Bacterioferritins, also known as cytochrome b1, are members of a broad superfamily of ferritin-like diiron-carboxylate proteins. Similar to ferritin in architecture, Bfr forms an oligomer of 24 subunits that assembles to form a hollow sphere with 432 symmetry. Up to 12 heme cofactor groups (iron protoporphyrin IX or coproporphyrin III) are bound between dimer pairs. The role of the heme is unknown, although it may be involved in mediating iron-core reduction and iron release. Each subunit is composed of a four-helix bundle which carries a diiron ferroxidase center; it is here that initial oxidation of ferrous iron by molecular oxygen occurs, facilitating the detoxification of iron, protection against dioxygen and radical products, and storage of ferric-hydroxyphosphate at the core. Some bacterioferritins are composed of two subunit types, one conferring heme-binding ability (alpha) and the other (beta) bestowing ferroxidase activity.