Moreover recent data indicate migration of iron atoms

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detail. Moreover, recent data indicate migration of iron atoms during the early stages of core formation and the possible persistence of Fe 2 + for periods of time up to 24 hours. When large numbers of Fe(n) atoms are added, the protein coat appears to stabilize the encapsulated Fe(n).34a,b Formation of the iron core
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16 1 / TRANSITION-METAL STORAGE, TRANSPORT, AND BIOMINERALIZATION of ferritin has analogies to surface corrosion, in which electrochemical gradients are known to occur. Whether such gradients occur during ferritin formation and how different protein coats might influence proton release or alter the structure of the core are subjects only beginning to be examined. 2. The storage of zinc, copper, vanadium, chromium, molybdenum, cobalt, nickel, and manganese Ions of nonferrous transition metals require a much less complex biological storage system, because the solubilities are much higher (210 - 8 M) than those for Fe 3+ . As a result, the storage of nonferrous transition metals is less obvious, and information is more limited. In addition, investigations are more difficult than for iron, because the amounts in biological systems are so small. Essen- tially nothing is known yet about the storage of vanadium, chromium, molyb- denum, cobalt, nickel, and manganese, with the possible exception of accumu- lations of vanadium in the blood cells of tunicates. Zinc and copper, which are used in the highest concentrations of any of the non-ferrous transition metals, are specifically bound by the protein metallothionein 35,36 (see Figure 1.10). Like the ferritins, the metallothioneins are a family of proteins, widespread in nature and regulated by the metals they bind. In contrast to ferritin, the amounts of metal stored in metallothioneins are smaller (up to twelve atoms per molecule), the amount of protein in cells is less, and the template (mRNA) is not stored. Because the cellular concentrations of the metallothioneins are relatively low and the amount of metal needed is relatively small, it has been difficult to study the biological fate of copper and zinc in living organisms, and to discover the natural role of metallothioneins. However, the regulation of metallothionein synthesis by metals, hormones, and growth factors attests to the biological importance of the proteins. The unusual metal environments of metallothioneins have attracted the attention of bioinor- ganic chemists. Metallothioneins, especially in higher animals, are small proteins 35,36 rich in cysteine (20 per molecule) and devoid of the aromatic amino acids phenylal- anine and tyrosine. The cysteine residues are distributed throughout the peptide chain. However, in the native form of the protein (Figure 1.10), the peptide chains fold to produce two clusters of -SH, which bind either three or four atoms of zinc, cadmium, cobalt, mercury, lead, or nickel. Copper binding is distinct from zinc, with 12 sites per molecule.
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