Ferritin, the cellular Fe store
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص737-738
2025-10-22
56
Ferritin, the cellular Fe store
Ferritin is the principal store of non-haem Fe in animals (most Fe is occupied in haemoglo bin and myoglobin) and, when fully loaded, contains 20 per cent Fe by mass. It occurs in all types of organism, from mammals to prokaryotes. In mammals, it is found particularly in the spleen and in blood. Ferritins have two components, a ‘mineral’ core that contains up to 4500 Fe atoms (mammalian ferritin) and a protein shell. Apoferritin (the protein shell devoid of Fe) can be prepared by treatment of ferritin with reducing agents and an Fe (II) chelating ligand (such as 1,10-phenanthroline or 2,2-bipyridyl). Dialysis then yields the intact shell. Apoferritins have average molar masses in the range 460 to 550 kg mol 1. The protein shell (Fig. 27.13) consists of 24 subunits that link together to form a hollow sphere with twofold, threefold (as shown in the illustration), and fourfold symmetry axes. Each sub unit consists of a bundle of four long and one short α helices, with a loop that forms a section of β sheet with a neighbouring subunit. The mineral core is composed of hydrated Fe (III) oxide with varying amounts of phosphate, which helps anchor it to the internal surface. The structure as revealed by X-ray or electron diffraction resembles that of the mineral ferrihydrite, 5Fe2O3 .9H2O, which is based on an hcp array of O2 and OH ions, with Fe (III) layered in both the octahedral and tetrahedral sites (20). The threefold and fourfold symmetry axes of apoferritin are, respectively, hydrophilic and hydrophobic pores. The threefold-axis pores are suited for the passage of ions. However, the ferrihydrite core is insoluble and Fe must be mobilized. The most feasible mechanism so far proposed for the reversible incorporation of Fe in ferritin involves its transport in and out as Fe (II), perhaps as the Fe2+ ion, which is soluble at neutral pH, but more likely some type of ‘chaperone’ complex. Oxidation to Fe (III) is thought to occur at specific di-iron binding sites known as ferroxidase centres, present in each of the subunits. Oxidation to Fe (III) involves the coordination of O2 and inner-sphere electron transfer:
2Fe (II)+O2+2H+→2Fe (III)+H2O2
The mechanism by which Fe is released almost certainly involves its reduction back to the more mobile Fe(II).
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