A cross species comparison of metabolic network functions.

We compare a large number of organisms with respect to their metabolic network functions. We measure such functions in terms of the synthesizing capacity of a network when it is provided with a few small chemical substances as external resources. We call this measure the scope and show that it is generally robust against structural alterations of the reaction network. Organisms can be separated into two groups, one with a small and one with a large scope. Networks with a high synthesizing capacity also show a high degree of robustness against structural changes, indicating that this network function has been a target in the evolutionary past of the corresponding organisms. A comparison between structural and functional similarities reveals that organisms with a similar structure do not necessarily show similar biological functions. The presented concepts allow for a systematic investigation of structure-function relationships of metabolic networks and may put forth valuable hints on the evolution of metabolic pathways.