Ferritin iron mineralization

We can briefly conclude that the mineralization process of iron in ferritin cores is a difficult process to follow experimentally. While we believe that iron is delivered for storage within the protein cavity as Fe(II), and that an oxidation step occurs in the formation of the ferritin iron core, it is not clear what percentage of iron oxidation occurs on the growing surface of the mineral and what at the catalytic ferroxidase... 

Ferritin iron cores, or polynuclear iron complexes in lipid vesicles or in matrices of protein and complex carbohydrates, appear to be the precursors of minerals such as hematite and magnetite that form in certain bacteria (31), marine Invertebrates (22), insects, and birds. The conversion from ferrltin-llke iron cores requires partial changes in the oxidation state of and/or ordering of the iron atoms, and may depend on some of the natural variations in ferritin core structure. 

Figure 10 Four helix bundle of the L chain of ferritin showing iron oxide nucleating site (Glu61, Glu64, and Glu67). Quaternary structure of ferritin displaying mineralized ferrihydrite particle...

Iron minerals are nucleated on the inner surface of ferritin, at clusters of conserved carboxylate residues [45]. Substitution of Glu residues with Ala in L-type ferritin slows the rate of mineralization and decreases the pattern of bound water, which is highly conserved [46]. The nucleation sites are constant in all known ferritin sequences, which contrasts with the presence of the ferroxidase site only in H-type ferritins [1, 2, 47]. 

The iron mineral is stable inside ferritin until the addition of reductant and/or chelators [48]. The rate at which the iron is released by reductive chelation (FMN/ NADH -I- bipyridyl) is relatively constant for recombinant H- and L-type ferritins [49] and for natural mixtures of subunits from different tissues, except when post-translationally modified [50, 51]. The site for iron release is unknown, but rates of release are similar in recombinant ferritins composed of all the ferritin subunit types currently known. 

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

The biological mineralizing systems for iron that have been studied the most extensively are the ferrihydrite (and, in prokaryotic ferritins, the amorphous... 

It has been suggested (Bozzi et ah, 1997 Grant et ah, 1998) that Dps and E. inocua ferritin represent examples of a family of ancestral dodecameric protein which had as function to trap, but not to mineralize, metal ions, and that the ability to oxidize and mineralize iron efficiently and to form fourfold interactions came later. The hollow-cored dodecameric motif exemplified by Dps and E. inocua ferritin has clearly been adapted to a number of functions, since in addition to DNA binding and iron storage, other family members include a novel pilin, a bromoperoxidase and several other proteins of unknown function (Grant et ah, 1998). 

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). 

Typically, mammalian ferritins can store up to 4500 atoms of iron in a water-soluble, nontoxic, bioavailable form as a hydrated ferric oxide mineral core with variable amounts of phosphate. The iron cores of mammalian ferritins are ferrihydrite-like (5Fe203 -9H20) with varying degrees of crystallinity, whereas those from bacterioferritins are amorphous due to their high phosphate content. The Fe/phosphate ratio in bacterioferritins can range from 1 1 to 1 2, while the corresponding ratio in mammalian ferritins is approximately 1 0.1. 

Why mammalian ferritin cores contain ferrihydrite-like structures rather than some other mineral phase is less easy to understand, and presumably reflects the way in which the biomineral is built up within the interior of the protein shell together with the geometry of the presumed nucleation sites. The phosphate content in the intracellular milieu can readily be invoked to explain the amorphous nature of the iron core of bacterioferritins and plants. Indeed, when the iron cores of bacterioferritins are reconstituted in the absence of phosphate, they are found to be more highly ordered than their native counterparts, and give electron diffraction lines typical of the ferrihydrite structure. Recently it has been reported that the 12 subunit ferritin-like Dps protein (Figure 19.6), discussed in Chapter 8, forms a ferrihydrite-like mineral core, which would seem to imply that deposition of ferric oxyhydroxides within a hollow protein cavity (albeit smaller) leads to the production of this particular mineral form (Su et al., 2005 Kauko et al., 2006). 

Fe(II) penetrates inside the spherical shell by the hydrophilic channels. After an oxidation on ferroxidase sites, located on H subunits, Fe(III) iron ions migrate to a nucleation site, situated on L subunits, where a crystal of hydrated iron oxide grows. Up to 4500 Fe(III) can be stored inside this mineral phase (31). The number of iron atoms contained in the ferritin molecule is called the loading factor (LF). 

The mineralized core of ferritin can be modeled by mixed valence species such as [Fe 4-Fe g02(0Me)i8(02CMe)6]-4.67MeCN, whose 3D close-packed layer structure mimics ferritin. This compound can be prepared by oxidizing a methanolic solution of iron(II) acetate and lithium methoxide with a slow stream of dioxygen it can be reduced to give [Fe 2Fe o02-(0Me)i8(02CMe)g]2-.i ... 

As the human body is able to store many minerals, deviations from the daily ration are balanced out over a given period of time. Minerals stored in the body include water, which is distributed throughout the whole body calcium, stored in the form of apatite in the bones (see p. 340) iodine, stored as thyroglobulin in the thyroid and iron, stored in the form of ferritin and hemosiderin in the bone marrow, spleen, and liver (see p. 286). The storage site for many trace elements is the liver. In many cases, the metabolism of minerals is regulated by hormones—for example, the uptake and excretion of H2O, Na, ... 

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