Non-heme iron enzymes: contrasts to heme catalysis

Abstract

Non-heme iron enzymes catalyze a wide range of O(2) reactions, paralleling those of heme systems. Non-heme iron active sites are, however, much more difficult to study because they do not exhibit the intense spectral features characteristic of the porphyrin ligand. A spectroscopic methodology was developed that provides significant mechanistic insight into the reactivity of non-heme ferrous active sites. These studies reveal a general mechanistic strategy used by these enzymes and differences in substrate and cofactor interactions dependent on their requirement for activation by iron. Contributions to O(2) activation have been elucidated for non-heme relative to heme ligand sets, and major differences in reactivity are defined with respect to the heterolytic and homolytic cleavage of O-O bonds.

Figure 1

Electronic structures/spectra of ferrous sites. (A) Ligand field theory predictions for different geometries. (B) LT MCD spectra of corresponding structures.

Figure 2

VTVH MCD. (A) MCD signal increases with increasing H. (B) Overlaying saturation magnetization curves for S = 1/2 Kramers system taken at different temperatures. (C) Nesting saturation magnetization curves for S = 2 non-Kramers ions at different temperatures.

Figure 3

VTVH MCD of PAH. (A) Effects of substrate (blue), cofactor (green), and both substrate + cofactor (red) binding to the resting Fe^II site. (B and C) Saturation magnetization of resting (black) and substrate + cofactor (red) bound sites. (D) Experimentally determined energy level diagram.

Scheme 1

General mechanistic strategy.

Figure 4

LT MCD of CS2. (A) α-KG cofactor (green) or substrate (blue) binding. (B) Both cofactor and substrate (three) binding.

Scheme 2

Fe^II⩵OH₂ bond contribution to hydroxylation vs. desaturation.

Figure 5

Heterolytic and homolytic cleavage of the O—O bond of ABLM.

Figure 6

Fe^IV⩵O bonding scheme.