Copper trails only iron and zinc as the most abundant metallic element in biological systems. The organometallic and bioinorganic chemistry of copper is dominated by Cu(I) and Cu(II) oxidation states. In enzymology, copper is somewhat similar to iron in that it functions in a series of oxidases (reviewed by Whittaker, 1999), oxygenases (reviewed by Blain et al, 2002), and low-molecular-weight electron transfer proteins that are reminiscent of ferredoxins (reviewed by Mogi et al, 1994). Furthermore, although one class of superoxide dismutases is known that contains iron, another class contains copper and zinc in which the copper undergoes redox change during catalysis (reviewed by Bannister et al, 1991).

Although copper is an essential element, free copper ions are highly reactive and catalyze formation of toxic reactive oxygen species inside cells (reviewed by Ercal et al, 2002). Subsequently, prokaryotes and eukaryotes have evolved mechanisms that regulate copper uptake, transfer to enzymes, sequestration, and efflux (reviewed by Pena et al, 1999). The mechanisms of copper homeostasis have been well characterized in the Gram-positive bacterium Enterococcus hirae (reviewed by Lu et al, 2003) and in yeast (reviewed by Labbe and Thiele, 1999). Periplasmic copper binding proteins appear to have a role in copper resistance in Gram-negative bacteria (reviewed by Cooksey, 1994). In Saccharomyces cerevisiae, reduction of Cu(II) to Cu(I) by metalloreductases on the cell surface is necessary for high-affinity copper uptake (Hassett and Kosman, 1995).

Sugio et al (1990) found that Thiobacillus ferrooxidans could reduce Cu(II) to Cu(I) using elemental sulfur as an electron donor. Biofilms composed of sulfate-reducing bacteria were shown to accumulate copper from solution as sulfides (White and Gadd, 2000). The potential for other microorganisms to immobilize soluble copper through bioaccumulation and biosorption has been explored (Pradhan and Rai, 2001; Tsekova et al, 2000).

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Sugio T, Tsujita Y, Inagaki K, Tano T. Reduction of cupric ions with elemental sulfur by Thiobacillus ferrooxidans. Appl. Environ. Microbiol. 1990;56:693-96.