Purification and characterization of a novel glutamate decarboxylase from Bacillus megatarium

Bacterial glutamate decarboxylase (GAD) is a useful enzyme to transform glutamate into γ-aminobytyric acid (GABA), a component of pharmaceuticals, functional foods, and the biodegradable plastic polyamide 4. This enzyme exhibits an acidic pH optimum (usually pH 4-5), and sharply loses activity at pH 6. However, the expanding activity of GAD at a near-neutral pH will be useful in industry. In the current study, we identified and characterized a homologue of glutamate decarboxylase from Bacillus megatarium. The gene product designated Bm-GAD exhibited homology to enzymes from E.coli and Lactobacillus brevis (50% and 23%, respectively). Bm-Gad was cloned, overexpressed in E.coli BL21(DE3), and purified to homogeneity. The purified recombinant Bm-GAD was approximately 55 kDa. The enzyme showed optimal activity in the range of pH 5-6 at 37 ^oC, and the enzyme had a specific activity of 27 U/mg at pH 6. The enzyme was stable under 40 ^oC for at least 4 hours, and the enzyme activity could be significantly inhibited by Cu²⁺. The expanding pH active of the Bm-GAD toward a near-neutral pH makes it better candidate for GABA production in industry. The production of GABA using E.coli whole-cell biocatalysis will be further investigated.