Structure and function of radical SAM enzymes

'Radical SAM' enzymes juxtapose a [4Fe-4S] cluster and S-adenosyl-L-methionine (SAM) to generate catalytic 5'-deoxyadenosyl radicals. The crystal structures of oxygen-independent coproporphyrinogen III oxidase HemN and biotin synthase reveal the positioning of both cofactors with respect to each other and relative to the surrounding protein environment. Each is found in an unprecedented coordination environment including the direct ligation of the [4Fe-4S] cluster by the amino nitrogen and one carboxylate oxygen of the methionine moiety of SAM, as observed for other members of the Radical SAM family by ENDOR. The availability of two protein structures supported by biochemical and biophysical data underscores common features, anticipating the structural elements of other family members. Remaining differences emphasize the plasticity of the protein scaffold in functionally accommodating 600 family members.