Metabolic flux analysis for a ppc mutant Escherichia coli based on 13C-labelling experiments together with enzyme activity assays and intracellular metabolite measurements

Abstract

The physiology and central metabolism of a ppc mutant Escherichia coli were investigated based on the metabolic flux distribution obtained by ¹³C-labelling experiments using gas chromatography-mass spectrometry (GC-MS) and 2-dimensional nuclear magnetic resonance (2D NMR) strategies together with enzyme activity assays and intracellular metabolite concentration measurements. Compared to the wild type, its ppc mutant excreted little acetate and produced less carbon dioxide at the expense of a slower growth rate and a lower glucose uptake rate. Consequently, an improvement of the biomass yield on glucose was observed in the ppc mutant. Enzyme activity measurements revealed that isocitrate lyase activity increased by more than 3-fold in the ppc mutant. Some TCA cycle enzymes such as citrate synthase, aconitase and malate dehydrogenase were also upregulated, but enzymes of glycolysis and the pentose phosphate pathway were downregulated. The intracellular intermediates in the glycolysis and the pentose phosphate pathway, therefore, accumulated, while acetyl coenzyme A and oxaloacetate concentrations decreased in the ppc mutant. The intracellular metabolic flux analysis uncovered that deletion of ppc resulted in the appearance of the glyoxylate shunt, with 18.9% of the carbon flux being channeled via the glyoxylate shunt. However, the flux of the pentose phosphate pathway significantly decreased in the ppc mutant.