FIGURE 3.

Forward fluxes for CS binding: implication of the ([L]·k₃)/k₂ and k₄/k₁ ratios. Simulated plots are for isolated CS binding. Microscopic rate constants are provided in Supporting Information Section S3. Data for t = 0 not presented. (A) The model complies with the traditional “rapid equilibrium binding” paradigm (i.e., with k₄ > k₁). Left side: forward fluxes versus time at low [L] = 0.2 nM. Such as for IF (Figure 2C), F_on is essentially dictated by F₃ at all times. Mid: forward fluxes versus time at higher [L] = 4 nM. F_on becomes essentially dictated by F₁ (i.e., Biv‐like, Figure 2C) as time goes by. Right side: forward fluxes versus [L] at equilibrium. The F_on versus [L] plot adopts a bell‐shaped pattern with a F₃‐ dictated ascending portion and a F₁‐ dictated descending portion. (B) Plots such as in (A) for the alternative CS binding model¹⁴, ¹⁷, ³² in where the conformational change proceeds faster (i.e., with k₄ < k₁). Low [L] = 5 nM and high [L] = 100 nM. The observations are quite similar as for the traditional model in (A). This contrasts with the hyperbolically decreasing k_obs versus [L] plot for the traditional model and hyperbolically decreasing plot for the alternative model (Supporting Information Section S4). Finally, F_on is not maximal when the F₁‐ and F₃ versus [L] plots intersect because of their different curvature (right side)