Chemistry & Biochemistry Theses and Dissertations
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Item THE INTERPLAY OF SUBSTRATE, PROTEIN AND ITS COFACTOR IN CONTROLLING THE CATALYTIC PROPERTIES OF HUMAN IODOTYROSINE DEIODINASE(2014) Hu, Jimin; Rokita, Steven E; Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Human iodotyrosine deiodinase (hIYD) belongs to nitro-FMN reductase superfamily and is responsible for recycling iodine from the I-Tyr (monoiodotyrosine) and I2-Tyr (diiodotyrosine) formed as byproducts of thyroid hormone synthesis. Heterologous expression of hIYD lacking its membrane domain in E. coli provided large quantities of highly pure hIYD that allowed for its physical and biochemical studies. Its kinetic parameters, binding constants and crystal structure were characterized. Substrate was able to induced dramatic effects on FMN coordination in hIYD, including the zwitterionic region of I-Tyr interacts with the N3 and O4 of the FMN, the OH of Thr239 moved close to N5 of FMN (from 4.8 Å to 3.1Å) and allowed the formation of a hydrogen bond. The aromatic ring of I-Tyr additionally stacks above the FMN. Accumulation of the neutral semiquinone was observed during the reduction of hIYD in the presence of substrate analogue 3-fluoro-L-tyrosine (F-Tyr). In the absence of ligand, only the oxidized and two-electron reduced forms of FMN were observed. Among all of the interactions to FMN in hIYD, H-bonding at the N5 FMN was identified as most important to control the redox of flavin. Mutagenesis of Thr239 to Ala removed this H-bonding as confirmed by the crystal structure of hIYDT239A*F-Tyr. As a result, no semiquinone was detected during the titration of hIYDT239A in the presence of F-Tyr. The deiodination activity of T239A was also dramatically decreased (10-fold). The zwitterion region of the substrate was found critical for binding to enzyme. Modifications of the zwitterion region resulted in at least a 500-fold increase in KD. The catalytic activity was unlikely determined by the zwitterion region since all of the modified substrates exhibited similar initial rates as the native substrate under conditions in which their concentration equaled their KD. The pH dependence of hIYD binding indicated that the phenolic group of I2-Tyr is also critical for binding and the hIYD prefers binding to the phenolate form of substrate. All the results presented in this thesis support the current proposed catalytic mechanism of IYD involving a stepwise electron transfer process.Item The Redox Chemistry of Dirhodium Carboxamidates: From Fundamental Structures to Catalytic Functions.(2008-03-24) Nichols, Jason M.; Doyle, Michael P.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Redox chemistry is the study of molecular structure and function associated with changes in oxidation state. In this manuscript, the structures and functions of dinuclear rhodium complexes in various oxidation states are investigated. In Chapter 1, probing the structural chemistry of dirhodium(II) carboxamidates reveals that an unprecedented, stable, dirhodium(III) complex can be synthesized and characterized. Bis(σ-phenyl)-tetrakis(μ-caprolactamato)dirhodium(III) [Rh2(cap)4Ph2] was prepared from Rh2(cap)4 by a copper catalyzed, aerobic oxidation with aryl transfer from sodium tetraphenylborate. Structural data was obtained by single crystal X-ray diffraction (XRD) of Rh2(cap)4Ph2 and related structures with systematic changes in oxidation state. X-ray photoelectron spectroscopy (XPS) was used to determine binding energies for the rhodium electrons in the complexes. The structural data and XPS binding energies indicate that the Rh-Rh bonding interaction does not exist in Rh2(cap)4Ph2. In Chapter 2, the synthesis of Rh2(cap)4Ph2 was made general by using aryl-boronic acids as the aryl transfer agent. The synthesis provided access to an array of bis(σ-aryl)-Rh2L4 complexes with varying substitution of the aryl ligands. X-ray structures, electrochemical, and computational analysis of complexes with substituents of varying electron-deficiency confirm the Rh Rh bond cleavage. A second-order Jahn-Teller effect is proposed as the basis for the observed Rh-Rh-C bond angle distortions in the X-ray crystal structures. The delocalization of the aromatic π-system through the Rh2-core was investigated and found to be absent, consistent with the calculated electronic structure. The final chapter explores the catalytic redox chemistry of Rh2(cap)4. The mechanism for the oxidative Mannich reaction catalyzed by Rh2(cap)4 in conjunction with tert-butyl hydroperoxide was investigated. This study revealed that iminium ions were formed by the oxidation of N,N-dialkylanilines with the Rh2(cap)4/TBHP system. Rh2(cap)4 was found to be a catalyst for the homolytic decomposition of TBHP to yield the tert-butylperoxyl radical (t BuOO) in a one-electron redox couple. Iminium ions were formed in a stepwise process from N,N-dialkylaniline via rate-limiting, hydrogen atom transfer to t-BuOO followed by rapid electron transfer to excess oxidant in situ. The net hydrogen atom transfer was found to be a step-wise electron transfer/proton transfer between the N,N-dialkylaniline and t BuOO providing evidence for a novel reactivity mode for peroxyl radicals. Nucleophilic capture of the iminium ion to complete the Mannich process was found to occur without association to Rh2(cap)4 under thermodynamic control.