On the Road to Bioremediation of “Dioxin”

Radical enzymes in anaerobes

This review describes enzymes that contain radicals and/or catalyze reactions with radical intermediates. Because radicals irreversibly react with dioxygen, most of these enzymes occur in anaerobic bacteria and archaea. Exceptions are the families of coenzyme B(12)- and S-adenosylmethionine (SAM)-dependent radical enzymes, of which some members also occur in aerobes. Especially oxygen-sensitive radical enzymes are the glycyl radical enzymes and 2-hydroxyacyl-CoA dehydratases. The latter are activated by an ATP-dependent one-electron transfer and act via a ketyl radical anion mechanism. Related enzymes are the ATP-dependent benzoyl-CoA reductase and the ATP-independent 4-hydroxybenzoyl-CoA reductase. Ketyl radical anions may also be generated by one-electron oxidation as shown by the flavin-adenine-dinucleotide (FAD)- and [4Fe-4S]-containing 4-hydroxybutyryl-CoA dehydratase. Finally, two radical enzymes are discussed, pyruvate:ferredoxin oxidoreductase and methane-forming methyl-CoM reductase, which catalyze their main reaction in two-electron steps, but subsequent electron transfers proceed via radicals.

Recent advances in oxygenase-catalyzed biotransformations

Oxygenases continue to be widely studied for selective biooxidation of organic compounds. Protein engineering has resulted in heme and flavin monooxygenases with widely altered substrate specificities, and attempts have been reported to scale-up reactions catalyzed by these enzymes. Cofactor regeneration is still a key issue in these developments. Protein engineering contributed to understanding of structure versus function in dioxygenases.