Effects of oxygen vectors on the synthesis and molecular weight of poly(γ-glutamic acid) and the metabolic characterization of Bacillus subtilis NX-2

The effects of different oxygen vectors on the synthesis and molecular weight of poly(γ-glutamic acid) (PGA) were investigated in the batch fermentation of Bacillus subtilis NX-2. n-Hexane, n-heptane, and n-hexadecane enhanced the PGA concentration and molecular weight. The PGA concentration reached a maximum of 39.4 ± 0.19 g L^-1, and the highest molecular weight obtained was (19.0 ± 0.02) × 10⁵ Da with the addition of 0.3% n-heptane. However, n-dodecane decreased the PGA concentration and molecular weight to final values of 20.1 ± 0.10 g L^-1 and (8.4 ± 0.02) × 10⁵ Da, respectively. Analysis of the intracellular nucleotide levels of B. subtilis NX-2 with n-heptane and n-dodecane additives showed that the lowest NADH/NAD⁺ ratio and ATP levels were obtained with the n-dodecane additives, which can explain the decreased PGA yield and molecular weight. The metabolic flux distribution of B. subtilis NX-2 with n-heptane and n-dodecane additives was also investigated. Flux distribution was primarily directed to the EMP and TCA cycles with n-heptane additives. The flux of 2-oxoglutarate to intracellular glutamate and the flux distribution from extracellular to intracellular glutamate both increased to improve PGA production.