Unique Iron Binding and Oxidation Properties of Human Mitochondrial Ferritin: A Comparative Analysis with Human H-chain Ferritin

Ferritins are ubiquitous iron mineralizing and storage proteins that play an important role in iron homeostasis. Although excess iron is stored in the cytoplasm, most of the metabolically active iron is processed in the mitochondria of the cell. Little is known about how these organelles
regulate iron homeostasis and toxicity. The recently discovered human
mitochondrial ferritin (MtF), unlike other mammalian ferritins, is a
homopolymer of 24 subunits that has a high degree of sequence homology
with human H-chain ferritin (HuHF). Parallel experiments with MtF and
HuHF reported here reveal striking differences in their iron oxidation and
hydrolysis chemistry despite their similar diFe ferroxidase centers. In
contrast to HuHF, MtF does not regenerate its ferroxidase activity after
oxidation of its initial complement of Fe(II) and generally has considerably
slower ferroxidation and mineralization activities as well. MtF exhibits sigmoidal kinetics of mineralization more characteristic of an L-chain than an H-chain ferritin. Site-directed mutagenesis reveals that serine 144, a residue situated near the ferroxidase center in MtF but absent from HuHF, is one player in this impairment of activity. Additionally only one-half of the 24 ferroxidase centers of MtF are functional, further contributing to its lower activity. Stopped-flow absorption spectrometry of Fe(II) oxidation by O2 in MtF shows the formation of a transient diiron(III) m-peroxo species (lmaxZ650 nm) as observed in HuHF. Also, as for HuHF, minimal hydroxyl radical is produced during the oxidative deposition of iron in MtF using O2 as the oxidant. However, the 2Fe(II)CH2O2 detoxification reaction found in
HuHF does not occur in MtF. The structural differences and the
physiological implications of the unique iron oxidation properties of MtF are discussed in light of these results.