Characterization and mutagenesis of a novel Mycobacterium smegmatis-derived glutamate decarboxylase active at neutral pH

γ-aminobutyric acid (GABA) has various physiological functions and is widely used in medicine, food, and other fields. Glutamate decarboxylase (GAD) is a key enzyme that catalyzes the decarboxylation of L-glutamate to synthesize GABA. However, the industrial application of microorganism-derived GAD is limited by its rapid loss of enzymatic activity with pH approaching neutrality. In this study, a novel glutamate decarboxylase, GAD MSM, from Mycobacterium smegmatis was overexpressed and purified. On the basis of homologous modeling and substrate molecular docking, several GAD MSM mutants were constructed, and their enzymatic properties were analyzed. The results showed that the optimal pH of wild-type GAD MSM is 5.4; at pH 6.2, 22.8% enzymatic activity was retained. The T211I replacement in GAD and C-terminal deletion mutant GAD MSMΔC showed relatively high catalytic activity in a pH range of 5.0–7.0. The V max and K m values of GAD MSMΔC were 14.69 and 5.70, respectively, at pH 5.5, and 9.87 and 6.17, respectively, at pH 7.0. Compared with the wild-type GAD, GAD MSMΔC maintained higher affinity and enzymatic activity of the substrate, maintaining 78.5% of the highest enzymatic activity even at pH 7.0, which is the highest reported activity retention for GAD under neutral pH condition. Therefore, GAD MSMΔC can be used for the transformation of high-yielding strains and industrial production of GABA.