Known cobalt compounds are largely composed of cobalt in oxidation states of I, II, and III. These are also biologically relevant oxidation states of cobalt. For example, Co(I)-B₁₂ is an important low-potential two-electron reduced form of an important cofactor found in bacterial enzymes. Cobalt is typically found in association with nickel in nature and it is in low abundance with respect to iron. Still, cobalt is an essential element in prokaryotes and is generally required in higher concentrations by anaerobes than by aerobes (reviewed by Santander et al, 1997).

The largest group of enzymes that require cobalt are the B₁₂ enzymes; however, several different types of cobalt-containing enzymes have been identified (reviewed by Kobayashi and Shimizu, 1999). B₁₂ and its derivatives (cobalamin and other cobalt corrinoids) mediate the bioalkylation of both metallic and non-metallic elements (reviewed by Pratt, 1993). B₁₂ biosynthesis is a complex multi-step process and the steps in the aerobic and anaerobic biosynthetic pathways have been determined (reviewed by Scott and Roessner, 2002). In addition, cobalt sometimes has a role as an enzymatic superacid and, in this regard, has proven useful as an experimental substitute for zinc as a probe of the metal coordination environment (Maret and Vallee, 1993).

Cobalt accumulation typically occurs via the non-specific CorA magnesium uptake system, and efflux is through RND-family transporters in Gram-negative bacteria and through CDF-family transporters in Gram-positive bacteria (reviewed by Nies, 1999). Several prokaryotes have been shown to couple the oxidation of organic and inorganic molecules to the reduction of Co(III) to Co(II) (Blessing et al, 2001; Liu et al, 2002; Roh et al, 2002).

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Pratt JM. Making and breaking the Co-alkyl bond in B₁₂ derivatives. Metal Ions in Biological Systems 1993;29:229-286.

ToxFAQs: Cobalt