Sulfur to α-carbon thioether-containing peptides (sactipeptides) are ribosomally synthesized post-translationally modified peptides with bacteriocidal activities. The thioether cross-link, which is required for biological activity, is installed by a member of the radical S-adenosyl-l-methionine (SAM) superfamily in the peptide substrate. Herein, we show that the radical SAM enzyme, SkfB, utilizes the 5′-deoxyadenosyl radical generated from the reductive cleavage of SAM to abstract a hydrogen atom from the α-carbon of the amino acid at position 12 in the substrate, SkfA, to initiate the installation of a thioether cross-link. The insights from this work can be applied to all radical SAM sactipeptide maturases.
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