ABSTRACT
The abstraction or transfer of a hydrogen atom (H•) from an unactivated bond in vivo is often carried out by transition metal-containing enzymes. Owing to its multiple biologically accessible oxidation states, iron is more often than not called upon to aid in hydrogen atom abstraction mechanisms. Under aerobic conditions, cytochrome P450 enzymes may utilize an oxyferryl heme group to abstract an H-atom from a C-H bond, eventually producing an alcohol [1]. This oxygen insertion reaction is used in solubilizing foreign hydrophobic chemicals and synthesizing biologically important chemicals. Under anaerobic conditions, the radical S-adenosyl-l-methionine (SAM) superfamily [2] is primarily responsible for H-atom abstraction events that initiate a wide array of biochemical transformations. Radical SAM enzymes all contain a [4Fe-4S] cubane cluster, whose irons are coordinated by sulfurs from three cysteine residues and in a bidentate fashion by the carboxyl and amine functional groups of SAM (Figure 2.1). More than 5000 anaerobic and aerobic organisms from all three domains of life possess radical SAM enzymes. Roughly 50,000 radical SAM enzymes have been putatively identied through bioinformatics analyses of sequences in the National Center for Biotechnology Information sequence database.
