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A photograph of microscopic apoferritin crystals obtained by the addition of polyethylene glycol.
This is the wireframe display of ferritin, extracted from the
bacteria Escherichia Coli . Ferritins
are a family of proteins found in animals, plants and bacteria and have
the function of storing iron. They are found in the liver, spleen and bone marrow
of animals. When iron is set free from its
transporting system (see the tour of Transferrin) and released within a
cell, it has to be either directly used or stored because free iron can
react with dioxygen to produce hazardous radicals:
Fe(II) + O2
Fe(III) + O2-.
Fe(II) + H2O2 Fe(III) + OH- +
The O2-. and OH. radicals are highly reactive and can damage the genetic material in
the cell nucleus. The transport and the storage of iron must be rapid and reversible
under physiological conditions.
heme group in bacterial ferritin that connects the two subunits through sulphurs from Met 52 (coloured green) from both chain A and B. Like myoglobin, it contains porphyrin b and the subunits are sometimes referred to as a cytochrome b1. The ferritin molecules from other organisms, like humans, lack this heme group (but they do contain the diiron nucleation centre). The heme might serve to assist the self-assembly of the protein sphere by acting as a connector between the two subunits. It could also function as an electron carrier, shuttling electrons from reducing agents outside the sphere to the diiron-dioxygen reduction centre.
Guided Tours of Metalloproteins). It is assumed that this 2-Fe2+ site binds O2 to produce the unstable complex
(his)(glu)FeIII( µ-OO2-)( µ-glu)2FeIII(his)(glu)
Hydrolysis of this complex gives (H2O)xFeIIIOFeIII(OH2)x which is released into the cavity where Fe(III) further hydrolyses to start the nucleation of the ferrihydrite mineral.
4Fe3+ + 8H2O
4Fe(O)OH + 12H+
This reaction shows that ferritin is a pH buffer as well as an Fe buffer.
2+, enters the protein through three-fold channels.
4Fe2+ + O2 + 6H2O
4Fe(O)OH + 8H+.
This process results in the formation of well-defined crystals of up to 75 Å in diameter.
The ferritin model complexes are synthesized and studied in order to elucidate the formation, structure and properties of the iron oxyhydroxide mineral that is formed inside the protein. In addition they are interesting nano-scale objects and they provide information on the formation of rust.
6( µ4-O)2( µ2-OMe)8(OMe)4(tren)2]2+ (tren = 2,2',2''-triaminotriethylamine, N(CH2CH2NH2)3) . It is formed as an orange crystalline solid
by solvolytic aggregation when a solution of Fe(CF3SO3)2 and tren in methanol is exposed to air.
If you would like to see more models for the mineral formed inside the ferritin cavity see:
Understanding the crystallization of minerals inside the ferritin cavity has lead to important advances in the synthesis of nanomaterials. Apoferritin can serve as a perfect "reaction vessel" for the growth of crystalline nanoparticles of CdS or ZnS of well-defined size. For the first steps towards such a synthetic procedure see Science (261), 1286 (Sept. 3, 1993).