Fig. 5.
Parameterization for models I, II, and III. (Ia) Simulated Scatchard plot for model I. In the absence of dimerization the plot is not able to demonstrate the existence of two separate binding affinities. Model inputs were: K = 2 nM, Kl = 100 nM, Ke = 30, and RT = 5 μM. The apparent affinity is 39 nM. (II a–c) Simulated Scatchard plots for model II show that in the absence of autoinhibition dimerization alone is not able to produce positive cooperativity. (IIa) Model inputs were: K = 1 nM, Kd = 1 μM, Ka = 1 nM, Kb = 1 nM, and RT = 5 μM. The apparent affinity is 1 nM. (IIb) Model inputs were: K = 2 nM, Kd = 1 μM, Ka = 1 nM, Kb = 1 nM, and RT = 5 μM. The apparent affinity is 1.15 nM. (IIc) Model inputs were: K = 2 nM, Kd = 1 μM, Ka = 2 nM, Kb = 1 nM, and RT = 5 μM. The apparent affinity is 2.12 nM. (III a–c) Simulated Scatchard plots for model III. Impaired dimerization can still produce concave-down Scatchard plots. (IIIa) Model inputs were: Kl = 100 nM, K = 0.5 nM, Kd = 1 μM, Ka = 0.5 nM, Kb = 0.5 nM, Ke = 30, and RT = 5 μM. The apparent affinity is ≈5–6 nM. (IIIb) Model inputs were: Kl = 100 nM, K = 2 nM, Kd = 10 μM, Ka = 2 nM, Kb = 1 nM, Ke = 30, and RT = 10 μM. The apparent affinity is ≈30–40 nM. (IIIc) Model inputs were: Kl = 100 nM, K = 2 nM, Kd = 5 μM, Ka = 2 nM, Kb = 1 nM, Ke = 30, and RT = 5 μM. The apparent affinity is ≈25–35 nM.