Immobilization and enzymatic properties of glutamate decarboxylase from Enterococcus faecium by affinity adsorption...

Glutamate decarboxylase (GAD, EC 4.1.1.15) is an important enzyme in gamma-aminobutyric acid biosynthesis and DL-glutamic acid resolution. In this study, the Enterococcus faecium-derived GAD was successfully immobilized by regenerated chitin (RC) via specific adsorption of cellulose-binding domain (CBD). The optimal binding buffer was 20 mmol/L phosphate buffer saline (pH 8.0), and the RC binding capacity was 1.77 ± 0.11 mg_cbd-gad/g_rc under this condition. The ratio of wet RC and crude enzyme solution used for immobilization was recommended to 3:50 (g/mL). To evaluate the effect of RC immobilization on GAD, properties of the immobilize GAD (RC-CBD-GAD) were investigated. Results indicated RC-CBD-GAD was relatively stable at pH 4.4-5.6 and temperature - 20-40 °C, and the optimal reaction pH value and temperature were pH 4.8 and 50 °C, respectively. When it was reacted with 5 mmol/L of follow chemical reagents respectively, the activity of RC-CBD-GAD was hardly affected by EDTA, KCl, and NaCl, and significantly inactivated by AgNO₃, MnSO₄, MgSO₄, CuSO₄, ZnSO₄, FeCl₂, FeCl₃, AlCl₃, CaCl_2, and Pb(CH₃COO)₂. The apparent K_m and V_max were 28.35 mmol/L and 147.06 μmol/(g_RC-CBD-GAD·min), respectively. The optimum time for a batch of catalytic reaction without exogenous pH control was 2 h. Under this reaction time, RC-CBD-GAD had a good reusability with a half-life of 23 cycles, indicating that it was very attractive for GABA industry. As a novel, efficient, and green CBD binding carrier, RC provides an alternative way to protein immobilization.