Free radical mechanisms in enzymology

Free radicals are involved in amino acid, carbohydrate, and hydrocarbon metabolism; detoxification of xenobiotics; coenzyme and cofactor biosynthesis; hormone metabolism; DNA biosynthesis and repair; and antibiotic biosynthesis. Most free radical mechanisms require coenzymes and/or cofactors such as flavins, heme, pterins, quinone cofactors, di-iron complexes, mononuclear iron, copper, adenosylcobalamin, S-adenosylmethionine/[Fe-4S], thiamine pyrophosphate, and pyridoxal-5-phospate. Although much has been learned about radical mechanisms in enzymology, most are poorly understood from the standpoint of the energetics of radical initiation. To date, the explosive growth in the number of enzymes employing S-adenosylmethionine/[4Fe-4S] or mononuclear iron chemistry has rekindled interest in free radical mechanisms in enzymology.