Fig. 1.

Comparisons of diiron sites for Fe substrate (ferritin) and cofactors reveal significant differences in iron ligands and second-shell residues. The ferritin Fe2 site ligands are Glu¹⁰³, Gln¹³⁷, and Asp¹⁴⁰, with a bridging carboxylate, Glu⁵⁸. The distinctive ferritin Gln¹³⁷ is compared with Glu²⁴³ in methane monooxygenase (MMO) and Glu²³⁸ in ribonucleotide reductase (RNR). Multiple histidine bonding interactions in diiron cofactor centers are absent from ferritin diiron substrate centers. The figure uses the frog M ferritin model [Protein Data Bank (PDB) file 1MFR, a crystal structure with Mg(II) as an Fe(II) homologue (18)] and CD/MCD analysis of diferrous binding (17). Diiron cofactor site structures include diferrous, as in MMO (1FYZ) (21) and RNR (1PFR) (22).