title: The γ-aminobutyric acid shunt contributes to closing the tricarboxylic acid cycle in Synechocystis sp. PCC 6803.
abstract: 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.
authors.affiliation: School of Life Sciences and Center for Bioenergy and Photosynthesis, Arizona State University, Tempe, Arizona, 85287-4501, USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Xiong
authors.forename: Daniel
authors.initials: D
authors.lastname: Brune
authors.forename: Wim F J
authors.initials: WF
authors.lastname: Vermaas
mesh_terms: D000613:Aminobutyrates / Q000378:metabolism*; D001711:Biotransformation; D002952:Citric Acid Cycle; D017353:Gene Deletion; D064688:Metabolic Flux Analysis; D050644:Succinate-Semialdehyde Dehydrogenase / Q000378:metabolism*; D019802:Succinic Acid / Q000378:metabolism*; D046939:Synechocystis / Q000201:enzymology* / Q000378:metabolism*; D000637:Transaminases / Q000378:metabolism*
chemical_list: D000613:Aminobutyrates; D019802:Succinic Acid; D050644:Succinate-Semialdehyde Dehydrogenase; D000637:Transaminases; C018731:acetylornithine transaminase