Figure 4.

Electrostatic potential surfaces at ionic strength of 150 mM: (A) structure #5; (B) structure #6. The active site gorge for subunit 3 is indicated by arrow and is used to highlight the correlation between reaction rate and variation in the electrostatic steering force due to structure dynamics. We find from Fig 3A that $k_{\text{on}}^{\text{s},3}$ = 2.40 × 10¹¹ M⁻¹ min⁻¹ in structure #5 (A) and 1.37 × 10¹¹ M⁻¹ min⁻¹ in structure #6 (B), with the difference accounting for ∼69% of the difference in the overall reaction rate of $k_{\text{on}}^{\text{t}}$ = 7.80 × 10¹¹ M⁻¹ min⁻¹ (structure #5) and 6.30 × 10¹¹ M⁻¹ min⁻¹ (structure #6). The $k_{\text{on}}^{\text{t}}$ values calculated with the neutral ACh are reversed, being 1.30 × 10¹¹ M⁻¹ min⁻¹ and 1.50 ± 10¹¹ M⁻¹ min⁻¹ for structures #5 and 6, respectively. The higher rate coefficient in structure #5 is entirely due to the stronger electrostatic effect, as can be seen from the relative intensities of the negative electrostatic potentials in panels A and B.