Fig. 5.

Insensitivity of torque at high load to rates of proton transfer and stepping. ΔpH = 2.5, pK_I = 7.1, pK_II = 5.4 for all simulations. Other parameters: (a) ${\mathrm{k}}_{{\mathrm{H}}^{+}}$= 10^11.5 M⁻¹⋅s⁻¹, k_step = 2 × 10³ s⁻¹; (b) ${\mathrm{k}}_{{\mathrm{H}}^{+}}$= 10¹² M⁻¹⋅s⁻¹, k_step = 5 × 10³ s⁻¹; (c) ${\mathrm{k}}_{{\mathrm{H}}^{+}}$= 10^12.5 M⁻¹⋅s⁻¹, k_step = 10⁴ s⁻¹; and (d) ${\mathrm{k}}_{{\mathrm{H}}^{+}}$= 10¹³ M⁻¹⋅s⁻¹, k_step = 2 × 10⁴ s⁻¹. At a rotation rate of 10 Hz (typical of a tethered-cell experiment), motors a−d would use 3.6, 6.4, 11, and 21 protons per step, respectively. Values of stall torque and zero-load speed for these and additional parameter sets are listed in Table S3.