L-γ-polyglutamate in Bacillus: stereocontrol by the PgsA epimerase and selective γ-PGA hydrolases

Poly-γ-glutamic acid (γ-PGA) occurs in three stereochemical forms-L-, DL-, and D-polymers-and the D/L ratio profoundly affects crystallinity, degradability, and materials performance. Bacillus subtilis and related secretors typically produce DL-PGA, whereas halophilic archaea yield stereoregular L-PGA and Bacillus anthracis forms a covalently anchored D-PGA capsule. Recent work has clarified how the PgsBCA/CapBCA modules access these outputs: PgsB and PgsC build and export a purely L-Glu polymer, while the MslH-like enzyme PgsA/CapA introduces D-Glu, establishing the D/L ratio. In the extracellular space, the D/L composition of γ-PGA is shaped by stereoselective hydrolases-PgdS, NlpC/P60 DL-endopeptidases, phage-derived Pgh enzymes, and GGT-family proteins. We integrate these biosynthetic and degradative pathways into a stereochemical framework and outline how tuning epimerase activity and controlling these hydrolases may enable high-molecular-weight, L-rich γ-PGA in Bacillus for coatings, absorbents, biomedical applications, and personal-care formulations where L-PGA surpasses DL-PGA in moisture retention and stability.