Application of ion-exchange resin as solid acid for buffer-free production of γ-aminobutyric acid using Enterococcus faecium cells

In this study, ion-exchange resin was applied as solid acid catalyst for the γ-aminobutyric acid (GABA) production using a strain of Enterococcus faecium (GZ2) by whole-cell bioconversion. It was observed that the accumulated GABA resulted in the inhibition of the bioconversion activity. To improve the productivity, a cation-exchange resin-based in situ product removal (ISPR) approach was developed. Three cation-exchange resins were tested to select a suitable adsorbent used in the bioconversion. The strong acidic cation-exchange resin IRP-69 exhibited the highest GABA production and remained helpful in reducing the free GABA concentration in the reaction solution. Compared to the bioconversion in sodium acetate buffer, the GABA production, with the addition of resin, was significantly improved by 2.12-fold. Furthermore, a fed-batch bioconversion strategy with the feeding of L-monosodium glutamate (L-MSG) was tested. The maximum production of GABA, 406.1 mM, was achieved after 12 h reaction with an initial substrate concentration of 200 mM, and a triple feeding of 100 mM L-MSG afterwards at 4 h, 6 h and 8 h. The result shows that the operation of bioconversion reaction in a feeding substrate batch mode is an effective approach to improve the efficiency and productivity of GABA.