  [Background] Several studies have mentioned network modularity -- that a
network can easily be decomposed into subgraphs that are densely connected
within and weakly connected between each other -- as a factor affecting
metabolic robustness. In this paper we measure the relation between network
modularity and several aspects of robustness directly in a model system of
metabolism. [Methodology/Principal Findings] By using a model for generating
chemical reaction systems where one can tune the network modularity, we find
that robustness increases with modularity for changes in the concentrations of
metabolites, whereas it decreases with changes in the expression of enzymes.
The same modularity scaling is true for the speed of relaxation after the
perturbations. [Conclusions/Significance] Modularity is not a general principle
for making metabolism either more or less robust; this question needs to be
addressed specifically for different types of perturbations of the system.
