FIGURE 4.

Competition between pathways of a thermodynamic cycle: IF versus CS. Competition between IF and CS is evaluated for three Cases. Both pathways comply with the classical frame of reference for Case A (see “Definition”). The two other cases diverge. The conformational change proceeds faster than the binding for CS For Case B (such as in Figure 3B) and Case C is based on kinetic data provided by Zhou et al.²⁴ The latter values are 10⁶‐ fold lower for the sake of comparison. Please see Supporting Information Section S3 for microscopic rate constants and important ratios thereof. (A) The relative contribution of IF to T*L, “Rc_‐IF” increases with [L] for the three Cases at equilibrium. Rc_‐IF can be expressed in terms of F_on (i.e., Rc_‐IF = F_on‐IF/(F_on‐IF + F_on‐CS)), F_off and the accumulation of T*L (same type of equations). These three modes of calculation yield nearly‐ overlapping plots for each Case (hence only one color. Rc_‐IF at very low [L] differs among the Cases: i.e., from <0.5 (CS‐ dominant) for Case A to nearly 1 (outspokenly IF‐ dominant) for Case C.²⁴ (B) The evolution of Rc_‐IF with time is shown for the three cases for [L] = 2.5 × K_D. Note that, while the three approaches provide different Rc_‐IF‐ values early on for each given case, they become closely the same at equilibrium. (C) Evolution of microscopic and macroscopic fluxes at equilibrium as a function of [L]. Such as for the individual pathways in Figures 2 and 3, F_on is dominated by F₃ for both IF and CS at low [L]. Hence, the different Rc_‐IF values low [L] relates to F_3‐IF << F_3‐CS for Case A, F_3‐IF ≈ F_3‐CS for Case B and F_3‐IF >> F_3‐CS for Case C. The bell‐shaped F_on versus [L] profile for CS is preserved when moving from the isolated CS pathway (Figure 3) to the cycle and the drop of F_on‐CS at higher [L] permits Rc_‐IF to increase