Fig. 2.

Energy landscape representation of electron-coupled proton pumping. Electron reduction induces “ratcheting” between left-leaning energy landscapes for protons (black) by lowering the barriers for proton uptake from the N side (states A, B, and D) and right-leaning landscapes favoring proton release to the P side (states C, E, and F). Proton and electron free energy surfaces (red; shifted vertically) are drawn to scale for model b of Table 1 (including barriers for an attempt frequency of 10⁹ s⁻¹). (A) Initial proton uptake from the N side. (B) Proton translocation to the pump site 2. (C) Electron transfer. (D) Uptake of the second proton from the N side. (E) Release of the pumped proton to the P side. (F) Formation of product “1/2H₂O.” Thick black arrows indicate the dominant steady state flux. The dashed blue arrow indicates “slip,” i.e., product formation without pumping. The dashed black arrow between F and A indicates that the reaction is reversible in principle, halting the pump eventually in a closed system together with depletion of substrates and buildup of product water. Only the six states with significant population are shown. With a “water gate,” internal proton transfer is accelerated, as indicated by the dashed barrier in B, thus increasing the pumping efficiency.