Spectroscopic and theoretical elucidation of structural contributions to reactivity in binuclear non-heme iron enzymes: Comparison of substrate versus cofactor active sites.

摘要

Binuclear non-heme iron enzymes are found in a variety of biological systems and are used to catalyze a number of critical reactions, including desaturation, ferroxidation, and hydroxlylation. Insight into the structures and mechanisms of these metalloenzymes allows the design of more efficient catalysts to carry out similar tasks, utilization of the enzymes in alternative applications such as bioremediation, and molecular insight into diseases where these enzymes malfunction. In this thesis a combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD are used to probe the Fe(II) binding to non-heme diiron active sites, as well as the zero-field splitting and the exchange-coupling between the irons. These data are then coupled with Density Functional Theory calculations to provide significant insight into how, despite largely conserved ligand motifs around the diiron active sites, geometric and electronic changes in the active sites alter their reactivity. In Methane Monooxygenase and Delta 9 desaturase, the non-heme diiron cofactor site does not react with O 2 until addition of a MMOB or stearoyl-ACP respectively, in contrast to the ribonucleotide reductase active site. CD/MCD studies have shown that in both systems, addition of their respective substrate structurally perturbs the site leading to an increase in reactivity. These spectroscopic studies have been used to calibrate DFT calculations and correlate the observed spectral perturbations with structural changes at the active site, providing an understanding of the necessary features of an O2 reactive site. While the active site ligands of cofactor sites retain a conserved ExxH motif on each iron, diiron substrate sites, in which the irons are released after reaction with O2, exhibit a much weaker ligand field environment. Extension of this CD/MCD methodology to the study of substrate sites found in Maxi and Mini-ferritins, has provided significant insight into the anaerobic Fe(II) binding and overall structure of the substrate ferroxidase sites. In both cases very weak ligand fields with at least two water ligands bind the iron(s), likely contributing to the transient nature of these sites and the divergence in O2 reaction mechanisms.