Stimulus-response curves for a 3-unit chain.

<p>Stimulus-response curves for a 3-unit chain involving only single phosphorylation (A) or with double phosphorylation in units 2 and 3, representing the MAPK cascade (B). The parameters are those indicated in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000041#pcbi-1000041-t001" target="_blank">Table 1</a>. The responses were obtained by both the mechanistic and the reduced mechanistic descriptions, which are in perfect agreement. The input stimulus is given by <i>E</i><sub>1<i>T</i></sub> , the total amount of kinase for the first unit. <i>y</i><sub>1</sub><i><sup>*</sup></i>, <i>y</i><sub>2</sub><i><sup>*</sup></i>, and <i>y</i><sub>3</sub><i><sup>*</sup></i> are plotted with black, blue, and red filled lines, respectively. GK-like model predictions are also included (dotted lines). The Hill coefficients characterizing each curve are listed in the legend.</p>