Peptide Epimerase Responsible for d-Amino Acid Introduction in Poly-γ-glutamic Acid Biosynthesis

Poly-γ-glutamic acid (PGA) is a natural polymer of d- and/or l-glutamic acid (Glu) linked by isopeptide bonds. We recently showed that PGA synthetase, an enzyme complex composed of PgsB, PgsC, and PgsA, uses only l-Glu for polymerization, and d-Glu residues are introduced by peptide epimerization. However, it remains unclear which of the three enzymes is responsible for epimerization because in vitro functional characterization of the membrane-associated PgsBCA complex has never been successful. Here, we performed gene exchange experiments and showed that PgsA is responsible for the epimerization. Additionally, we identified a region in PgsA that modulates epimerization activity based on homology modeling from the recently solved structure of MslH, which showed 53% identity to PgsA. Our results suggested that d/l-ratios of the PGA product can be altered by introducing amino acid substitutions in this region, which will be useful for the production of PGA with controlled d/l-ratios.