Fig. 5
The modulator concentration that is optimal for signaling. a Titration curves of the free regulator [R] as a function of the total modulator concentration [MT] in the absence of the signal (black line) and at saturating signal concentrations. For positive cooperativity the titration curve shifts to the left (α = 0.1, red line) and for negative cooperativity to the right (blue line, α = 10). The shaded red (blue) regime indicates all possible signaling outputs that can be obtained by varying S in each case. The difference between the two titration curves (vertical line) determines the output dynamic range at a fixed modulator concentration. Other parameters: [RT] = 1 μM for K1 = 5 μM. b The relative response amplitude A as a function of the modulator concentration for the two systems shown in a. The amplitude reaches an optimum Aopt at a particular Mopt in each case (vertical line)