Iron core of ferritin

Physicochemical Characteristics of the Nonheme Iron Cores of Ferritins... 

The generally accepted structure for the crystalline iron core of ferritin is the ferrihydrite structure proposed by Towe Bradley.This consists of oxygen layers with iron in octahedral sites between the layers. Ford et al and PowelF have sununarized the evidence in support of this model for ferritin, which includes electronic spectroscopic data and EXAFS measurements confirming the presence of six-coordinate Fe(III) and indicating that four-coordinate Fe(III) is present, if at aU, at low levels only. This latter point is important because the X-ray powder diffraction patterns of... 

Some of the characteristics of the nonheme iron cores of ferritins and hacfers are given in Table II. As can be seen there is a wide variation in properties, though these do not seem to depend solely on overall core size. The mean core diameters measured by electron microscopy for human ferritin (84) and P. aeruginosa hacfer (100) were found to he 70-75 and 60-65 A, respectively, with, in both cases, a distribution of sizes between 55 and 80 or 85 A. The maximum core attainable for human or horse ferritin corresponds to 4500 atoms of Fe per molecule (49), or —33% of the mass of the fully loaded protein. The bacfer core contains less iron and thus is considerably less densely packed. 

There is still great uncertainty surrounding the formation and breakdown of the iron core of ferritin. For relatively rapid iron uptake and release the iron needs to be in the ferrous state. Thus, because the iron in the core is mostly in the ferric state, most attention has been directed toward redox-linked iron uptake and release. However, Fe + is stable in the core for considerable periods of time 115,144), >16 hr in some studies (116), leading to the suggestion that in vivo not all the iron in the core is Fe ". ... 

Electron microscopy and Mossbauer spectroscopy show (39) that the iron in hemosiderin is in the form of mineral phases, much like the iron core of ferritin. However, hemosiderin is insoluble in water at pH 7 and thus has not been chemically characterized to the same extent as ferritin. Nevertheless, the available evidence favors the formation of hemosiderin from the degradation and aggregation of ferritin (3,148). 

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). 

Hemosiderin is a much less studied molecule. It is generally considered a product of the degradation of ferritin [IT]. The diameters of the iron cores of ferritin and of hemosiderin were determined by electron microscopy on ferritin and hemosiderin isolated from the tissues. As presented in the table, the average diameters of the cores are significantly smaller in brain ferritin and hemosiderin than in liver ferritin and hemosiderin (3.1-3.7 nm versus 6.0 nm and 2.0 05 nm versus 3.5 nm, respectively) [19]. 

The smaller diameter of the iron cores of ferritin in brain compared to that in liver tissues fits well with the blocking temperatures measured by Mossbauer spectroscopy. The blocking temperature determined by MS for SN is about I5 K [20] compared to about 35-40 K for human liver [21 ]. Figure 16.4 presents the Mossbauer spectra obtained from SN at 20 K, I OK, and 4.1 K, from which the blocking temperature was estimated. 

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