As we are considering reversible reactions as two independent unidirectional reactions, we set vmin to zero. Problems like Equation 1 can be efficiently solved using linear programming. In order to avoid thermodynamically infeasible loops, we utilized pFBA [39], effectively using the solution of Equation 1, to fix the objective to its maximum value and minimize the L1-norm of all other fluxes in a second optimization.

3.3. Combinatorial Minimal Media and Reaction Essentiality Combinatorial minimal media were constructed using the following procedure. (i) All experimentally verified nutrients in the Inhibitors,research,lifescience,medical iAF1260 model were classified as sources for elemental carbon, nitrogen, sulfur and phosphate (see also Supplementary Table S1). Some compounds fall hereby into multiple categories, e.g., glucose-6-phosphate is both a carbon and a phosphate source. (ii) Combinations of nutrients were then chosen such that only one of

each elemental selleck chemicals source was included in the medium, e.g., no additional Inhibitors,research,lifescience,medical phosphate source was provided in a medium containing glucose-6-phosphate. Steady-state fluxes that optimize biomass production have been calculated Inhibitors,research,lifescience,medical for all possible substrate combinations leading to a total of 72468 analyzed minimal-media conditions. For each simulation, the essentiality of all active reactions was determined by fixing the respective fluxes to zero and recomputing the maximal biomass flux for the mutants. A reaction was classified as essential if the biomass flux dropped to zero. Inhibitors,research,lifescience,medical 3.4. Blocked Reactions We removed all globally blocked reactions from the model to give the topological methods described in this article

(UPUC, SA) the opportunity to work on the same information content as their dynamical counterpart (MC). A high (not as high as the default flux boundaries vmax) maximal uptake and secretion rate was assigned to all available transporters in the system and then blocked reactions were confirmed by flux variability analysis [32]. These globally blocked reactions cannot carry a flux under any Inhibitors,research,lifescience,medical environmental conditions and consequently are not available to methods that use FBA. 3.5. Metabolic Core Reactions are assigned to the metabolic core if they were active in all wild-type simulation, following the definition of Almaas et al. [21]. In contrast to [21], however, we use a finite number unless of combinatorial minimal media instead of randomly sampled conditions. Consequently, the size of the metabolic core in this study is larger than in the original work. 3.6. Synthetic Accessibility Reactions The synthetic accessibility of all reactions in the system was computed according to [20]. The needed outputs were defined to be the substrates of the biomass function and the ingredients of a glucose minimal medium were defined to be the inputs of the system.