This is a very simple model of a reversible covalent modification after Goldbeter-Koshland. E1, E2 stand for the enzymes, a kinase and a phosphatase respectively. W stands for the substrate. When W is in excess, the steady state shows ultrasensitivity, meaning a slight excess of E1 over E2 will result in a large excess of W* (the phosphorylated form) over W (the unphosphorylated one). This is indeed what we can see in the simulation results.