  Spatially or chemically isolated functional modules composed of several
cellular components and carrying discrete functions are considered fundamental
building blocks of cellular organization, but their presence in highly
integrated biochemical networks lacks quantitative support. Here we show that
the metabolic networks of 43 distinct organisms are organized into many small,
highly connected topologic modules that combine in a hierarchical manner into
larger, less cohesive units, their number and degree of clustering following a
power law. Within Escherichia coli the uncovered hierarchical modularity
closely overlaps with known metabolic functions. The identified network
architecture may be generic to system-level cellular organization.
