Xiong Wei, Brune Daniel, Vermaas Wim F J
School of Life Sciences and Center for Bioenergy and Photosynthesis, Arizona State University, Tempe, Arizona, 85287-4501, USA.
Mol Microbiol. 2014 Aug;93(4):786-96. doi: 10.1111/mmi.12699. Epub 2014 Jul 16.
A traditional 2-oxoglutarate dehydrogenase complex is missing in the cyanobacterial tricarboxylic acid cycle. To determine pathways that convert 2-oxoglutarate into succinate in the cyanobacterium Synechocystis sp. PCC 6803, a series of mutant strains, Δsll1981, Δslr0370, Δslr1022 and combinations thereof, deficient in 2-oxoglutarate decarboxylase (Sll1981), succinate semialdehyde dehydrogenase (Slr0370), and/or in γ-aminobutyrate metabolism (Slr1022) were constructed. Like in Pseudomonas aeruginosa, N-acetylornithine aminotransferase, encoded by slr1022, was shown to also function as γ-aminobutyrate aminotransferase, catalysing γ-aminobutyrate conversion to succinic semialdehyde. As succinic semialdehyde dehydrogenase converts succinic semialdehyde to succinate, an intact γ-aminobutyrate shunt is present in Synechocystis. The Δsll1981 strain, lacking 2-oxoglutarate decarboxylase, exhibited a succinate level that was 60% of that in wild type. However, the succinate level in the Δslr1022 and Δslr0370 strains and the Δsll1981/Δslr1022 and Δsll1981/Δslr0370 double mutants was reduced to 20-40% of that in wild type, suggesting that the γ-aminobutyrate shunt has a larger impact on metabolite flux to succinate than the pathway via 2-oxoglutarate decarboxylase. (13) C-stable isotope analysis indicated that the γ-aminobutyrate shunt catalysed conversion of glutamate to succinate. Independent of the 2-oxoglutarate decarboxylase bypass, the γ-aminobutyrate shunt is a major contributor to flux from 2-oxoglutarate and glutamate to succinate in Synechocystis sp. PCC 6803.
蓝藻的三羧酸循环中缺少传统的2-氧代戊二酸脱氢酶复合物。为了确定在集胞藻PCC 6803中2-氧代戊二酸转化为琥珀酸的途径，构建了一系列缺乏2-氧代戊二酸脱羧酶（Sll1981）、琥珀酸半醛脱氢酶（Slr0370）和/或γ-氨基丁酸代谢（Slr1022）的突变菌株，即Δsll1981、Δslr0370、Δslr1022及其组合。与铜绿假单胞菌一样，由slr1022编码的N-乙酰鸟氨酸转氨酶也被证明可作为γ-氨基丁酸转氨酶，催化γ-氨基丁酸转化为琥珀酸半醛。由于琥珀酸半醛脱氢酶将琥珀酸半醛转化为琥珀酸，因此集胞藻中存在完整的γ-氨基丁酸分流途径。缺乏2-氧代戊二酸脱羧酶的Δsll1981菌株的琥珀酸水平为野生型的60%。然而，Δslr1022和Δslr0370菌株以及Δsll1981/Δslr1022和Δsll1981/Δslr0370双突变体的琥珀酸水平降至野生型的20-40%，这表明γ-氨基丁酸分流途径对琥珀酸代谢通量的影响大于通过2-氧代戊二酸脱羧酶的途径。（13）C稳定同位素分析表明，γ-氨基丁酸分流途径催化谷氨酸转化为琥珀酸。与2-氧代戊二酸脱羧酶旁路无关，γ-氨基丁酸分流途径是集胞藻PCC 6803中2-氧代戊二酸和谷氨酸向琥珀酸通量的主要贡献者。
