Crystal structure of bullfrog M ferritin at 2.8 angstrom resolution: analysis of subunit interactions and the binuclear metal center

Ferritins concentrate and store iron as a mineral in all bacterial, plant, and animal cells. The two ferritin subunit types, H or M (fast) and L (sIow ), differ in rates of iron uptake and mineralization and assemble in vivo t o form heteropolymeric protein shells made up of 24 subunits; H/L subunit r atios reflect cell specificity of H and L subunit gene expression. A diferr ic peroxo species that is the initial reaction product of Fe(II) in H-type ferritins, as well as in ribonucleotide reductase (R2) and methane monooxyg enase hydroxylase (MMOH), has recently been characterized, exploiting the r elatively high accumulation of the peroxo intermediate in frog H-subunit ty pe recombinant ferritin with the M sequence. The stability of the diferric reaction centers in R2 and MMOH contrasts with the instability of diferric centers in ferritin, which are precursors of the ferric mineral. We have de termined the crystal structure of the homopolymer of recombinant frog M fer ritin in two crystal forms: P4(1)2(1)2, a = b = 170.0 Angstrom and c = 481. 5 Angstrom; and P3(1)2(1), a = b = 210.8 Angstrom and c = 328.1 Angstrom. T he structural model for the trigonal form was refined to a crystallographic R value of 19.0% (R-free = 19.4%); the two structures have an r.m.s.d. of similar to 0.22 Angstrom for all C-alpha atoms. Comparison with the previou sly determined crystal structure of frog L ferritin indicates that the subu nit interface at the molecular twofold axes is most variable, which may rel ate to the presence of the ferroxidase site in H-type ferritin subunits. Tw o metal ions (Mg) from the crystallization buffer were found in the ferroxi dase site of the M ferritin crystals and interact with Glu23, Glu58, His61, Glu103, Gln137 and, unique to the M subunit, Asp140. The data suggest that Gln137 and Asp140 are a vestige of the second GluxxHis site, resulting fro m single nucleotide mutations of Glu and His codons and giving rise to Ala1 40 or Ser140 present in other eukaryotic H-type ferritins, by additional si ngle nucleotide mutations. The observation of the Gln137xxAsp140 site in th e frog M ferritin accounts for both the instability of the diferric oxy com plexes in ferritin compared to MMOH and R2 and the observed kinetic variabi lity of the diferric peroxo species in different H-type ferritin sequences.