The flux-sums of essential metabolites before and after enzyme deletions. The flux-sum values of the 12 essential metabolites before and after enzyme knockout are shown. The x-axis: metabolites; y-axis: the flux-sum values. The blue bars: the flux-sum values in the original system; the red bars: the flux-sum values in the system after enzyme knockout. (A - C) Flux-sums of the essential metabolites upon the knockouts of TKb, PGK and TIS respectively. All metabolites suffer devastating flux reductions. (D - E) Flux-sums of the essential metabolites upon the knockouts of PGI and PGM respectively. All metabolites' flux-sums can be sustained at a high level compared with the original values. The observations support our conclusion that the former enzymes are more critical than the latter ones. See Additional file 6 for abbreviations of metabolites.