... work was supported by a grant from Ministry of Science and Technology of the Spanish Government (SAF 2002–02785), INTASgrant (97 150 4), and the Netherlands’ Organization for ScientificResearch. ... negative p aths in the c ycle of subgraphs here reflect the competition of two r eactions for a single species. For example, the competition of protein X and the enzyme E2 for the acetyl group in pyruvatedehydrogenase ... kshave two kinds of vertices, i.e. one kind for species (hereshown by open circles) a nd one kind for reactions (shown by closed circles). The circles are connected by arrows. For example, the...