Abstract
Pyropia haitanensis is an important laver species in China. Its quality traits are closely related to the content of glutamic acid. Glutamate dehydrogenase (GDH) is a crucial enzyme in the glutamic acid metabolism. In this study, two GDH genes from P. haitanensis, PhGDH1 and PhGDH2, were cloned and successfully expressed in Escherichia coli. The in vitro enzyme activity assay demonstrated that the catalytic activity of PhGDHs is mainly in the direction of ammonium assimilation. The measured Km values of PhGDH1 for NADH, (NH4)2SO4, and α-oxoglutarate were 0.12, 4.99, and 0.16 mM, respectively, while the corresponding Km values of PhGDH2 were 0.02, 3.98, and 0.104 mM, respectively. Site-directed mutagenesis results showed that Gly193 and Thr361 were important catalytic residues for PhGDH2. Moreover, expression levels of both PhGDHs were significantly increased under abiotic stresses. These results suggest that PhGDHs can convert α-oxoglutarate to glutamic acid, and enhance the flavor and stress resistance of P. haitanensis.
Highlights
IntroductionP. haitanensis is cultivated on a large scale and shows the highest yield among all laver species [1]
Thr361 were important catalytic residues for PhGDH2. Expression levels of both PhGDHs were significantly increased under abiotic stresses. These results suggest that PhGDHs can convert α-oxoglutarate to glutamic acid, and enhance the flavor and stress resistance of P. haitanensis
The results of subcellular localization showed that PhGDH1 was potentially located in the cytoplasm, while PhGDH2 was presumed to be in the chloroplast
Summary
P. haitanensis is cultivated on a large scale and shows the highest yield among all laver species [1] It is extensively distributed in the coastal areas of Guangdong, Fujian, Zhejiang, and Jiangsu provinces [3]. The annual output of P. haitanensis is about 88,000 tons (dry weight), accounting for 75% of the total output of nori in China and more than 50% of the total output of nori in the world [1].
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