Ferritin: Mechanistic Studies and Electron Transfer Properties

• Main Author: Zhang, Bo
• Format: Others
• Published: BYU ScholarsArchive 2006
• Subjects: Ferritin; iron deposition; electron transfer; mechanism; kinetics; Biochemistry; Chemistry
• Online Access: https://scholarsarchive.byu.edu/etd/766; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=1765&context=etd

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 Fe2+/O2 stoichiometry, the kinetics and the molecular mechanism of iron deposition are discussed. 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 ([FeIIFeIII(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.