Iron storage proteins, ferrihydrite mineralization

Ferricytochrome c, 32 49 NIR MCD spectrum, 36 233-234 Ferrihemoproteins, reduction rates, 36 430-431 Ferrihydrite, 36 422 chemical composition, 36 455 inside apoferritin cavity, 36 459 mineralization in iron storage proteins, 36 161-164... 

Ferritin, the iron storage protein foimd in a variety of living organisms, serves two important functions. It catalyzes the oxidation of the cytotoxic Fe + ions to the less toxic Fe " " ions and it stores the insoluble Fe " " ions within its inner protein cavity in the form of a ferrihydrite-phosphate mineral core ([13, 14], see Chapters 12 and 15). The oxidation of ferrous ions is known as ferroxidase activity and the conversion of the oxidized Fe + ions into a solid state iron core is termed mineralization. The ferritin molecule is composed of 24 subunits, which are arranged to form a... 

Once the iron is inside the sphere, a site of mineral nu-cleation is located—step 2 in the biomineralization process. Although the site(s) of nucleation are not known, the walls are lined with carboxylic acids from aspartate and glutamate residues. These are the probable sites of nucleation. After nucleation, the mineral begins to form. The shape is controlled by the protein shell. Studies of the resulting mineral are consistent with octahedrally coordinated iron(III) ions joined by bridging oxide and/or hydroxide ions. A mineral termed ferrihydrite has a similar postulated structure. Because ferritin is used for iron storage agents, the supramolecular structures described in step 4 of Section III.B are not formed. 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

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