Date of Award

Fall 12-17-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Aimin Liu

Second Advisor

Ivaylo Ivanov

Third Advisor

Jun Yin

Abstract

Hemoenzymes are prevalent in nature and participate in a wide range of biological activities. Frequently, high-valence iron intermediates are involved in the catalytic events of these enzymes, especially when the activation of peroxide or dioxygen is involved. Building on the fundamental framework of iron-oxygen chemistry, the mechanistic understandings of these enzymes and their reactive intermediates constantly attract attention from the enzymology community.

This dissertation work focused on the mechanistic studies on two hemoenzymes, tryptophan 2,3-dioxygenase (TDO) and MauG, both of which catalyze unique chemical transformations, i.e., tryptophan oxidation. TDO and MauG are structurally distinct from each other; they catalyze different types of oxidization reactions on tryptophan via diverse strategies. TDO catalyzes the ring-cleavage dioxygenation reaction of free L-tryptophan, incorporating both oxygen atoms from dioxygen into the substrate. MauG uses hydrogen peroxide as the oxidant to catalyze a complex posttranslational-modification reaction on two tryptophan residues from a protein substrate. It utilizes radical chemistry to perform a 40-Å long-range catalytic event. Despite the differences in their catalytic behavior, both enzymes are suggested to employ high-valence iron intermediates in their reactions.

A collection of biochemical and spectroscopic approaches was employed to obtain detailed insight into the electronic and structural contributions to the formation and stabilization of high-valence iron intermediates, and into the heme-dependent tryptophan-oxidizing mechanisms. In the study TDO, we solved the long-standing mystery of how the active Fe(II) enzyme is generated from the resting Fe(III) form by hydrogen peroxide. A peroxide-driven reactivation mechanism was established based on the identification of a compound ES-type ferryl intermediate. Additional efforts were dedicated to clarify the controversy in the literature regarding the catalytic roles of a distal histidine residue. Chemical-rescue approaches were used to specify the catalytic contributions of the target residue. In the study of MauG, we discovered an unprecedented tryptophan-mediated charge-resonance phenomenon in the bis-Fe(IV) redox state. This discovery provides the molecular basis for the chemical reactivity and stability of the catalytically competent bis-Fe(IV) intermediate. Together with our collaborators, we also outlined the mechanism of the MauG-mediated long-range catalysis by identifying the catalytic functions of several important residues along the reaction pathway.

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