The iron storage protein ferritin is composed of 24 subunits of two ty
pes, H and L, which assemble to form a hollow protein shell capable of
encapsulating up to 4500 Fe atoms in the form of a hydrous ferric oxi
de mineral core. Iron is oxidized for incorporation into the mineraliz
ed core by either a protein enzymatic mechanism involving a putative d
imeric Fe ferroxidase site on the H chain subunit or a mineral surface
mechanism. The net stoichiometric reactions for the two kinetic pathw
ays are given by the following equations: 2Fe(2+) + O2 + 4H(2)O --> 2F
eOOH(core) + H2O2 + 4H(+) 4Fe(2+) + O-2 + 6H(2)O --> 4FeOOH(core) + 8H
(+) The L chain subunit appears to be involved mostly in mineralizatio
n of the core but modulates the ferroxidase activity of the H chain su
bunit as well. A reaction mechanism in which Fe(II) oxidation occurs i
n one electron steps accounts for the observed kinetics. Several react
ion intermediates have been identified using UV-visible, electron para
magnetic resonance and Mossbauer spectroscopies but their roles in the
mechanism of iron deposition in the protein remain to be determined.