The Ferritin Protein Nanocage and Biomineral, from Single Fe Atoms to FeO Nanoparticles: Starting with EXAFS

Abstract

Ferritins are protein nanocages that use iron and oxygen chemistry to concentrate iron and trap dioxygen or hydrogen peroxide in biominerals of hydrated ferric oxides, 5–8 nm in diameter, inside the cages. The proteins are found in nature from archea to humans. Protein catalytic sites are embedded in the protein cage and initiate mineralization by oxido‐reduction of ferrous ions and dioxygen or hydrogen peroxide to couple two iron ions through a peroxo bridge, followed by decay to diferric oxo/hydroxyl mineral precursors; ferritin protein subdomains that fold/unfold independently of the protein cage control recovery of ferrous ions from the mineral. Early EXAFS (1978) was extremely useful in defining the ferritin mineral. More recent use of rapid freeze quench (RFQ) EXAFS spectroscopies, coupled with RFQ Mössbauer, Resonance Raman and rapid mixing UV‐vis spectroscopy, have identified and characterized unusual ferritin protein catalytic intermediates and mineral precursors. EXAFS spectroscopy can play an important role in the future understanding of protein catalysis in metalloproteins such as ferritin, ribonucleotide reductase and methane monooxygenases. Needed are instrumentation improvements that will provide rapid‐scan fluorescence spectra with high signal/noise ratios.