title: Natural and Synthetic Variants of the Tricarboxylic Acid Cycle in Cyanobacteria: Introduction of the GABA Shunt intoSynechococcussp. PCC 7002.
abstract: For nearly half a century, it was believed that cyanobacteria had an incomplete tricarboxylic acid (TCA) cycle, because 2-oxoglutarate dehydrogenase (2-OGDH) was missing. Recently, a bypass route via succinic semialdehyde (SSA), which utilizes 2-oxoglutarate decarboxylase (OgdA) and succinic semialdehyde dehydrogenase (SsaD) to convert 2-oxoglutarate (2-OG) into succinate, was identified, thus completing the TCA cycle in most cyanobacteria. In addition to the recently characterized glyoxylate shunt that occurs in a few of cyanobacteria, the existence of a third variant of the TCA cycle connecting these metabolites, the γ-aminobutyric acid (GABA) shunt, was considered to be ambiguous because the GABA aminotransferase is missing in many cyanobacteria. In this study we isolated and biochemically characterized the enzymes of the GABA shunt. We show thatN-acetylornithine aminotransferase (ArgD) can function as a GABA aminotransferase and that, together with glutamate decarboxylase (GadA), it can complete a functional GABA shunt. To prove the connectivity between the OgdA/SsaD bypass and the GABA shunt, thegadAgene fromSynechocystissp. PCC 6803 was heterologously expressed inSynechococcussp. PCC 7002, which naturally lacks this enzyme. Metabolite profiling of sevenSynechococcussp. PCC 7002 mutant strains related to these two routes to succinate were investigated and proved the functional connectivity. Metabolite profiling also indicated that, compared to the OgdA/SsaD shunt, the GABA shunt was less efficient in converting 2-OG to SSA inSynechococcussp. PCC 7002. The metabolic profiling study of these two TCA cycle variants provides new insights into carbon metabolism as well as evolution of the TCA cycle in cyanobacteria.
authors.affiliation: 403C Althouse Laboratory, Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park PA, USA.
authors.forename: Shuyi
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Waksman Institute of Microbiology, Rutgers, The State University of New Jersey, Piscataway NJ, USA.
authors.forename: Xiao
authors.initials: X
authors.lastname: Qian
authors.affiliation: Waksman Institute of Microbiology, Rutgers, The State University of New Jersey, Piscataway NJ, USA.
authors.forename: Shannon
authors.initials: S
authors.lastname: Chang
authors.affiliation: Waksman Institute of Microbiology, Rutgers, The State University of New Jersey, PiscatawayNJ, USA; Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, PiscatawayNJ, USA.
authors.forename: G C
authors.initials: GC
authors.lastname: Dismukes
authors.affiliation: 403C Althouse Laboratory, Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University ParkPA, USA; Department of Chemistry and Biochemistry, Montana State University, BozemanMT, USA.
authors.forename: Donald A
authors.initials: DA
authors.lastname: Bryant
keywords: 2-oxoglutaric acid; GABA shunt; Synechococcus sp. PCC 7002; Synechocystis sp. PCC 6803; TCA cycle; cyanobacteria; photosynthesis; succinic acid semialdehyde