Ferritin EXAFS

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 ... 

Ferritin has also been compared with iron-dextran by the EXAFS technique.1108 The apoferritin controls the deposition of the core. Reconstitution of ferritin under a range of conditions always gives the same structure, which is not the case in the absence.of apoferritin. There are metal-binding sites on the protein shell. There is evidence for the binding of iron to apoferritin, probably by carboxyl groups, but there is little detailed information on these sites.1098 On the other hand, other metal ions inhibit the formation of ferritin and may do this by binding at or close to the iron sites. Of most significance appear to be results on Tb3+, Zn2+ and V02+,... 

EXAFS studies of A. vinelandii bacterioferritin have shown that there are fewer Fe-Fe contacts in the mineral core than occur with native horse ferritin, and those that do occur are at a greater interatomic distance. Moreover, the EXAFS data showed that the core iron of bacterioferritin had 5 to 6 phosphorus atoms no more than 3.17 A distant, thus supporting a model for the core of an amorphous Fe(III)-phosphate complex in which some of the phosphate bridges Fe(III) ions and some of it is nonbridging. 

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... 

Ferritin has also been compared with iron-dextran by the EXAFS technique." ... 

Iron Core Only a small fraction of the iron atoms in ferritin bind directly to the protein. The core contains the bulk of the iron in a polynuclear aggregate with properties similar to ferrihydrite, a mineral found in nature and formed experimentally by heating neutral aqueous solutions of Fe(III)(N03)3. X-ray diffraction data from ferritin cores are best fit by a model with hexagonal close-packed layers of oxygen that are interrupted by irregularly incomplete layers of octahedrally coordinated Fe(III) atoms. The octahedral coordination is confirmed by Mossbauer spectroscopy and by EXAFS, which also shows that the average Fe(III) atom is surrounded by six oxygen atoms at a distance of 1.95 A and six iron atoms at distances of 3.0 to 3.3 A. 

---