Ferritin iron cores

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... 

Different Mossbauer and magnetometric studies have shown the superparamagnetism of ferritin iron cores. This property is also typical of ferro-... 

Variations in ferritin iron cores include the number of iron atoms, composition, and the degree of order (3,6,21-23). Size variations of the iron core range from 1-4500 Fe atoms and appear to be under biological control (e.g. Ref. 15). The distribution of iron core sizes in a particular ferritin preparation can be easily observed after sedimentation of ferritin through a gradient of sucrose. 

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). 

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. 

Iron Clusters and the Early Stages of Ferritin Iron Core Formation... 

The sequence of steps in the biosynthesis of the ferritin iron core has been studied by analyzing the incorporation of Fe into ferritin during synthesis of the protein vivo. Ferritin, collected at various intervals after the induction of synthesis, was fractionated according to iron core size by sedimentation through gradients of sucrose (32). Fe appeared first in ferritin with small amounts of Fe, and later, the Fe appeared in fractions further down the gradient as the core size and the ratio... 

Towe, K.M. (1990) Phosphorus and the ferritin iron core Function-balanced biomineralization. In Crick, R.E. (ed.) Origin, evolution, and modem aspects of biomineralization in plants and animals. Plenum Press, New York, 265-272... 

Chemical Composition of Horse Spleen Ferritin Iron Cores"... 

A further development in the field of Mossbauer spectra fitting and analysis is expected regarding the explanation of applicability of either continuous distributions of quadrupole splitting and hyperfine field or a superposition of discrete quadrupole doublets and magnetic sextets with models of multidomain and multilayer structures of the ferritin iron core. In this case, application of Mossbauer spectroscopy with a high velocity resolution may be used because it leads to a lower instrumental error in the determination of hyperfine parameters (this allows small variations of hyperfine parameters to be distinguished) as well as to a more reliable fitting of complicated Mossbauer spectra (see reviews [32,34-39, 128]). 

However, ferritins isolated from the bacterium Pseudomonos aeruginosa (Mann et ah, 1986) and from the chiton Acanthopleura hirtosa (St. Pierre et ah, 1990) have iron cores of limited crystallinity, despite having P Fe ratios of around 1 40, perhaps suggesting that core crystallinity is influenced by the rate of iron deposition as well as by the composition of the medium. The way in which phosphate may influence core development is discussed below. 

On the basis of a number of physico-chemical methods (Mossbauer spectroscopy, electron diffraction, EXAFS) the iron cores of naturally occurring haemosiderins isolated from various iron-loaded animals and man (horse, reindeer, birds and human old age) were consistently shown to have ferrihydrite-like iron cores similar to those of ferritin (Ward et ah, 1992, 2000). In marked contrast, in the tissues of patients with two pathogenic iron-loading syndromes, genetic haemochromatosis and thalassaemia, the haemosiderins isolated had predominantly amorphous ferric oxide and goethite cores, respectively (Dickson etah, 1988 Mann etah, 1988 ... 

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). 

Fig. 13. On the left, representation of the ferritin molecule the subunits form a spherical shell containing a ferrihydrite crystal. On the right, a Transmission Electron Microscope picture showing the iron core contained inside the proteic shell, appearing as electron-dense dark spots.

Variations in ferritin protein coats coincide with variations in iron metabolism and gene expression, suggesting an Interdependence. Iron core formation from protein coats requires Fe(Il), at least experimentally, which follows a complex path of oxidation and hydrolytic polymerization the roles of the protein and the electron acceptor are only partly understood. It is known that mononuclear and small polynuclear Fe clusters bind to the protein early in core formation. However, variability in the stoichiometry of Fe/oxidant and the apparent sequestration and stabilization of Fe(II) in the protein for long periods of time indicate a complex microenvironment maintained by the protein coats. Full understanding of the relation of the protein to core formation, particularly at intermediate stages, requires a systematic analysis using defined or engineered protein coats. 

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