  Local perturbations of individual metabolic reactions may result in different
levels of lethality, depending on their roles in metabolism and the size of
subsequent cascades induced by their failure. Moreover, essentiality of
individual metabolic reactions may show large variations within and across
species. Here we quantify their essentialities in hundreds of species by
computing the growth rate after removal of individual and pairs of reactions by
flux balance analysis. We find that about 10% of reactions are essential, i.e.,
growth stops without them, and most of the remaining reactions are redundant in
the metabolic network of each species. This large-scale and cross-species study
allows us to determine ad hoc ages of each reaction and species. We find that
when a reaction is older and contained in younger species, the reaction is more
likely to be essential. Such correlations of essentiality with the ages of
reactions and species may be attributable to the evolution of cellular
metabolism, in which alternative pathways are recruited to ensure the stability
of important reactions to various degrees across species.
