Iron-storage protein that displays ferroxidase activity, catalyzing the oxidation of Fe(2+) ions into Fe(3+) ions, that can then be deposited as a ferric-oxide mineral core within the central cavity of the protein complex. 4 Fe(2+) + 4 H(+) + O2 = 4 Fe(3+) + 2 H2O Homooligomer of 24 subunits that are packed together to form an approximately spherical molecule with a central cavity, in which large amounts of iron can be stored. Belongs to the ferritin family. Prokaryotic subfamily.