Figure 3. Effect of an intermediate voltage-independent step on the free energy estimates. (A) A six-state linear model of activation of a voltage-dependent ion channel with five closed state and a single final open state. The voltage dependence of the equilibrium constants is given by Ki=Ki0exp(ziFVβ) (i = 1, 2, 3, 4, or 5), where Ki0 is the chemical component of the equilibrium constant Ki and zi is its voltage dependence. The third step (shown in bold) is voltage independent, and thus z3 = 0. ΔG−C is the sum of the chemical free energy change of each of the steps and will equal −RTln∏i=15Ki0. (B) QmaxFVm is plotted against ΔG−C for different values of log K30 (−5 to 5) and two values of z2 (closed squares, z2 = 5; and open triangles, z2 = 0). The remaining parameters of the model were arbitrarily chosen as K10 = 1, K20 = 75, K40 = 25, K50 = 5, z1 = 2, z3 = 0, z4 = 2.5, and z5 = 1. (C and D) Variation of ΔG−C (C) and Vm (D) with changing values of z2 (0–5) and log K30.
Image published in: Chowdhury S and Chanda B (2012)
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