Figure 10.
Evaluation of how the binding of two ATP molecules for channel activation and the independent contribution of three subunits to the final pore opening can converge using Markov models. The voltage-induced activation phase of the simulation was compared with experimental data for the lowest activating [ATP]. State diagrams for different Markov models (A–C), which were based on the assumption that each subunit is independent and passes through two transitions, with a fast ATP-binding and a rate-limiting voltage-dependent step. (Details of the model are given in Materials and methods.) (A) 10-step model with three intact ATP-binding sites, which requires the binding of three ATP molecules for full activation. (B) 10-step model with three intact ATP-binding sites that can be activated by the binding of two ATP molecules. (C) Six-step model with only two intact ATP-binding sites, which can be activated by the binding of two ATP molecules. Next to each model diagram, simulation traces for voltage-induced activation at various [ATP] are shown (traces for 3, 10, 30, 100, and 300 µM ATP are shaded in red, orange, blue, green, and black, respectively). Dashed black lines show single-exponential fittings of the voltage-induced activation phase at 3 µM ATP. Note the apparent sigmoidicity of voltage-induced activation for model A, which could not be fitted by a single-exponential function (shown by the arrow). Simulation traces of the models in B and C were almost successfully fitted by a single-exponential function.