that is responsible for catalyzing the γ -PGA synthesis in γ -PGA
synthetase complex was also assayed.
As shown in Fig. 5, the carbon flux from iso-citrate to
α-oxoglutarate increased and from α-oxoglutarate to succinylCoA decreased with KCl added. Thus, the carbon flux from α-
oxoglutarate to glutamate reached 19.2 mmol · g/DCW · h when
KCl added, which was 3.84-fold higher than that of without KCl.
The changes of enzyme activity of ICDH, ODHC, GDH and GOGAT
were consistent with the flux distribution. Generally, GDH and
GOGAT are responsible for the shunt from α-oxoglutarate to glutamate in many bacteria [12]. In this case, the activity of GOGAT
was 4.9 × 10−2 U/mg when KCl added, which was 2.88-fold higher
than that of without KCl. However, the activity of GDH was improved 2.13-fold by KCl added. The result showed that the contribution of GOGAT to the conversion of α-oxoglutarate to glutamate was slightly more than that of the GDH. Furthermore, the
extracellular glutamate uptake rate between A and B are not significantly different. Therefore, the higher yield of γ -PGA reached
25.62 g/L with KCl added, which mainly resulted from the increase of intracellular glutamate. In addition, transcriptional analysis showed that the expression of pgsB was improved 9.6-fold by
KCl added, which might be another reason for the improved γ -PGA
production.
Previously reported that the conversion of L-glutamate to
D-isomer in B. subtilis was mainly mediated by glutamate racemase [21]. Thus, in order to investigate the possible explanation for
the observed stereochemical modulation by KCl in B. subtilis GXA-
28, the metabolic flux and enzyme activities of glutamate node
were analyzed. Results showed that the flux from L-glutamate
to D-isomer increased from 22.9 to 37.6 mmol · g/DCW · h and
the activities of GLR increased from 0.42 to 0.53 U/mg when
KCl added. Thereby, the carbon flux from D-glutamate to γ -PGA
reached 35.2 mmol · g/DCW · h, which was increased 78.7% than
that without KCl. These results indicated that GLR plays an important role during the process of γ -PGA stereochemical regulation by
KCl.