Catalyzes the oxidation of ferrous iron(II) to ferric iron(III) and stores iron in a soluble, non-toxic, readily available form. Important for iron homeostasis. Iron is taken up in the ferrous form and deposited as ferric hydroxides after oxidation. 4 Fe(2+) + 4 H(+) + O2 = 4 Fe(3+) + 2 H2O Homooligomer of 24 subunits. The functional molecule is roughly spherical and contains a central cavity into which the polymeric mineral iron core is deposited (By similarity). Belongs to the ferritin family.