Radical enzymes in anaerobes

Annu Rev Microbiol. 2006:60:27-49. doi: 10.1146/annurev.micro.60.080805.142216.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Amino Acids / metabolism
  • Bacteria, Anaerobic / enzymology*
  • Cobamides / physiology*
  • Flavins / metabolism
  • Free Radicals / metabolism*
  • Glycine / metabolism*
  • Hydro-Lyases / physiology
  • Methane / biosynthesis
  • Pyruvate Synthase / physiology
  • S-Adenosylmethionine / metabolism*


  • Amino Acids
  • Cobamides
  • Flavins
  • Free Radicals
  • S-Adenosylmethionine
  • Pyruvate Synthase
  • Hydro-Lyases
  • cobamamide
  • Methane
  • Glycine