Modeling Computation A 26 Hierarchical Organization of Modularity in Metabolic Networks
The identification and characterization of system level features of biological organization is a key issue of post genomic biology. An elegant proposal addressing the cells functional architecture is offered by the concept of modularity, assuming that the cell can be partitioned into a collection of modules. Each module, a discrete entity of several elementary components, performs an identifiable biological task, separable from the functions of other modules. Yet, it is now widely recognized that the thousands of components of the metabolism are dynamically connected to one another, such that the cells functional properties are ultimately encoded into a complex metabolic web of molecular interactions. Within this network, however, modular organization and clear boundaries between sub networks are not immediately apparent. Indeed, recent studies have demonstrated that metabolic networks have a scale free topology. A distinguishing feature of such scale free networks is the existence of a few hubs, highly connected metabolites such as pyruvate or CoA, which participate in a very large number of metabolic reactions. With a large number of links, these hubs integrate all substrates into a single, integrated web in which the existence of fully separated modules is prohibited.