Iron occupies a central region in the first row of transition-metal elements in the periodic table and, likewise, is central to many metabolic processes in bacteria. This prevalence of iron-containing proteins derives from the availability of iron in nature, where it exists in concentrations of 5 X 105 ppm in primary igneous rocks. Thus biological systems are constantly exposed to iron and evolutionary processes have capitalized on this metal's abundance in devising enzymes that catalyze a wide variety of reactions. In addition to its availability, iron also possesses chemical properties that allow it to conform to a diverse set of metabolic functions. Although inorganic complexes of Fe1+ and Fe4+ are known, biologically relevant free iron typically exists in Fe2+ and Fe3+ forms. Both Fe2+ and Fe3+ exist in low- and high-spin configurations and the facility of interconversion between the low- and high-spin forms has important biochemical implications, as discussed in detail by Brady et al (1968). The energy difference between low- and high-spin forms for given oxidation state can be very small. Thus the modulation of spin state by ligand charges and differences in geometry can be important in catalytic mechanisms.

Microorganisms have evolved a variety of highly regulated mechanisms to acquire ferric and ferrous iron. Both fungi and bacteria secrete ferric iron-chelating molecules ("siderophores") to facilitate iron uptake from the environment or to strip iron ions from other iron chelators (Clarke et al, 2001; Haas, 2003). Membrane-bound reductases that reduce Fe3+ to the more soluble Fe2+ form for transport have been described in bacteria and yeast (reviewed by Byers and Arceneaux, 1998; Kosman, 2003). Siderophore-independent ferric iron transport systems have also been discovered in some bacteria (Angerer et al, 1992; Saken et al, 2000). Although iron is required for growth by nearly all microorganisms, free Fe2+ ions are efficient catalysts that can rapidly generate toxic oxygen free radicals within cells. This problem is addressed in prokaryotes and eukaryotes by ferritins, which are iron-binding proteins that function to store surplus iron for later use and to prevent the formation of iron-catalyzed free radicals (reviewed by Arosio and Levi, 2002).

Dissimilatory reduction of ferric to ferrous iron coupled to the oxidation of organic and inorganic substrates has been observed in several prokaryotic genera (reviewed by Lovley, 1993). Some chemolithotrophic prokaryotes can derive energy for growth through the oxidation of ferrous iron minerals (reviewed by Straub et al, 2001,). Iron-oxidizing bacteria are used in the mining industry for the extraction of gold and other metals from ores (Rawlings et al, 2003). Shewanella putrefaciens, an iron-reducing bacterium, was found to accumulate microscopic particles of an iron oxide when grown in a defined medium with iron provided at typical environmental concentration (Glasauer et al, 2002).

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