Heme-dependent Tryptophan Oxidation: Mechanistic Studies on Tryptophan 2,3-Dioxygenase and MauG

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

Tryptophan-mediated charge-resonance stabilization in the bis-Fe(IV) redox state of MauG

The diheme enzyme MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor protein to generate a tryptophan tryptophylquinone cofactor. The MauG-catalyzed reaction proceeds via a bis-Fe(IV) intermediate in which one heme is present as Fe(IV)=O and the other as Fe(IV) with axial histidine and tyrosine ligation. Herein, a unique near-infrared absorption feature exhibited specifically in bis-Fe(IV) MauG is described, and evidence is presented that it results from a charge-resonance-transition phenomenon. As the two hemes are physically separated by 14.5 angstrom, a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes is reversibly oxidized and reduced to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(IV) MauG. Analysis of the MauG structure reveals that electron transfer via this mechanism is rapid enough to enable a charge-resonance stabilization of the bis-Fe(IV) state without direct contact between the hemes. The finding of the charge-resonance-transition phenomenon explains why the bis-Fe(IV) intermediate is stabilized in MauG and does not permanently oxidize its own aromatic residues

Inhibition of PPARγ by BZ26, a GW9662 derivate, attenuated obesity-related breast cancer progression by inhibiting the reprogramming of mature adipocytes into to cancer associate adipocyte-like cells

Obesity has been associated with the development of 13 different types of cancers, including breast cancer. Evidence has indicated that cancer-associated adipocytes promote the proliferation, invasion, and metastasis of cancer. However, the mechanisms that link CAAs to the progression of obesity-related cancer are still unknown. Here, we found the mature adipocytes in the visceral fat of HFD-fed mice have a CAAs phenotype but the stromal vascular fraction of the visceral fat has not. Importantly, we found the derivate of the potent PPARγ antagonist GW9662, BZ26 inhibited the reprogramming of mature adipocytes in the visceral fat of HFD-fed mice into CAA-like cells and inhibited the proliferation and invasion of obesity-related breast cancer. Further study found that it mediated the browning of visceral, subcutaneous and perirenal fat and attenuated inflammation of adipose tissue and metabolic disorders. For the mechanism, we found that BZ26 bound and inhibited PPARγ by acting as a new modulator. Therefore, BZ26 serves as a novel modulator of PPARγ activity, that is, capable of inhibiting obesity-related breast cancer progression by inhibiting of CAA-like cell formation, suggesting that inhibiting the reprogramming of mature adipocytes into CAAs or CAA-like cells may be a potential therapeutic strategy for obesity-related cancer treatment

Enhanced efficiency in Concentrated Parabolic Solar Collector (CPSC) with a porous absorber tube filled with metal nanoparticle suspension

In this study, effects of different nanoparticles and porosity of absorber tube on the performance of a Concentrating Parabolic Solar Collector (CPSC) were investigated. A section of porous-filled absorber tube was modeled as a semi-circular cavity under the solar radiation which is filled by nanofluids and the governing equations were solved by FlexPDE numerical software. The effect of four physical parameters, nanoparticles type, nanoparticles volume fraction (φ), Darcy number (Da) and Rayleigh number (Ra), on the Nusselt number (Nu) was discussed. It turns out that Cu nanoparticle is the most suitable one for such solar collectors, compared to the commonly used Fe3O4, Al2O3, TiO2. With the increased addition of Cu nanoparticles all the parameters φ, Da and Ra shows a significant increase against the Nu, indicates the enhanced heat transfer in such cases. As a result, low concentration of Cu nanoparticle suspension combined with porous matrix was supposed to be beneficial for the performance enhancement of concentrating parabolic solar collector. Keywords: Concentrating parabolic solar collector, Porous absorber tube, Nanofluid, Nusselt number, Finite Element Metho

Simulation of OH– and oxygen transport in the air–cathode catalyst layer of microbial fuel cells

The mass transfer of OH– and oxygen within an air–cathode is critical for the performance of microbial fuel cells (MFCs). Improving the understanding of this complex transportation mechanism could help guide the design of air-cathodes to enhance the power density of MFCs. Herein, a 2D-agglomerate model is developed to study OH– and O2 transfer, and electrochemical performance within an air–cathode MFC. The effects of key variables (binder volume fraction, and Pt/carbon mass ratio) on OH– and oxygen transport behavior have been investigated. Simulation results reveal that the OH– and oxygen concentrations within the catalyst layer are closely related to the catalyst layer structure. A lower volume fraction of Nafion® binder is beneficial to OH– and oxygen transfer, while the Pt/carbon mass ratio has complex effects on OH– and oxygen transfer and reactions. This work aims to improve our understanding of OH– and oxygen mass transfer and offers an effective approach to constructing high-performance air–cathode MFCs