Chen Hui, Zheng Yanli, Zhan Jiao, He Chenliu, Wang Qiang
Key Laboratory of Algal Biology, Institute of Hydrobiology, The Chinese Academy of Sciences, Wuhan, 430072 Hubei China.
University of the Chinese Academy of Sciences, Beijing, 100039 China.
Biotechnol Biofuels. 2017 Jun 15;10:153. doi: 10.1186/s13068-017-0839-4. eCollection 2017.
Microalgae are a promising feedstock for biofuel production. Microalgal metabolic pathways are heavily influenced by environmental factors. For instance, lipid metabolism can be induced by nitrogen-limiting conditions. However, the underlying mechanisms of lipid biosynthesis are unclear. In this study, we analyzed the global metabolic profiles of three genetically closely related strains (C1, C2, and C3) with significant differences in lipid productivity to identify the contributions of key metabolic pathways to lipid metabolism. We found that nitrogen obtained from amino acid catabolism was assimilated via the glutamate-glutamine pathway and then stored as amino acids and intermediate molecules (particularly proline, alanine, arginine, succinate, and gamma-aminobutyrate) via the corresponding metabolic pathways, which led to carbon-nitrogen disequilibrium. Excess carbon obtained from photosynthesis or glycolysis was re-distributed into carbon-containing compounds, such as glucose-6-phosphate, fructose-6-phosphate, phosphoenolpyruvate, lactate, citrate, 3-hydroxybutyrate, and leucine, and then diverted into lipid metabolism for the production of storage lipids via the gamma-aminobutyrate pathway, glycolysis, and the tricarboxylic acid cycle. These results were substantiated in the model green alga by analyzing various mutants deficient in glutamate synthase/NADH-dependent, glutamate synthase/Fd-dependent, glutamine synthetase, aspartate aminotransferase, alanine aminotransferase, pyruvate kinase, and citrate synthase. Our study suggests that not only carbon but also nitrogen assimilation and distribution pathways contribute to lipid biosynthesis. Furthermore, these findings may facilitate genetic engineering efforts to enhance microalgal biofuel production.
微藻是一种很有前景的生物燃料生产原料。微藻的代谢途径受到环境因素的严重影响。例如，脂质代谢可由氮限制条件诱导。然而，脂质生物合成的潜在机制尚不清楚。在本研究中，我们分析了三株遗传关系密切、脂质生产率存在显著差异的菌株（C1、C2和C3）的整体代谢谱，以确定关键代谢途径对脂质代谢的贡献。我们发现，从氨基酸分解代谢获得的氮通过谷氨酸-谷氨酰胺途径被同化，然后通过相应的代谢途径作为氨基酸和中间分子（特别是脯氨酸、丙氨酸、精氨酸、琥珀酸和γ-氨基丁酸）储存起来，这导致了碳氮失衡。从光合作用或糖酵解获得的过量碳被重新分配到含碳化合物中，如6-磷酸葡萄糖、6-磷酸果糖、磷酸烯醇丙酮酸、乳酸、柠檬酸、3-羟基丁酸和亮氨酸，然后通过γ-氨基丁酸途径、糖酵解和三羧酸循环进入脂质代谢，用于储存脂质的生产。通过分析谷氨酸合酶/NADH依赖性、谷氨酸合酶/Fd依赖性、谷氨酰胺合成酶、天冬氨酸转氨酶、丙氨酸转氨酶、丙酮酸激酶和柠檬酸合酶缺陷的各种突变体，在模式绿藻中证实了这些结果。我们的研究表明，不仅碳，而且氮的同化和分配途径也有助于脂质生物合成。此外，这些发现可能有助于通过基因工程提高微藻生物燃料的产量。
