Ferritin mammalian

One form of antioxidant defense may be the binding of excess Fe3+ and other transition metal ions, preventing Fe3+, and other transition metal pro-oxidants from catalyzing free radical reactions. Most intracellular Fe3+ is stored in ferritin. Mammalian ferritins consist of a hollow protein shell 12-13 nm outside diameter... 

Perhaps the best-understood process of biomineralization is the storage of iron within ferritin. Mammalian ferritins consist of a protein shell made up of 24 individual peptide subunits (Fig. 10). The resulting structure is roughly a hollow sphere where iron can be stored as ferric oxide. 

In normal human subjects, some 25 % of total body iron (800-1000 mg) is present in the storage forms, mostly as ferritin. Whereas it is likely that all mammalian cell types contain some ferritin, haemosiderin in normal subjects is essentially restricted to cells of the reticuloendothelial system. Ferritin turns out to be almost universal in its distribution ferritin and ferritin-like proteins have been found in all organisms except for one or two archaebacteria. In contrast, haemosiderin has not been found to any extent outside of iron-loaded animals, except for a brief report of a phytosiderin in pea seeds (Laulhere et ah, 1989). 

Table 6.1 Amino-acid sequence alignment of four mammalian ferritins (Horse L chain, HoL Human L chain, HuL Human H chain, HuH Rat H, RaH) and of one of the ferritins, FTN, and the bacterioferritin, BFR of...

Figure 6.6 E helices around the fourfold axes in four ferritins and EcoBFR. (a) Superposition of -carbon traces for three mammalian ferritins, HuHF, HoLF and RaLF (b) BfLF (c) EcBFR. (a) is viewed perpendicular to and (b) and (c) along the fourfold axis. Reprinted from Harrison et al., 1998, by courtesy of Marcel Dekker, Inc.

The cores of individual mammalian ferritin molecules are clearly visible by transmission electron microscopy as well defined nanoparticle crystallites encapsulated within the protein shell which can attain a size close to that of the 8 nm interior diameter of the protein shell (Massover, 1993). This is illustrated in Figure 6.13 for a sample of human ferritin. The amount of iron in the core is variable, and can range from zero to a maximum of approximately 4500 atoms (Fischbach and Anderegg, 1965) this corresponds to the capacity of the internal cavity for Fe(III) as... 

Plants contain phytoferritins, which accumulate in non-green plastids1 in conditions of iron loading. They are targeted to the plastids by a putative-transit peptide at their N-terminal extremity, and possess the specific residues for ferroxidase activity and iron nucleation, found in mammalian H-type or L-type ferritin subunits. 

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. 

Iron incorporation into mammalian ferritins is thought to involve the following steps (Crichton, 2001) (1) Uptake of Fe2+ into the protein shell, most probably through the hydrophilic three fold channels. (2) Oxidation of ferrous iron by the dinuclear ferroxidase... 

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). 

Known compositional variations of ferritin iron cores only Involve phosphate, which can range from as much as 80% (21) to as little as 5% of the iron (21) in normal mammalian liver or spleen, the amount of phosphate in the ferritin iron core is ca. 12% of the iron (24). When the phosphate content is high, the distribution of phosphate is clearly throughout the core rather than on the surface. However, interior locations for phosphate are also suggested when the phosphate content is lower, by data on an Fe(III)ATP model complex (P Fe 1 4) (25) or by phosphate accessibility studies in horse spleen ferritin (P Fe = 1 8) (24). Based on model studies, other possible variations in core composition could Include H2O or sulfate (26). 

Iron occurs in every mammalian cell and is vital for life processes. It is bound to various proteins and found in blood and tissues. The iron-porphyrin or heme proteins include hemoglobin, myoglobin and various heme enzymes, such as cytochromes and peroxidases. Also, it occurs in non heme compounds, such as ferritin, siderophilin, and hemosiderin. Hemoglobin, found in the red blood cells, is responsible for transport of oxygen to the tissue cells and constitutes about two-thirds (mass) of all iron present in the human body. An adult human may contain about 4 to 6 grams of iron. 

Two proteins are important for iron metabolism in mammalian cells the transferrin receptor (TFR) md ferritin. Ferritin is a protein for the storage of iron. The production and its level is increased when more iron is available. 

Transport and storage processes involving iron are by far the best understood, both for mammalian systems (transferrin and ferritin) and for microbes (the siderophores in iron transport). In addition, knowledge of the transport and storage of copper and zinc in mammalian systems is advancing steadily, although awareness of microbial transport systems in general is poor. 

Grady, J.K., J. Shao, P. Arosio, P. Santambrogio, and N.D. Chasteen. 2000. Vanadyl(IV) binding to mammalian ferritins. An EPR study aided by site-directed mutagenesis. J. Inorg. Biochem. 80 107-113. 

Iron regulatory proteins (IRPs) regulate the cellular iron level in mammalian cells. IRPs are known as cytosol mRNA binding proteins which control the stability or the translation rate of mRNAs of iron metabolism-related proteins such as TfR, ferritin, and 5-aminolevulinic acid synthetase in response to the availability of cellular iron [19-21] after uptake [5]. The regulatory mechanism involves the interaction between the iron-responsive element (IRE) in the 3 or 5 untranslated regions of the transcripts and cytosolic IRPs (IRP-1 and -2). IRP-1 is an iron-sulfur (Fe-S) protein with aconitase activity containing a cubane 4Fe-4S cluster. When Fe is replete, IRP-1 prevails in a 4Fe-4S form as a holo-form and is an active cytoplasmic aconitase. As shown in Fig. 3, when Fe is deplete, it readily loses one Fe from the fourth labile Fe in the Fe-S cluster to become a 3Fe-4S cluster and in this state has little enzymatic activity [22, 23]. 

As mentioned above, there are the profound relationships between Al and Fe metabolism in mammalian cells Al can bind proteins bound to Fe. Apo-Tf binds to Al to form di-Al-Tf (Al2Tf). Al2Tf is recognized by TfR to be taken up by brain cells. Al binds Fe storage protein, ferritin and also influences the expression of ferritin mRNA. If this is a reliable phenomenon, Al is required to interact with IRPs which post-trascriptionally regulate the expression of ferritin or TfR mRNAs. 

The spontaneous assembly of small molecular fragments into larger, high-symmetry clusters has been accomplished in Nature for more than a billion years. Examples in the natural world include the protein ferritin. This very ancient protein is found in bacteria, plants, and animals. Mammalian ferritin is a 24-mer with octahedral symmetry (such that each of the asymmetric subunits is related to the other 23 by one of the symmetry operations of the pure rotation group O and its 24 symmetry elements), but there is a microbial ferritin with 12 subunits and T symmetry. An illustration of this structure is shown in Fig. 8. 

Mammalian ferritins are heteropolymers of H- and L-chains. These subunits are very closely related, with an a-carbon rmsd of 0.5 A and 55% sequence identity conservation of primary sequence rises to 79% when considering those residues responsible for intersubunit interactions. Subunit assembly appears to take place via partially structured monomers associating to form fully structured homodimers, which then aggregate further. Upon chemical denaturation and refolding, heterodimers are rarely observed. ... 

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