Figure 5.

Performance of an ion pump built using truncated AQP. (a) Simulation system with parts depicted as in Figure 1c. In this double-membrane system, the bottom truncated AQP tetramer is oriented as in Figure 1c, while the top complex faces opposite. Because of the opposing orientations, the gate residues of one tetramer remain closed while those of the other are open. Note that the use of periodic boundary conditions means that the upper and lower partitions depicted here function as single chamber, referred to as the cathode chamber. (b–c) The number of ions permeated through the top (b) and bottom (c) truncated AQP tetramers of the double-membrane system as a function of the simulation time. The step-function black trace above panel b indicates the applied bias. Ion permeation across either membrane in the cis to trans direction (defined in panel a) increases the ion count; transport in the opposite direction decreases it. Na⁺ overwhelmingly travels from the anode (center) chamber to the cathode (outer two partitions) chamber in response to applied bias for as long as the gates remain effective. Dotted lines serve as guides for the eye. (d) Molarity of NaCl in the anode (green) and cathode (black) chambers. The step-function black trace above panel d indicates the applied bias. (e) Ratio of Na⁺ (blue trace) and Cl⁻ (red trace) molarity in the cathode chamber to that of the anode chamber.