  Metabolic networks have two properties that are generally regarded as
unrelated: One, they have metabolic reactions whose single knockout is lethal
for the organism, and two, they have correlated sets of reactions forming
functional modules. In this review we argue that both essentiality and
modularity seem to arise as a consequence of the same structural property: the
existence of low degree metabolites. This observation allows a prediction of
(a) essential metabolic reactions which are potential drug targets in
pathogenic microorganisms and (b) regulatory modules within biological
networks, from purely structural information about the metabolic network.
