Interest ingly certain bacteria azotobacter have

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denum cofactor, or FeMoCo) and in an iron-sulfur center (Chapter 7). Interest- ingly, certain bacteria (Azotobacter) have alternative nitrogenases, which are produced when molybdenum is deficient and which contain vanadium and iron or only iron. All other known Mo-proteins are also Fe-proteins with iron cen- ters, such as tetrapyrroles (heme and chlorins), Fe-sulfur clusters, and, appar- ently, non-heme/non-sulfur iron. Some Mo-proteins contain additional cofactors such as the Havins, e.g., in xanthine oxidase and aldehyde oxidase. The number of redox centers in some Mo-proteins exceeds the number of electrons trans- ferred; reasons for this are unknown currently. B. Chemical Properties Relative to Storage and Transport 1. Iron Iron is the most abundant transition element in the Earth's crust and, in general, in all life forms. An outline of the distribution of iron in the Earth's crust 20 ,21 is shown in Table 1.2. As can be seen, approximately one-third of the Earth's mass is estimated to be iron. Of course, only the Earth's crust is relevant for life forms, but even there it is the most abundant transition element. Its concentration is relatively high in most crustal rocks (lowest in limestone, which is more or less pure calcium carbonate). In the oceans, which constitute 70 percent of the Earth's surface, the concentration of iron is low but increases with depth, since this iron exists as suspended particulate matter rather than as a soluble species. Iron is a limiting factor in plankton growth, and the rich 5
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6 1 / TRANSITION-METAL STORAGE, TRANSPORT, AND BIOMINERALIZATION Table 1,2 Iron: Its terrestrial distribution. a One third of Earth's mass, most abundant element by weight Distribution in crustal rocks (weight %): igneous 5.6 shale 4.7 sandstone 1.0 limestone 0.4 Ocean (70% of Earth's surface): 0.003--0.1 ppb, increasing with depth; limiting factor in plankton growth Rivers: 0.07-7 ppm K sp for Fe(OH)3 is approximately 10 -39, hence at pH 7 [Fe3+] 10 -IS M a Data from References la and 20. fisheries associated with strong up-welling of ocean depths result at least in part from the biological growth allowed by these iron supplies. Properties that dom- inate the transport behavior of most transition metal ions are: (l) redox chem- istry, (2) hydrolysis, and (3) the solubility of the metal ions in various com- plexes, particularly the hydroxides. As an example of the effects of solubility, consider the enormous variation in the concentration of iron in rivers, depending on whether the water is from a clear mountain stream running over rock or a muddy river carrying large amounts of sediment. However, the amount of dissolved iron in the form of free ferric ion or its hydrolysis products, whatever the source of water, is extremely low. As can be seen from the solubility of hydrated Fe(III) (K s ~ 10 -18 M) (Table 1.2), the concentration of free ferric ion is extraordinarily low at neutral pH; so significant concentrations of soluble iron species can be attained only by strong complex formation.
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