This is in contrast to the standard notion of essentiality, which is assigned to a gene or reaction whose single knockout abolishes a phenotype. k-essential links between genes/reactions and better systems-level functions arise from synergistic epistasis between parallel pathways in the network. Complex MCSs found using our method yield many k-essential reactions. To quantify novel k-essential links between reactions and objectives, we compared the numbers of k-essential reactions to the number of 1-essential reactions obtained from a brute-force single knockout analysis of the human metabolic network. Figure Figure44 shows how many reactions were deemed k-essential for each objective, with the numbers of reactions shown to be 1-essential for the objective shown in parentheses next to the metabolite label.

We found that for most objectives we were able to associate many more k-essential reactions with the production of a given metabolite than were able to be found using a single knockout analysis. In many cases, this difference was profound, such as for sphingomyelin, whose producibility we were able to epistatically link to 235 reactions in the metabolic network. Figure 4 Histogram showing number of k-essential reactions discovered for each biosynthetic objective tested in our study. A reaction is k-essential for an objective if it contributes to at least one MCS for that objective. The number of reactions found to be … MCSs span multiple compartments and metabolic subsystems MCSs discovered by our analysis span a breadth of cellular compartments.

However, the actual distributions of compartment span vary distinctly between specific metabolite classes (Fig. (Fig.5).5). In particular, amino acid-targeting MCSs discovered by our method employ the fewest number of compartments, drawing from cytoplasmic fluxes alone or a combination of cytoplasmic and mitochondrial reactions. MCSs targeting core metabolites span between two and three compartments, consisting of primarily cytoplasmic and mitochondrial reactions, however often also employing peroxisomal fluxes. Nucleotide-targeting MCSs sometimes employ cytoplasmic reactions only, however more often pull combinations of reactions from two or three of the following compartments: cytoplasm, mitochondria, lysosome, and nucleus.

Across all metabolite classes studied, membrane-lipid-targeting MCSs are the most diverse: they harness up to five compartment combinations that employ reactions Dacomitinib from the cytoplasm, endoplasmic reticulum, Golgi apparatus, nucleus, and peroxisome. Figure 5 Histogram showing number of compartments spanned by MCSs targeting the four metabolite classes. Frequencies are calibrated separately for each metabolite class. There are also metabolite class differences in the subsystem span of discovered MCSs (Fig. (Fig.6).6). Nucleotide and amino acid-targeting MCSs span between one and five subsystems.