Iron-storage protein. 4 Fe(2+) + 6 H2O + O2 = 12 H(+) + 4 iron(III) oxide-hydroxide Homooligomer of 24 subunits that assemble into a spherical protein shell (12 +/- 1 nM diameter) that can sequester at least 2000 iron atoms. Belongs to the ferritin family. Prokaryotic subfamily.