D-amino acids endow peptides with diverse, desirable properties, but the post-translational and site-specific epimerization of L-amino acids into their D-counterparts is rare and chemically challenging. Bottromycins are ribosomally synthesized and post-translationally modified peptides that have overcome this challenge and feature a D-aspartate (D-Asp), which was proposed to arise spontaneously during biosynthesis. We have identified the highly unusual alpha/beta-hydrolase (ABH) fold enzyme BotH as a peptide epimerase responsible for the post-translational epimerization of L-Asp to D-Asp during bottromycin biosynthesis. The biochemical characterization of BotH combined with the structures of BotH and the BotH-substrate complex allowed us to propose a mechanism for this reaction. Bioinformatic analyses of BotH homologs show that similar ABH enzymes are found in diverse biosynthetic gene clusters. This places BotH as the founding member of a group of atypical ABH enzymes that may be able to epimerize non-Asp stereocenters across different families of secondary metabolites.
