Efficient Biosynthesis of Low-Molecular-Weight Poly-γ-glutamic Acid Based on Stereochemistry Regulation in Bacillus amyloliquefaciens

Low-molecular-weight poly-γ-glutamic acid (LMW-γ-PGA) has attracted much attention because of its many potential applications in food, agriculture, medicine, and cosmetics. Enzymatic degradation is an efficient way for the synthesis of LMW-γ-PGA. However, the stereochemistry of γ-PGA limits the degradation of γ-PGA. This study identifies the role of γ-PGA synthase (pgsA) and glutamate racemase (racE) in the regulation of γ-PGA stereochemistry and demonstrates their combinational use for LMW-γ-PGA synthesis. First, the expression of pgsA and racE was enhanced, leading to improvements both in the molecular weight (Mw) and the d-glutamate proportion of γ-PGA. Then, an optimal combination of pgsA, racE, and γ-PGA hydrolase pgdS was constructed by exchanging the gene origins for the synthesis of LMW-γ-PGA. Finally, the Mw of γ-PGA was decreased to 6-8 kDa, which was much lower compared with the case without stereochemistry switching (20-30 kDa). This study provides a novel strategy to control the Mw of γ-PGA based on stereochemistry regulation and lays a solid foundation for synthesis of LMW-γ-PGA.