated hydrocarbons. These enzymes share the same Fe coordination, only differing by the axial ligation, e.g., Cys in P450 oxygenases, Tyr in catalases, and His in peroxidases. By examining ~200 heme-iron proteins, we show that the protein hosts exert highly specific intramolecular electric fields on the active sites, and there is a strong correlation between the direction and magnitude of this field and the protein function. In all heme proteins, the field is preferentially aligned with the Fe‒O bond (<b><i>F<sub>z</sub></i></b>). The Cys-ligated P450 oxygenases have the highest average <b><i>F<sub>z</sub></i></b> of 28.5 MV cm<sup>-1</sup>, i.e., most enhancing the oxyl-radical character of the oxo group, and consistent with the ability of these proteins to activate strong C‒H bonds. In contrast, in Tyr-ligated proteins, the average <b><i>F<sub>z</sub></i></b> is only 3.0 MV cm<sup>-1</sup>, apparently suppressing single-electron off-pathway oxidations, and in His-ligated proteins, <b><i>F<sub>z</sub></i></b> is –8.7 MV cm<sup>-1</sup>. The operational field range is given by the trade-off between the low reactivity of the Fe<sup>IV</sup>O Compound I at the more negative <b><i>F<sub>z</sub></i></b>, and the low selectivity at the more positive <b><i>F<sub>z</sub></i></b>. Consequently, a heme-iron site placed in the field characteristic of another heme-iron protein class loses its canonical function, and gains an adverse one. Thus, electric fields produced by the protein scaffolds, together with the nature of the axial ligand, control all heme-iron chemistry.

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate via a Preorganized Secondary Coordination Sphere

“…Enzyme active sites leverage effects such as hydrogen bonding, electron transfer pathways, and electrostatic effects to precisely tune the reactivity of metallocofactors. [1][2][3][4][5][6][7][8][9][10][11] One example illustrating the importance of the secondary coordination sphere is in oxidase chemistry. For example, the terminal oxidant in cytochrome P450 enzymes, Compound I, consists of an Fe(IV)-oxo which is generated from O2 via the delivery of proton and electron equivalents from cofactors and the protein superstructure.…”

“…Single crystals suitable for XRD were grown via vapor diffusion of petroleum ether into a concentrated solution of product in toluene overnight at room temperature. 1 Reactivity with PPh3, DHA, and diphenylhydrazene (DPH). A 0.35 mM solution of 1 in toluene was prepared in the glovebox in an air-tight cuvette with a septa.…”