Nonheme Ferritin domain, found in archaea and bacteria, is a member of a broad superfamily of ferritin-like diiron-carboxylate proteins. The ferritin protein shell is composed of 24 protein subunits arranged in 432 symmetry. Each protein subunit, a four-helix bundle with a fifth short terminal helix, contains a dinuclear ferroxidase center (H type). Unique to this group of proteins is a third metal site in the ferroxidase center. Iron storage involves the uptake of iron (II) at the protein shell, its oxidation by molecular oxygen at the ferroxidase centers, and the movement of iron (III) into the cavity for deposition as ferrihydrite.