Figure 2. Modular organization of human metabolic network.

A) Hierarchical clustering of the network by the steps between 4204 metabolic reactions, where the warmer color codes for fewer steps. Each red island represents a cluster of closely connected reactions. B) An insert by the while arrow in A. This demonstrates that network modules and pathways correlate with but not equate to each other. C) When measured by reaction steps between metabolites, most metabolites are connected in no more than four steps. This serves as a practical guide in searching subnetworks in the total metabolic network.