ABSTRACT
The
responses of Escherichia coli central carbon metabolism to
knockout mutations in phosphoglucose isomerase and glucose-6-phosphate
(G6P) dehydrogenase genes were investigated by using glucose- and
ammonia-limited chemostats. The metabolic network structures and
intracellular carbon fluxes in the wild type and in the knockout
mutants were characterized by using the complementary methods of flux
ratio analysis and metabolic flux analysis based on
[U-13C]glucose labeling and
two-dimensional nuclear magnetic resonance (NMR) spectroscopy of
cellular amino acids, glycerol, and glucose. Disruption of
phosphoglucose isomerase resulted in use of the pentose phosphate
pathway as the primary route of glucose catabolism, while flux
rerouting via the Embden-Meyerhof-Parnas pathway and the nonoxidative
branch of the pentose phosphate pathway compensated for the G6P
dehydrogenase deficiency. Furthermore, additional, unexpected flux
responses to the knockout mutations were observed. Most prominently,
the glyoxylate shunt was found to be active in phosphoglucose
isomerase-deficient E. coli. The Entner-Doudoroff pathway also
contributed to a minor fraction of the glucose catabolism in this
mutant strain. Moreover, although knockout of G6P dehydrogenase had no
significant influence on the central metabolism under glucose-limited
conditions, this mutation resulted in extensive overflow metabolism and
extremely low tricarboxylic acid cycle fluxes under ammonia limitation
conditions.