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

Abstract

Conspectus Mononuclear non-heme Fe (NHFe) enzymes play key roles in antibiotic biosynthesis, hypoxic response, DNA repair, anticancer therapy and many other biological processes. On a molecular level 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. These ferrous active sites had been inaccessible to traditional spectroscopic methods. A methodology has been developed that provides detailed geometric and electronic structure insight for these NHFeII active sites. This has defined a general mechanistic strategy utilized by a wide range of these enzymes to control O2 activation by FeII coordination unsaturation only in the presence of cosubstrates to limit autooxidation and self-hydroxylation. Depending on the type of enzyme, O2 activation either involves a 2e− reduced FeIII–OOH intermediate or a 4e− reduced FeIV=O intermediate. The nature of these intermediates has been defined in terms of geometric structure using nuclear resonance vibrational spectroscopy (NRVS) and electronic structure using magnetic circular dichroism (MCD) to define the frontier molecular orbitals (FMOs) that control reactivity. For FeIII–OOH intermediates the anticancer drug Activated Bleomycin is shown to be the non-heme Fe analog of compound 0 in heme (e.g. P450) chemistry but undergoes different reactivity where the low-spin (LS) FeIII–OOH can directly abstract an H atom from DNA. It is also shown that the transition states of LS and high-spin (HS) FeIII–OOH are fundamentally different in that the former goes through a hydroxyl radical while the latter is activated for EAS without O-O cleavage, which is important in one class of NHFe enzymes that utilizes a HS FeIII–OOH intermediate in dioxygenation. For FeIV=O intermediates the LS form is shown to have a π-type FMO activated for attack perpendicular to the Fe–O bond while the HS form (present in the NHFe enzymes) has both π and σ FMOs that are activated for attack both perpendicular to and along the Fe–O bond, respectively. For the NHFe enzymes these π vs σ FMOs direct reactivity for EAS vs H-atom abstraction, and for the latter halogenation vs hydroxylation. This study emphasizes that experimental spectroscopy is critical in evaluating the results of electronic structure calculations and thus key to bridging structure and reactivity with mechanism.

Mono- and binuclear non-heme iron chemistry from a theoretical perspective

“…[6,136,137,138,139,140] It was demonstrated that three different mechanisms exist for HAA, depending on the spin state of the complex (S = 1 vs. S = 2). [81,141,142] The triplet state, as the ground spin state of many synthetic NHFe IV =O compounds, effectively operates only through a " channel" with the C─H bond ideally oriented perpendicular to the Fe─oxo axis that allows the overlap of the substrate C─H orbital with one of the Fe=O d* FMOs.…”

“…[5] Typically, these enzymes contain mono-or binuclear iron sites (NHFe and NHFe2) with (O,N)-containing ligands and catalyze a broad set of oxidation reactions including H-atom abstraction for hydroxylation, halogenation, desaturation, peroxidation, ring closure of a substrate, electrophilic aromatic substitution for mono-or dioxygenation, or even phosphate-bond hydrolysis. [5,6] It is not the purpose of this minireview to provide an exhaustive report on non-heme iron chemistry and spectroscopy which can be found in extensive reviews (e.g. Ref.…”

NRVS Studies of the Peroxide Shunt Intermediate in a Rieske Dioxygenase and Its Relation to the Native FeII O2 Reaction

“…3 These NHFe II enzymes are divided into subclasses based on the source of the electrons used for the multielectron reduction of O 2 , the nature of the substrate, and whether a cofactor is required. 2 There has been much interest in characterizing the activated O 2 intermediates responsible for reacting with substrate. The most well-defined O 2 intermediates are S = 2 Fe IV =O intermediates, 6 which have been trapped in the pter-independent hydroxylases 7 and the α-ketoglutarate-dependent enzymes.…”

Detection of Indistinct Fe−N Stretching Bands in Iron(V) Nitrides by Photodissociation Spectroscopy

“…[1][2][3][4][5] The spectroscopic data on iron-oxo compounds, including the FeÀO stretching frequency, are well established; [6,7] however, the information about FeÀN bonds is still scarce. Spectral signatures of the FeÀN bond depend on both the oxidation state of iron centre and the symmetry of the surrounding ligand field.…”