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Ferritin: Mechanistic Studies and Electron Transfer Properties

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Brigham Young University BYU ScholarsArchive Theses and Dissertations 2006-08-08 Ferritin: Mechanistic Studies and Electron Transfer Properties Bo Zhang Brigham Young University - Provo Follow this and additional works at: https://scholarsarchive.byu.edu/etd Part of the Biochemistry Commons, and the Chemistry Commons BYU ScholarsArchive Citation Zhang, Bo, "Ferritin: Mechanistic Studies and Electron Transfer Properties" (2006). Theses and Dissertations. 766. https://scholarsarchive.byu.edu/etd/766 This Dissertation is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. FERRITIN: MECHANISTIC STUDIES AND ELECTRON TRANSFER PROPERTIES by Bo Zhang A dissertation submitted to the faculty of Brigham Young University In partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemistry and Biochemistry Brigham Young University August 2006 BRIGHAM YOUNG UNIVERSITY GRADUATE COMMITTEE APPROVAL of a dissertation submitted by Bo Zhang This dissertation has been read by each member of the following graduate committee and by majority vote has been found to be satisfactory. ________________________________ ________________________________ Date Gerald D. Watt, Chair ________________________________ ________________________________ Date Roger G. Harrison ________________________________ ________________________________ Date Barry M. Willardson ________________________________ ________________________________ Date Craig D. Thulin ________________________________ ________________________________ Date S. Scott Zimmerman ii BRIGHAM YOUNG UNIVERSITY As chair of the candidate’s graduate committee, I have read the dissertation of Bo Zhang in its final form and have found that (1) its format, citations, and bibliographical style are consistent and acceptable and fulfill university and department style requirements; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the final manuscript is satisfactory to the graduate committee and is ready for submission to the university library. ________________________________ ________________________________ Date Gerald D. Watt Chair, Graduate Committee Accepted for the Department ________________________________ Paul B. Farnsworth Department Chair Accepted for the College ________________________________ G. Rex Bryce Associate Dean, College of Physical and Mathematical Sciences iii ABSTRACT FERRITIN: MECHANISTIC STUDIES AND ELECTRON TRANSFER PROPERTIES Bo Zhang Department of Chemistry and Biochemistry Doctor of Philosophy Ferritins are ubiquitous iron storage proteins in living systems. Although much is known about the iron deposition process in ferritin and a mechanism has been developed, several important issues still remain unknown. One lingering question is the less than stoichiometric quantities of hydrogen peroxide detected in previous studies on animal ferritins. Extensive experimental data on identifying the species in competition for peroxide equivalents point to a surprising conclusion that H2O2 generated in the ferroxidase reaction is consumed by amine buffers that are commonly employed in in vitro ferritin studies, while non-nitrogen containing buffers, such as acetate, phosphate, and carbonate, do not react with H2O2. The effects of amine buffer oxidation on the 2+ Fe /O2 stoichiometry, the kinetics and the molecular mechanism of iron deposition are discussed. iv The ~2 nm ferritin shell surrounding the ~4000 Fe(O)OH mineral core was originally thought to isolate the core from the environment. However, synthesized Co- and Mn(O)OH cores in horse spleen and bacteria ferritins are shown to be rapidly reduced by ascorbic acid and horse spleen ferritin containing a reduced Fe(II) core (Fe(II)-HoSF) presumably without direct contact. Further experiments demonstrate that both Fe(II)-HoSF and Co-/Mn-ferritins bind to gold electrodes and exchange electrons through the metallic conductor. These results provide the first direct evidence for electron transfer (ET) through the ferritin shell. The nature of the ET pathway is further investigated by loading iron into native and recombinant ferritins using large oxidants that are too big to enter the ferritin interior and must accept electrons from Fe2+ through this pathway. Experimental results suggest that the endogenous redox center in heteropolymeric animal ferritins and the heme groups in bacteria ferritins mediate ET through the protein shell. Finally, the diffusion properties of ferritin pores are examined toward iron (2+ and 3+) and anion transfer. Iron transfer is studied by the formation of Prussian blue II III - ([Fe Fe (CN)6] ) encapsulated