Radical S-Adenosylmethionine Peptide Epimerases: Detection of Activity and Characterization of d-Amino Acid Products.

The identification of the polytheonamide (poy) gene cluster led to the discovery of the enzyme PoyD, a member of the radical S-adenosylmethionine superfamily capable of introducing d-amino acids into a ribosomally synthesized peptide. This enzyme was used as a starting point to identify additional radical S-adenosylmethionine peptide epimerases in other cyanobacterial genomes, which show different epimerization patterns compared to PoyD. During the course of studying these enzymes by heterologous expression in Escherichia coli, we developed a two-step strategy to (1) detect epimerase activity and (2) localize where epimerization occurs based on an in vivo deuterium labeling strategy. The procedures for these two methods are described in the following chapter and will set the stage for further study of these enzymes.