Control of carbon flux through enzymes of central and intermediary metabolism during growth of Escherichia coli on acetate

During aerobic growth of Escherichia coli on acetate, the component parts of the ‘acetate switch’ are turned-on as a consequence of direct competition, on the one hand, between phosphotransacetylase (PTA) and α-ketoglutarate dehydrogenase (α-KGDH) for their common co-factor free-CoA (HS-CoA) and, on the other hand, between isocitrate lyase (ICL) and isocitrate dehydrogenase (ICDH) for their common substrate isocitrate. Flux analysis revealed that competitions at both junctions in central metabolism are resolved in a precise way, so that the fraction of HS-CoA flux processed through PTA for biosynthesis relative to that processed through α-KGDH for energy generation, matches that observed for isocitrate flux through ICL relative to ICDH at the junction of isocitrate. Whereas the mechanism involved in the partition of carbon flux at the level of HS-CoA in central metabolism remains to be unravelled, the competition at the junction of isocitrate is resolved by the reversible phosphorylation/inactivation of ICDH and the operation of the glyoxylate bypass, the expression of which is subject to regulation at the transcriptional and translational levels as well as being dependent on growth rate.