Particularly significant as a model for ferritin

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particularly significant as a model for ferritin, because the structure of ferritin cores varies with the phosphate content. An asymmetric trinuclear (Fe30) 7+ complex 57 and an (FeO)l1 complex (Figure 1.21) have been prepared; these appear to serve as models for later stages of core nucleation (or growth). 59 Models for the full iron core of ferritin include ferrihydrite, which matches the ordered regions of ferritin cores that have little phosphate; however, the site vacancies in the lattice structure of ferrihydrite [FeO(OH)] appear to be more regular than in crystalline regions of ferritin cores. A polynuclear complex of iron and microbial dextran (a-l,4-D-glucose)n has spectroscopic (M6ssbauer, EXAFS) properties very similar to those of mammalian ferritin, presumably because the organic ligands are similar to those of the protein (-OH, -COOH). In contrast, a polynuclear complex of iron and mammalian chondroitin sulfate (a-l ,4-[a-1 ,3-D-glucuronic acid-N-acetyl-D-galactosamine-4-sulfate]n) contains two types of domains: one like mammalian ferritin [FeO(OH)] and one like hematite (a-Fe203), which was apparently nucleated by the sulfate, emphasizing
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32 06 032 "()033 Figure 1.21 The structure of a model for a possible intermediate in the formation of the ferritin iron core. The complex consists of 11 Fe(III) atoms with internal oxo-bridges and a coat of benzoate li- gands; the Fe atoms define a twisted, pentacapped trigonal prism. See Reference 53. the importance of anions in the structure of iron cores. 60 Finally, a model for iron cores high in phosphate, such as those from bacteria, is Fe-ATP (4: 1), in which the phosphate is distributed throughout the polynuclear iron complex, providing an average of 1 or 2 of the 6 oxygen ligands for iron. 61 The microenvironment inside the protein coat of ferritin has recently been modeled by encapsulating ferrous ion inside phosphatidylcholine vesicles and studying the oxidation of iron as the pH is raised. The efficacy of such a model is indicated by the observation of relatively stable mixtures of Fe(II)/Fe(III) inside the vesicles, as have also been observed in ferritin reconstituted experi- mentally from protein coats and ferrous ion. 43 ,54 Models for iron in ferritin must address both the features of traditional metal- protein interactions and the bulk properties of materials. Although such model- ing may be more difficult than other types of bioinorganic modeling, the diffi- culties are balanced by the availability of vast amounts of information on Fe-
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III. SUMMARY 33 protein interactions, corrosion, and mineralization. Furthermore, powerful tools such as x-ray absorption, Mossbauer and solid state NMR spectroscopy, scan- ning electron and proton microscopy, and transmission electron microscopy re- duce the number of problems encountered in modeling the ferritin ion core.
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