Enzyme systems for biodegradation of polychlorinated dibenzo-p-dioxins.

The angular dioxygenase, cytochrome P450, lignin peroxidase, and dehalogenase are known as dioxin-metabolizing enzymes. All of these enzymes have metal ions in their active centers, and the enzyme systems except for peroxidase have each distinct electron transport chain. Although the enzymatic properties of the angular dioxygenase, lignin peroxidase, and cytochrome P450 have been studied well, the information about dehalogenase is much less than other enzyme systems due to its instability under the aerobic conditions. However, this enzyme system appears to be quite promising from the viewpoint of practical use for bioremediation, because dehalogenases are capable of degradation of polychlorinated dibenzo-p-dioxins (PCDDs) with more than four chlorine substituents, whereas the other three enzyme systems prefer low-chlorinated PCDDs. On the other hand, protein engineering of angular dioxygenase, lignin peroxidase, and cytochrome P450 based on their tertiary structures has great potential to generate highly efficient dioxin-metabolizing enzymes. Actually, we successfully generated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-metabolizing enzyme by site-directed mutagenesis of cytochrome P450. We hope that recombinant microorganisms harboring genetically engineered dioxin-metabolizing enzymes will be used for bioremediation of soil contaminated with PCDDs and polychlorinated dibenzofurans in the near future.