CHAPTER 13: Ferritin and Its Role in Iron Homeostasis

Nature uses ferritins (protein cages around iron biominerals) to concentrate iron and consume pro-oxidants. Fe²⁺ released from dissolved ferritin mineral forms cofactors for iron proteins. Ferritin mineral substrates Fe²⁺, O₂ or H₂O₂ are pro-oxidants, making ferritins one of the antioxidant response proteins encoded in ARE genes. The two ferritin cage sizes are 12 subunit mini-ferritins (prokaryote Dps proteins) and 24 subunit maxi-ferritins (eukaryotes and prokaryotes); subunits fold into 4α-helix bundles and self-assemble into cages featuring multiple sites for iron chemistry. The four known activities of ferritin proteins are: 1. Fe²⁺ entry/exit (via ion channels); 2. multi-site, enzymatic (ferroxidase) oxidation of Fe²⁺ by O₂ or H₂O₂; 3. post-enzymatic hydrolysis yielding Fe₂O₃·H₂O precursors (eukaryotic nucleation channels); 4. protein-controlled reductive dissolution of ferritin mineral. Fe²⁺ is both the precursor and the product of ferritin chemistry and also regulates ferritin biosynthesis (mRNA translation) by binding ferritin IRE-RNA. An inhibitor protein IRP dissociates while an enhancer protein eIF-4F associates with the Fe²⁺/IRE-RNA complex. When newly synthesized ferritin protein converts Fe²⁺ to caged ferritin mineral, elevated ferritin protein synthesis stops, shutting down the iron feedback loop. Ferritin proteins hold considerable promise as nano-vessels for sensor and drug delivery, for nano-material syntheses and nano-catalysis. The ferritin future is huge.