Electron density map of apoferritin at 2.8-A resolution.

Abstract

THE structure of ferritin is of considerable interest because of its widespread occurrence in higher organisms, signifying a general need to store iron and remove this essential, but toxic element. Horse spleen apoferritin has a molecular weight (MW) of about 444,000 and is composed of 24 subunits (MW 18,500) each containing about 163 amino acids1. These are arranged in 432 symmetry to form a nearly spherical hollow shell with outside and inside diameters approximately 130 A and 75 A respectively1–3. The large cavity inside the molecule can store up to 4,500 Fe atoms, packaged in a microcrystalline inorganic component of approximate composition (FeOOH)8 (FeO:OPO3H2) (refs 4–6). The atomic structure of the micro-crystals is not specifically related to the surrounding protein structure6. Apoferritin catalyses the oxidation of Fe(II) which it retains inside the molecule as the ferric hydrolysate7,8. Our 6-A resolution structure of apoferritin3 showed the presence of several rods of electron density, tentatively assigned as α helices, and channels passing through the shell, which could provide an access route for Fe atoms. These features have been confirmed at 2.8-A resolution and we can now also provide a plausible subunit conformation and quaternary structure.