Radical Mechanisms of Enzymatic Catalysis¹

Two classes of enzymatic mechanisms that proceed by free radical chemistry initiated by the 5′-deoxyadenosyl radical are discussed. In the first class, the mechanism of the interconversion of L-lysine and L-β-lysine catalyzed by lysine 2,3-aminomutase (LAM) involves four radicals, three of which have been spectroscopically characterized. The reversible formation of the 5′-deoxyadenosyl radical takes place by the chemical cleavage of S-adenosylmethionine (SAM) reacting with the [4 Fe–4S]⁺ center in LAM. In other reactions of SAM with iron-sulfur proteins, SAM is irreversibly consumed to generate the 5′-deoxyadenosyl radical, which activates an enzyme by abstracting a hydrogen atom from an enzymatic glycyl residue to form a glycyl radical. The glycyl radical enzymes include pyruvate formate-lyase, anaerobic ribonucleotide reductase from Escherichia coli, and benzylsuccinate synthase. Biotin synthase and lipoate synthase are SAM-dependent [4 Fe–4S] proteins that catalyze the insertion of sulfur into unactivated C–H bonds, which are cleaved by the 5′-deoxyadenosyl radical from SAM. In the second class of enzymatic mechanisms using free radicals, adenosylcobalamin-dependent reactions, the 5′-deoxyadenosyl radical arises from homolytic cleavage of the cobalt-carbon bond, and it initiates radical reactions by abstracting hydrogen atoms from substrates. Three examples are described of suicide inactivation through the formation of exceptionally stable free radicals at enzymatic active sites.