Introduction of d‐Amino Acids in Minimalistic Peptide Substrates by an S‐Adenosyl‐l‐Methionine Radical Epimerase

Post‐translational modifying enzymes from the S‐adenosyl‐l‐methionine (AdoMet) radical superfamily garner attention due to their ability to accomplish challenging biochemical reactions. Among them, a family of AdoMet radical epimerases catalyze irreversible l‐ to d‐amino acid transformations of diverse residues, including 18 sites in the complex sponge‐derived polytheonamide toxins. Herein, the in vitro activity of the model epimerase OspD is reported and its catalytic mechanism and substrate flexibility is investigated. The wild‐type enzyme was capable of leader‐independent epimerization of not only the stand‐alone core peptide, but also truncated and cyclic core variants. Introduction of d‐amino acids can drastically alter the stability, structure, and activity of peptides; thus, epimerases offer opportunities in peptide bioengineering. Leader‐free to d: Post‐translational modifying enzymes from ribosomally synthesized peptide natural products offer new opportunities for small‐molecule bioengineering. The in vitro analysis of an S‐adenosyl‐l‐methionine radical enzyme is reported that generates d‐amino acids in the core region of its native precursor protein substrate, as well as in leaderless linear and cyclic cores.