Figure 9.

Thermodynamic impact of the c₁ subunit on ATP synthesis and ion transport. (a) Hypothetical free-energy profiles of the c-ring/subunit-a complex, coupled to F₁, for a prototypical F-type c₁₁-ring with no missing ion-binding sites, and for the A. woodii c-ring. The profiles illustrate the microscopic states, free-energy differences and kinetic barriers employed in the kinetic model of the rotational cycle described in the text. These states are individually identified by the configuration of the S_P and S_N sites, i.e. whether they feature a glutamate (E) or glutamine (Q), whether they are occupied by Na⁺ (E⁺) or not (E, Q), and whether they are paired with the arginine on subunit-a (E^R, Q^R) (Methods). The profiles shown represent an equilibrium condition in which the sodium-motive force (blue arrow) and the phosphorylation potential (red arrow) are exactly balanced. The simulations are, however, initiated out of equilibrium, i.e. the landscapes are heavily tilted in one or other direction, favoring ion-driven ATP synthesis or ATP-driven uphill transport, reaching equilibrium gradually. (b) Production of ATP driven by a variable transmembrane Na⁺ gradient (30-fold initially), under a constant membrane potential of 180 mV i.e. mimicking a voltage-clamp electrophysiological experiment in which the potential is set with K⁺ and valinomycin. The values of [ATP], [Na⁺]_P and [Na⁺]_N at equilibrium are compared with those set initially; percentage differences between the values calculated for an ATP synthase with a prototypical c₁₁ ring and the A. woodii enzyme are indicated. (c) Generation of a transmembrane Na⁺ gradient, under a constant membrane potential of 0 mV, driven by an excess of ATP over ADP and P_i. Equilibrium values for a prototypical c₁₁ ring and that in A. woodii are again compared. (d) Initial rotation rates under the same conditions used in (b) and (c). The data are averages over 100 independent kinetic Monte-Carlo trajectories of 20 million steps each (Supplementary Fig. 13).