in the ferritin cavity, and is consistent with a binding- dissociation model proposed previously for iron transfer through the three-fold channels. When native HoSF is reduced by methyl viologen in saline solutions, small anions such as F-, Cl-, and Br-, accumulate in the ferritin interior while phosphate is released. No anion transfer is observed during the reduction of reconstituted HoSF with no phosphate in the core. The possibility of ferritin as an anion pump in vivo is proposed. v ACKNOWLEDGMENTS I am deeply grateful for Dr. Gerald D. Watt for his mentorship during my graduate studies. Through his example and teaching he has not only helped me become an insightful scientist but also a more open-minded human being. I hope to use what I have learned from him to benefit others in my future career. I appreciate the valuable assistance from my graduate committee. Their insightful questions and suggestions have assisted my approach and analysis of my research. Thanks in particular to Dr. John N. Harb of Department of Chemical Engineering and Dr. Richard K. Watt at BYU. I am also indebted to several graduate and undergraduate students I have worked with: Dr. Phillip Wilson, Dr. Tim Miller, Dr. Tom Lowry, Ammon Poesy, Jason Knealey, and Jared Bunker, Garrett Davis, and Kyrsten Crawford. I would like to thank sources of funding during my stay including NASA and Department of Chemistry & Biochemistry at BYU. The department has provided me the opportunity to be a graduate student and supported me with tuition, teaching assistantships, and fellowships. Finally, to my family and most importantly, to my wife, Li Li, I express my deepest appreciation for their support and patience during my graduate studies. I could never have completed this work without them. vi TABLE OF CONTENTS LIST OF TABLES………………………………………………………………………………..xi LIST OF FIGURES……………………………………………………………………………...xiii LIST OF SCHEMES……………………………………………………………………………xvii ABBREVIATIONS…………………………………………………………………………… xviii CHAPTER 1: Introduction to Ferritin…………………………………………………1 CHAPTER 2: Ferritin-Catalyzed Consumption of H2O2 by Amine Buffers Causes 2+ Variable Fe to O2 Stoichiometry of Iron Deposition in Horse Spleen Ferritin…...17 Abstract……………………...…………………………………………………...19 Introduction………………………………………………………………………20 Materials and Methods…………………………………………………………...23 Results……………………………………………………………………………26 Discussion…………...…………………………………………………………...43 CHAPTER 3: Synthesis and Characterization of Co- & Mn-Oxyhydroxide Cores Formed in Horse Spleen Ferritin………………………………………………………51 Abstract……………………...…………………………………………………...53 Introduction………………………………………………………………………54 Materials and Methods…………………………………………………………...56 Results……………………………………………………………………………61 Discussion…………...…………………………………………………………...73 vii CHAPTER 4: Electron Exchange between Fe(II)-HoSF and Co(III)/Mn(III) Reconstituted Horse Spleen and Azotobacter vinelandii Ferritins…………………...77 Abstract……………………...…………………………………………………...79 Introduction………………………………………………………………………80 Materials and Methods…………………………………………………………...83 Results……………………………………………………………………………87 Discussion…………...…………………………………………………………...97 CHAPTER 5: Anaerobic Iron Deposition into Horse Spleen, Recombinant Human Heavy and Bacteria Ferritins by Large Oxidants…………………………………...105 Abstract……………………...………………………………………………….107 Introduction……………………………………………………………………..108 Materials and Methods………………………………………………………….112 Results…………………………………………………………………………..115 Discussion…………...………………………………………………………….131 CHAPTER 6: Rate of Iron Transfer through the HoSF Shell Determined by Formation of Prussian Blue and Fe-Desferal within the Ferritin Cavity………….137 Abstract……………………...………………………………………………….139 Introduction……………………………………………………………………..140 Materials and Methods………………………………………………………….144 Results…………………………………………………………………………..149 Discussion…………...………………………………………………………….161 viii CHAPTER 7: Anions Transfer in Ferritin during Reduction of the Mineral Cores by Methyl Viologen …………………………………………………………………...169 Abstract……………………...………………………………………………….171 Introduction……………………………………………………………………..172 Materials and Methods………………………………………………………….174 Results…………………………………………………………………………..178 Discussion…………...………………………………………………………….186 REFERENCES………………………………………………………………………...191 ix x LIST OF TABLES 2+ 2+ Table 2-1. Fe /O2 stoichiometry vs buffers type and concentrations at 22 Fe /HoSF ………………………………….………………………………….…………………27 Table 4-1. First-order rate constants of electron transfer from Fe(II)-HoSF to Co- HoSF/AvBF in the absence and presence of a surface………………………………96 Table 5-1. Amount of iron in ferritin interior after Fe2+ deposition using large oxidants ………………………………….………………………………….………………..118 Table 5-2.
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