Figure 5
Effects of the modulation of the negative feedback on Ira2 (reaction constant c 36 ). The figure shows the simulated dynamics of cAMP (left) and Ras2-GTP (right) resulting from a PSA-1D on the value of the reaction constant that modulates the phosphorylation of Ira2 by means of active PKA. The varied parameter is constant c36(see Table 1), within the interval [1.0×10−6, 1.0], being 1.0×10−3 the reference value (represented with the black thick line). The reaction that describes the feedback on Ira2 is able to control the establishment of oscillatory regimes: for values of the reaction constant lower than the reference value, only stable steady states can be reached. In particular, cAMP and Ras2-GTP reach a noisy steady state around 60,000 and 10,000 molecules, respectively. Conversely, by increasing the value of the constant, that is, enhancing the GTPase activity of Ras2 proteins by means of the phosphorylation of Ira2, the amplitude of oscillations of cAMP and Ras2-GTP is reduced from around 40,000 to 25,000 molecules and from 600 to 140 molecules, respectively, as well as their average amounts, that decrease from around 40,000 to 20,000 and from around 270 to 50 molecules, respectively. For values of constant c36greater than 1.0×10−2 the oscillatory regime is lost and the average values of cAMP and Ras2-GTP drop to around 5,000 and 20 molecules, respectively.