Radical mechanisms of S-adenosylmethionine-dependent enzymes

Abstract

Publisher Summary This chapter discusses the family of S-adenosylmethione (SAM)-dependent enzymes that make use of the 5’-deoxyadenosyl radical, with special reference to the mechanism by which SAM is cleaved reversibly at the active site. One member of the family, lysine 2,3-aminomutase, makes use of the 5’-deoxyadenosyl radical to initiate the molecular rearrangement of a substrate, and the radical is subsequently regenerated in the catalytic cycle, such that SAM functions in a catalytic role as a true coenzyme. The other family members use SAM as a substrate, and the 5’-deoxyadenosyl free radical appears to be an intermediate in an irreversible hydrogen abstraction reaction. The latter enzymes include the pyruvate formate-lyase activating enzyme, the anaerobic ribonucleotide reductase from escherichia coli , biotin synthase, and lipoic acid synthase. The chapter also considers the functions of the 5’-deoxyadenosyl radical and the mechanisms of these diverse reactions. Photolytic or thermal cleavage of adenosylcobalamin produces cob(II)alamin and the 5’- deoxyadenosyl radical through homolytic scission of the Co-C bond. Spectral and kinetic evidence indicate that this same cleavage is brought about at the active sites of adenosylcobalamin-dependent enzymes, and the ensuing 5’-deoxyadenosyl radical is thought to initiate the free radical-based reaction mechanisms catalyzed by those enzymes.