Zinc is found exclusively as Zn(II) in biological systems, and is an essential component of select prokaryotic and eukaryotic proteins. A zinc-containing bacteriochlorophyll has also been discovered in photosynthetic bacteria of the genus Acidiphilium (reviewed by Hiraishi and Shimada, 2001). Zinc is essential for DNA synthesis (reviewed by MacDonald, 2000) and a healthy immune system (reviewed by Dardenne, 2002) in humans. However, high concentrations of zinc in an organism can lead to zinc toxicity (ToxFAQs: Zinc).

"Zinc finger" transcription factors regulate a variety of cellular processes in prokaryotes and eukaryotes (reviewed by Bouhouche et al, 2000). The Zn(II)2Cys6 binuclear cluster DNA binding motif, which is unique to fungal DNA-binding proteins, features six cysteine residues that coordinate two zinc ions (reviewed by Todd and Andrianopulos, 1997). Zinc metalloproteases (Miyoshi and Shinoda, 2000) and zinc metallophospholipases C (Titball, 1993) have been identified as virulence factors in some pathogenic bacteria. Other microbial zinc-containing enzymes include superoxide dismutases (reviewed by Klotz et al, 2003) and carbonic anhydrases (reviewed by Smith and Ferry, 2000).

Zinc does not participate in redox reactions under physiological conditions, but free zinc ions can inactivate proteins and cofactors through inappropriate binding and are a hazard in the cellular environment. Therefore, both prokaryotic and eukaryotic cells have mechanisms to regulate the transport and compartmentalization of zinc ions. Metallothioneins that bind and sequester zinc ions in yeast (reviewed by Borrelly et al, 2002) and some bacteria (reviewed by Robinson et al, 2001) have been described. In bacteria, high-affinity zinc uptake occurs via ABC-type transporters and zinc efflux occurs through P-type ATPases and RND multi-drug efflux pumps (reviewed by Hantke, 2001). In eukaryotes, the regulation and mechanisms of zinc uptake and intracellular transport have been most extensively characterized in Saccharomyces cerevisiae (reviewed by Eide, 2003).

Dissimilatory bioreduction of zinc has not been documented. However, microorganisms can immobilize soluble zinc through biosorption and bioaccumulation. Kjaegaar et al (2001) reported a method for improving cell-surface zinc binding of Escherichia coli cells through creation of a display library in which peptide sequences within type I fimbriae were varied. Intracellular zinc accumulation has also been observed in bacteria (Kasan and Stegmann, 1987). Biomass from fungi and algae can remove zinc ions from solution through biosorption (reviewed by Volesky and Holan, 1995).

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