School of Agriculture, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa, 214-8571, Japan.
Sci Rep. 2021 Aug 24;11(1):17131. doi: 10.1038/s41598-021-96432-2.
A unicellular cyanobacterium Synechocystis sp. PCC 6803 possesses a unique tricarboxylic acid (TCA) cycle, wherein the intracellular citrate levels are approximately 1.5-10 times higher than the levels of other TCA cycle metabolite. Aconitase catalyses the reversible isomerisation of citrate and isocitrate. Herein, we biochemically analysed Synechocystis sp. PCC 6803 aconitase (SyAcnB), using citrate and isocitrate as the substrates. We observed that the activity of SyAcnB for citrate was highest at pH 7.7 and 45 °C and for isocitrate at pH 8.0 and 53 °C. The K value of SyAcnB for citrate was higher than that for isocitrate under the same conditions. The K value of SyAcnB for isocitrate was 3.6-fold higher than the reported K values of isocitrate dehydrogenase for isocitrate. Therefore, we suggest that citrate accumulation depends on the enzyme kinetics of SyAcnB, and 2-oxoglutarate production depends on the chemical equilibrium in this cyanobacterium.
单细胞蓝藻集胞藻 PCC 6803 拥有独特的三羧酸（TCA）循环，其细胞内柠檬酸水平大约比其他 TCA 循环代谢物高 1.5-10 倍。 aconitase 催化柠檬酸和异柠檬酸的可逆异构化。在此，我们使用柠檬酸和异柠檬酸作为底物，对集胞藻 PCC 6803 aconitase（SyAcnB）进行了生化分析。我们观察到 SyAcnB 对柠檬酸的活性在 pH 7.7 和 45°C 时最高，对异柠檬酸的活性在 pH 8.0 和 53°C 时最高。在相同条件下，SyAcnB 对柠檬酸的 K 值高于对异柠檬酸的 K 值。SyAcnB 对异柠檬酸的 K 值比报道的异柠檬酸脱氢酶对异柠檬酸的 K 值高 3.6 倍。因此，我们认为柠檬酸的积累取决于 SyAcnB 的酶动力学，而 2-酮戊二酸的产生则取决于该蓝藻中的化学平衡。
