Voit et al., In Silico Biol. 5, 0010 (2004); Figure 1

Figure 1: Two linear pathways that differ in a feedback signal with which X4 inhibits the production of X1. In pathway a, the production is constant at a rate α1. With inhibition, the production becomes dependent on X4, and this dependence is quantified by the parameter g14, which is negative and has a magnitude that directly reflects the strength of inhibition. The rate constant may also be different, because it reflects the unit of X4. If the inhibition decreases, g14 approaches 0, and in the limit, pathways a and b along with the representations of the production of X1 become equivalent, demonstrating that the S-system model for pathway a is always a numerical special case of the model for pathway b.