Apoferritin cavity

Fig. 13. Schematic representation of [GdHPD03A(H20)] entrapped into the Apoferritin cavity.

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... 

Once ferrihydrite particles have formed inside the apoferritin cavity they provide alternative oxidation centers for Fe(II) on the iron core particle surface 15). Evidence for this includes the following observations (1) the stoichiometry of Fe(II) oxidation by dioxygen increases from one to approximately four Fe(II)/02 as a core formation proceeds (55) (2) Fe(II) oxidation can be effected by oxidants other than O2 once a core is present 48) (3) Fe(II) can bind directly to core surfaces, as shown hy Mbsshauer spectroscopy, and the bound Fe(II) can be oxidized (56, 57) (4) added Fe binds preferentially to any existing iron core clusters rather than to the protein shell, again as shown by analysis of Mbssbauer spectra (see Table III and Fig. 3) (57) (5) after addition of an excess of Fe(II) to a small Fe(III) core, ag = 1.87 EPR signal... 

Galvez, N., Sanchez, P., Dominguez-Vera, J. M. (2005). Preparation of Cu and CuFe Prussian Blue derivative nanoparticles using the apoferritin cavity as nanoreactor. Dalton Trans., 7, 2492—2494. 

Unfortunately, attempts to increase the relaxivity per particle failed, as the preparation method used does not allow the inclusion of more than 8 10 Gd-HPD03A molecules per apoferritin cavity. The task of internalizing a much greater number of paramagnetic centres inside the apoferritin cavity has been pursued by dissolving the solid P-MnOOH phase that one may create by exposing the protein to a concentrated solution of Mn(ll) salts at... 

Mn + ions entrapped inside the apoferritin cavity (Figure 22.15). The Mn—apoferritin system shows markedly... 

Shin Y, Dohnalkova A, Lin Y. Preparation of homogeneous gold-silver alloy nanoparticles using the apoferritin cavity as a nanoreactor. J Phys Chem C 2010 114 5985-9. 

In order to deal with a system whose structural characteristics were unaltered by the loading with Gd(III) chelates, we choose Apoferritin because it allows the Imaging Probes to be entrapped inside its inner cavity (60). The exterior of such Gd(III)-loaded Apoferritin is exactly the same as in the parent Ferritin and then, once administered intravenously, it is quickly cleared-up by the proper receptors on hepatocytes (172). The process of loading Apoferritin with [GdHPD03A(H20)] consists first of the dissociation of the protein into subunits at pH 2, followed by its reforming at pH 7, thereby trapping the solution components (e.g., [GdHPD03A(H20)])... 

Since the (Fen05(0H)6> unit is stable, it has been speculated(8b,17b) that it might also be present in the ferritin core. Since the majority of phosphate in ferritin is adventitious, surface bound and the metallic core can be reconstituted in the absence of phosphate groups with no change in the X-ray powder diffraction pattem(l), replacement of bridging phosphate by bridging carboxylate groups should not influence the three dimensional structure of the core. Calculations show that -409 Fell nnits could fill the apoferritin inner cavity. Further details can be found in reference 17. 

Apoferritin consists of a hollow, spherical shell of external diameter 125 A, which provides an inner cavity of maximum diameter about 80 A for the storage of iron. 

The inner cavity is filled with small crystalline particles of composition [Fe(0)(0H)]g-[Fe(0)(0P03H2)], which is 57% iron and contains up to about 4500 iron atoms per ferritin molecule. While the core stores iron as Fe111, it is generally accepted that deposition and mobilization involve Fe11. Apoferritin rapidly accumulates iron in a solution containing Fe11 and 02, but Fe111... 

The extensive series of studies on in vitro core formation reported by Harrison and co-workers (32, 63, 82, 140) and others (133) has led to the development of a three-step hypothesis for iron uptake. In the first step, iron entry through the channels, Fe + passes from the outside of the protein through the channels in the apoferritin coat to the interior cavity. The second step, nucleation, involves iron binding to groups on the inner surface of the protein in such a way that a small cluster of coupled Fe ions is formed. The final step, formation of the core, involves the extension of a small nucleating cluster by the addition and oxidation of Fe +. This stage is characterized by an initial catalytic phase, during which the small cluster rapidly expands, followed by a reduced rate of expansion once the core has attained a particular size (—1000-1500 iron atoms per molecule). 

While the structures of apoferritin and ferritin are fairly well established, the manner in which iron is transported in and out of the protein cavity is still under investigation. It is proposed that iron enters as Fe and is oxidized once inside the protein. The formation of the crystalline core is an example of biomineralization and it is a remarkable achievement of evolution that iron can be stored in mammals effectively as hydrated iron(III) oxide, i.e. a form closely related to rust ... 

Multimeric ferritin is nicely suited for sequestering iron in a readily available nontoxic form. The shell formed by the subunits is 12 nm in diameter and less than 2 nm thick, so that up to 4000 iron atoms can be stored in the iron core. Ferritin is formed by the nucleation of a polynuclear ferric iron core in its cavity outlined by the inner surface of an apoferritin coat. The iron core of ferritin varies in size and in the number of iron crystallites, which are believed to be polynuclear hydrous oxide (5Fe203 9H2O). 

The same concepts are present in nature that has evolved examples of natural supramolecular capsules deputed to the transport and/or the storage of important chemical species. Examples are the tobacco mosaic virus characterized by a rodlike appearance and composed of 2130 molecules of coat protein that surround one molecule of genomic RNA 6400 bases long. Another important representative is apoferritin, which is a capsular aggregate with octahedral symmetry formed by 24 identical peptides that present a cavity of over 230 that can store up to 4500 iron atoms as ferric hydrous oxides. 

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