Ferritins are the primary iron storage proteins of most living organisms and members of a broad superfamily of ferritin-like diiron-carboxylate proteins. The iron-free (apoferritin) ferritin molecule is a protein shell composed of 24 protein chains arranged in 432 symmetry. Iron storage involves the uptake of iron (II) at the protein shell, its oxidation by molecular oxygen at the dinuclear ferroxidase centers, and the movement of iron (III) into the cavity for deposition as ferrihydrite; the protein shell can hold up to 4500 iron atoms. In vertebrates, two types of chains (subunits) have been characterized, H or M (fast) and L (slow), which differ in rates of iron uptake and mineralization. Bacterial non-heme ferritins are composed only of H chains. Fe(II) oxidation in the H/M subunits take place initially at the ferroxidase center, a carboxylate-bridged diiron center, located within the subunit four-helix bundle. In a complementary role, negatively charged residues on the protein shell inner surface of the L subunits promote ferrihydrite nucleation. Most plant ferritins combine both oxidase and nucleation functions in one chain: they have four interior glutamate residues as well as seven ferroxidase center residues.