A simple network model. (a) A simple network with 6 states and 4 reactions. Straight arrows connect substrate to product and the dotted arrows indicate enzymatic activity. Details of model equations and parameter values are given in Table S1 (see Additional file 1: Supplementary Table S1). (b) Activation of x
under a constant stimulus of x1 = 1: complete network (+), indirect pathway knock-out (*) and direct pathway knockout (x).