Figure 8.

Conformational changes during the anion-transporting cycle of pHR. In the resting state (left panels), the cytoplasmic half of helix C has a kink at Thr-216 to accommodate a halide ion in the extracellular vicinity of the retinal Schiff base. In the N state (middle panels), the extracellular half of helix C moves inward, a halide ion is translocated to the extracellular vicinity of the Schiff base, and the cytoplasmic half of helix F has a kink at Lys-215 to create a water channel in the cytoplasmic interhelical space. In the X(N′) state, the translocated halide ion is released into the cytoplasmic medium and the retinal Schiff base interacts with the nearby aspartate (Asp-252). In the O state (right panel), the cytoplasmic interhelical space is closed, whereas the extracellular half of helix C remains to take on an unbent conformation. The small red spheres represent water molecules. To see this figure in color, go online.