ABSTRACT
Twenty-four subunits (24-mer) constitute the protein shell of relative molecular mass (Mr) ranging from 450,000 to 500,000 with internal and external diameters of about 80 and 120 A, respectively. The geometry of the shell is approximately that of a rhombic dodecahedron (432 point-group symmetry), the faces of which consist of two subunits related by a twofold symmetry axis at its center. Channels that traverse the protein shell are present at the fourfold axis, where the E helices interact, and at the threefold axis, near the N termini of the subunits (Fig. IB). Iron is stored in the protein shell of ferritin as a hydrous ferric oxide nanoparticle with a structure similar to that of the mineral ferrihydrite. Eight hydrophilic channels at the threefold axes are thought to be the primary avenues by which iron gains entry to the interior of eukaryotic ferritins ([23] and references therein). The H chain contains a dinuclear
ferroxidase site that is located in the four-helix bundle of the subunit; it catalyzes the oxidation of ferrous iron by 0 2, producing H20 2. The L subunit lacks this site but contains additional glutamate residues on the interior surface of the protein shell that produce a microenvironment that facilitates mineralization and the turnover of iron(III) at the H subunit ferroxidase site [23].
