应用拉氏图信息提高短乳杆菌谷氨酸脱羧酶催化性能

Enhancing glutamate decarboxylase activity by site-directed mutagenesis: an insight from Ramachandran plot

摘要:

谷氨酸脱羧酶 (Glutamate decarboxylase，GAD) 是用于催化L-谷氨酸脱羧合成γ-氨基丁酸 (γ-aminobutyrate，GABA) 的唯一酶，提高GAD的催化活力或热稳定性，有利于GABA的高效制备和生产。以热稳定性和活性为筛选目标，通过研究短乳杆菌GAD1407三维模拟结构的拉氏图，确定不稳定氨基酸残基位点K413，采用定点突变的方法构建该位点的突变体，并测定野生型酶和突变酶的热稳定性和活力。结果表明突变酶K413A和突变酶K413I分别在热稳定性和酶活力上获得了提高，突变酶K413A在50 ℃的半衰期为105 min，是野生酶的2.1倍；突变酶K413I热稳定性没有明显的提高，但其酶活力却得到了有效提高，约为野生型的1.6倍。因此，通过拉氏图提供的结构信息可为利用理性设计提高GAD活性和热稳定性提供指导。

Abstract:

Glutamate decarboxylase (GAD) can catalyze the decarboxylation of glutamate into γ-aminobutyrate (GABA) and is the only enzyme of GABA biosynthesis. Improving GAD activity and thermostability will be helpful for the highly efficient biosynthesis of GABA. According to the Ramachandran plot information of GAD1407 three-dimensional structure from Lactobacillus brevis CGMCC No. 1306, we identified the unstable site K413 as the mutation target, constructed the mutant GAD by site-directed mutagenesis and measured the thermostability and activity of the wide type and mutant GAD. Mutant K413A led to a remarkably slower inactivation rate, and its half-life at 50 ℃ reached 105 min which was 2.1-fold higher than the wild type GAD1407. Moreover, mutant K413I exhibited 1.6-fold higher activity in comparison with the wide type GAD1407, although it had little improvement in thermostability of GAD. Ramachandran plot can be considered as a potential approach to increase GAD thermostability and activity.