Mechanism of hexavalent chromium detoxification by microorganisms and bioremediation application potential: A review

Chromium has been widely used in various industries. Hexavalent chromium (Cr6+) is a priority toxic, mutagenic and carcinogenic chemical, whereas its reduced trivalent form (Cr3+) is much less toxic and insoluble. Hence, the basic process for chromium detoxification is the transformation of Cr6+ to Cr3+. A number of aerobic and anaerobic microorganisms are capable of reducing Cr6+. In the presence of oxygen, microbial reduction of Cr6+ is commonly catalyzed by soluble enzymes, except in Pseudomonas maltophilia O-2 and Bacillus megaterium TKW3, which utilize membrane-associated reductases. Recently, two soluble Cr6+ reductases, ChrR and YieF, have been purified from Pseudomonas putida MK1 and Escherichia coli, respectively. ChrR catalyzes an initially one-electron shuttle followed by a two-electron transfer to Cr6+, with the formation of interniediate(s) Cr5+ and/or Cr4+ before further reduction to Cr3+. YieF displays a four-electron transfer that reduces Cr6+ directly to Cr3+. The membrane-associated Cr6+ reductase of B. megaterium TKW3 was isolated, but its reduction kinetics is as yet uncharacterized. Under anaerobic conditions, both Soluble and membrane-associated enzymes of the electron transfer system were reported to mediate Cr6+ reduction as a fortuitous process coupled to the oxidation of an electron donor substrate. In this process, Cr6+ serves as the terminal electron acceptor of an electron transfer chain that frequently involves cytochromes (e.g., b and c). An expanding array of Cr6+ reductases allows the selection of enzymes with higher reductive activity, which genetic and/or protein engineering may further enhance their efficiencies. With the advancement in technology for enzyme immobilization, it is speculated that the direct application of Cr6+ reductases may be a promising approach for bioremediation of Cr6+ in a wide range of environments. (c) 2006 Published by Elsevier Ltd.