Iron-storage protein, whose ferroxidase center binds Fe(2+) ions, oxidizes them by dioxygen to Fe(3+), and participates in the subsequent Fe(3+) oxide mineral core formation within the central cavity of the protein complex. May act as one of the electron carriers in the reverse electron-transport system from cytochrome c-552 to NADP(+). 4 Fe(2+) + 4 H(+) + O2 = 4 Fe(3+) + 2 H2O Binds 1 heme b (iron(II)-protoporphyrin IX) group per dimer. Binds 2 iron ions per subunit. The catalytic dinuclear iron-binding site within each subunit is known as the ferroxidase center. Homooligomer of 24 subunits, arranged as 12 dimers, that are packed together to form an approximately spherical molecule with a central cavity, in which large amounts of iron can be deposited. Belongs to the bacterioferritin family.