Geometric and Electronic Structure Contributions to Function in Non-heme Iron Enzymes

Buy research chemicals – Mononuclear non-heme Fe (NHFe) enzymes play key roles in DNA repair, the biosynthesis of antibiotics, the response to hypoxia, cancer therapy, and many other biological processes. These enzymes catalyze a diverse range of oxidation reactions, including hydroxylation, halogenation, ring closure, desaturation, and electrophilic aromatic substitution (EAS).

Most of these enzymes use an FeII site to activate dioxygen, but traditional spectroscopic methods have not allowed researchers to insightfully probe these ferrous active sites. We have developed a methodology that provides detailed geometric and electronic structure insights into these NHFeII active sites. Using these data, we have defined a general mechanistic strategy that many of these enzymes use: they control O2 activation (and limit autoxidation and self-hydroxylation) by allowing FeII coordination unsaturation only in the presence of cosubstrates.

Depending on the type of enzyme, O2 activation either involves a 2e reduced FeIII–OOH intermediate or a 4e reduced FeIV═O intermediate. Nuclear resonance vibrational spectroscopy (NRVS) has provided the geometric structure of these intermediates, and magnetic circular dichroism (MCD) has defined the frontier molecular orbitals (FMOs), the electronic structure that controls reactivity. This Account emphasizes that experimental spectroscopy is critical in evaluating the results of electronic structure calculations. Therefore these data are a key mechanistic bridge between structure and reactivity.

Research Chemicals – For the FeIII–OOH intermediates, the anticancer drug activated bleomycin (BLM) acts as the non-heme Fe analog of compound 0 in heme (e.g., P450) chemistry. However BLM shows different reactivity: the low-spin (LS) FeIII–OOH can directly abstract a H atom from DNA. The LS and high-spin (HS) FeIII–OOHs have fundamentally different transition states. The LS transition state goes through a hydroxyl radical, but the HS transition state is activated for EAS without O–O cleavage. This activation is important in one class of NHFe enzymes that utilizes a HS FeIII–OOH intermediate in dioxygenation.

For FeIV═O intermediates, the LS form has a π-type FMO activated for attack perpendicular to the Fe–O bond. However, the HS form (present in the NHFe enzymes) has a π FMO activated perpendicular to the Fe–O bond and a σ FMO positioned along the Fe–O bond. For the NHFe enzymes, the presence of π and σ FMOs enables enzymatic control in determining the type of reactivity: EAS or H-atom extraction for one substrate with different enzymes and halogenation or hydroxylation for one enzyme with different substrates.