Figure 2.

Schematic of the Cl^–/H⁺ antiport mechanism proposed by Basilio et al.,^1,6 with numbered conformation of the protein representing a distinct “state”. Double-arrows indicate that both transport directions are possible. In this work, we define a “positive” transport direction as Cl^– transport from extracellular to intracellular bulk (with H⁺ transport in the opposite direction). In the postulated mechanism above, the “positive” direction corresponds to moving between states in the following order: from an initial state (1) with both E148 and E203 deprotonated, E203 is protonated by a proton from the intracellular solution moving to state 2. To create state 3, a proton is transferred from E203 to E148. While E148 is protonated (highlighted orange), two Cl^– ions enter the protein from the extracellular solution and bind to S_ext and S_cen to create state 4. With these chloride ions present, E148 is deprotonated and H⁺ is released to the extracellular solution to create state 5, which also involves the opening of the internal gate at Y445 (also called Tyr_cen) by the conformational change of helix O (not shown here). State 6 is reached after two Cl^– ions at S_ext and S_cen are released to the intracellular solution. To return to the initial state and complete the cycle, the internal gate at Y445 closes.