  The exploration of the structural topology and the organizing principles of
genome-based large-scale metabolic networks is essential for studying possible
relations between structure and functionality of metabolic networks.
Topological analysis of graph models has often been applied to study the
structural characteristics of complex metabolic networks.In this work,
metabolic networks of 75 organisms were investigated from a topological point
of view. Network decomposition of three microbes (Escherichia coli, Aeropyrum
pernix and Saccharomyces cerevisiae) shows that almost all of the sub-networks
exhibit a highly modularized bow-tie topological pattern similar to that of the
global metabolic networks. Moreover, these small bow-ties are hierarchically
nested into larger ones and collectively integrated into a large metabolic
network, and important features of this modularity are not observed in the
random shuffled network. In addition, such a bow-tie pattern appears to be
present in certain chemically isolated functional modules and spatially
separated modules including carbohydrate metabolism, cytosol and mitochondrion
respectively. The highly modularized bow-tie pattern is present at different
levels and scales, and in different chemical and spatial modules of metabolic
networks, which is likely the result of the evolutionary process rather than a
random accident. Identification and analysis of such a pattern is helpful for
understanding the design principles and facilitate the modelling of metabolic
networks.
