Figure 4.

Multisite modification. (A and B) The kinase and phosphatase pairs are different for the two modification steps (K1, P1 and K2, P2). (A) [X^∗∗] is plotted against the width of the localized patch (expressed as a fraction of total domain length): [X^∗∗] is always higher when the modifications occur in the same location. The X^∗∗ concentration increases with the width of the patch. (B) The average (over the full domain) and local concentrations of X^∗ are compared in these cases. The average concentration of X^∗ is greater when modifications are in different compartments, implying a higher total production of X^∗ in this case. (C) When the kinases for both modification steps are different but the phosphatases are the same (and the modifications take place in separate compartments), at steady state, the concentration of X^∗∗ will be zero if X^∗ is the only communicating species (dash-dotted line), leaving only X in the second compartment. A way to overcome this locational constraint is by having both X and X^∗ diffuse in the domain (solid line).