The ?-Aminobutyric Acid Shunt Contributes to Closing the Tricarboxylic Acid Cycle in Synechocystis sp. PCC 6803.

Molecular microbiology

PubMedID: 24989231

Xiong W, Brune D, Vermaas WF. The ?-Aminobutyric Acid Shunt Contributes to Closing the Tricarboxylic Acid Cycle in Synechocystis sp. PCC 6803. Mol Microbiol. 2014;.
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, ?sll1981, ?slr0370, ?slr1022 and combinations thereof, deficient in 2-oxoglutarate decarboxylase (Sll1981), succinate semialdehyde dehydrogenase (Slr0370), and/or in ?-aminobutyrate metabolism (Slr1022) were constructed. Like in Pseudomonas aeruginosa, N-acetylornithine aminotransferase, encoded by slr1022, was shown to also function as ?-aminobutyrate aminotransferase, catalyzing ?-aminobutyrate conversion to succinic semialdehyde. As succinic semialdehyde dehydrogenase converts succinic semialdehyde to succinate, an intact ?-aminobutyrate shunt is present in Synechocystis. The ?sll1981 strain, lacking 2-oxoglutarate decarboxylase, exhibited a succinate level that was 60% of that in wild type. However, the succinate level in the ?slr1022 and ?slr0370 strains and the ?sll1981/?slr1022 and ?sll1981/?slr0370 double mutants was reduced to 20-40% of that in wild type, suggesting that the ?-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 ?-aminobutyrate shunt catalyzed conversion of glutamate to succinate. Independent of the 2-oxoglutarate decarboxylase bypass, the ?-aminobutyrate shunt is a major contributor to flux from 2-oxoglutarate and glutamate to succinate in Synechocystis sp. PCC 6803.