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C4
C4
C4
C3
C3
3 x C4 axes4 x C3 axes O symmetry(no mirror planes)
24 subunuits of4 a-helix bundles
Like an iron malted milk ball!
Fe3+Ox(OH)y core
One bundle
Fe3+O(OH)
Fe2+ oxid
Fe2+ exit
C4
C4
C3
C3
Fact: At pH 7, [Fe3+] = 10-18 M why?
• Ferritin manages to concentrate ferric ion to mM concentrations (10-3 M).• Lack of gene for ferritin lethal.
Fe3+2O3(H2O)1000 + 5000 H+
Exits from C3 pores
The reaction
2000 Fe2+(H2O)6 + O2 1000 Fe3+—O-O—Fe3+ 2000 Fe3+(H2O)6 + H2O2
fast, msec slow, minutes or hours Enters at C3 pores; ‘translocating mineral precursor’
rxn occurs in subunits
How and where iron exits from ferritin for cellular use is uncertain. Proline substitution for conserved leucine 134 (L134P) allowed normal assembly but increased iron exit rates. X-ray crystallography of H-L134P ferritin revealed localized unfolding at the 3-fold axis, also iron entry sites, consistent with shared use sites for iron exit and entry. The junction of three ferritin subunits appears to be a dynamic aperture with a "shutter" that cytoplasmic factors might open or close to regulate iron release in vivo.
Localized unfolding at the junction of three ferritin subunits. A mechanism for iron release?Takagi, H., Shi, D., Ha, Y., Allewell, N.M., Theil, E.C. (1998) J.Biol.Chem. 273: 18685-18688
