Radical species in the catalytic pathways of enzymes from anaerobes

Radicals are reactive intermediates of growing importance in enzymatic cata lysis. There are reactions in which neutral radicals participate and those where radical anions are involved. The former class is illustrated by lysin e 2,3-aminomutase and also by enzymes dependent on coenzyme Bit, that catal yse carbon skeleton rearrangements (e.g. glutamate mutase). A substrate-bas ed radical for both lysine 2,3-aminomutase and glutamate mutase has been ch aracterised by EPR spectroscopy. Representatives of the second class are 2- hydroxyglutaryl-CoA dehydratase, benzoyl-CoA reductase, DNA photolyase and chorismate synthase, all of which may generate the radical anion by one-ele ctron reduction. 4-Hydroxybutyryl-CoA dehydratase, pyruvate formate lyase, and the coenzyme B-12-dependent eliminases (ribonucleotide reductase, ethan olamine ammonia lyase and diol dehydratase) could be examples of radical an ion formation by one-electron oxidation. The electron-rich ketyl-like radic al anions cause the elimination of an adjacent group. The advantages of usi ng radicals as intermediates in enzymatic transformations are their high re activity and special properties. However, this reactivity includes rapid bi molecular combination with dioxygen and radicals are therefore primarily ut ilised as intermediates by anaerobic organisms. (C) 1999 Federation of Euro pean Microbiological Societies. Published by Elsevier Science B.V. All righ ts reserved.