Radical species in the catalytic pathways of enzymes from anaerobes

Abstract

Radicals are reactive intermediates of growing importance in enzymatic catalysis. There are reactions in which neutral radicals participate and those where radical anions are involved. The former class is illustrated by lysine 2,3-aminomutase and also by enzymes dependent on coenzyme B12, that catalyse carbon skeleton rearrangements (e.g. glutamate mutase). A substrate-based radical for both lysine 2,3-aminomutase and glutamate mutase has been characterised 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-electron reduction. 4-Hydroxybutyryl-CoA dehydratase, pyruvate formate lyase, and the coenzyme B12-dependent eliminases (ribonucleotide reductase, ethanolamine ammonia lyase and diol dehydratase) could be examples of radical anion formation by one-electron oxidation. The electron-rich ketyl-like radical anions cause the elimination of an adjacent group. The advantages of using radicals as intermediates in enzymatic transformations are their high reactivity and special properties. However, this reactivity includes rapid bimolecular combination with dioxygen and radicals are therefore primarily utilised as intermediates by anaerobic organisms.