Acidic inactivation of LacY, FucP and XylE is due to the pH at the proton release site.

<p>The transient currents under symmetrical and asymmetrical pH conditions for LacY, FucP and XylE before and after addition of monensin are shown. The pH values were chosen so that inactivation of symporter activity at acidic pH values could be observed (compare <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0156392#pone.0156392.g003" target="_blank">Fig 3e</a>). The pH values for the resting solution (R) and the nonactivating as well as activating solutions (NA and A) are indicated inside circles symbolizing the proteoliposomes and outside these circles respectively. The dashed circles symbolize leaky proteoliposomes due to monensin addition. There the pH gradient rapidly dissipates during the first flow of nonactivating solution (compare <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0156392#pone.0156392.g005" target="_blank">Fig 5</a>) and the intraliposomal pH adjusts to the outer pH. For all symporters a constant pH gradient |ΔpH| = 3.6 was chosen. All solutions were prepared as described in Materials and Methods and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0156392#pone.0156392.g003" target="_blank">Fig 3</a>. The peak currents representing turnover rates are influenced mainly by the inner pH where proton release takes place and are independent of the outer pH where proton binding occurs. This is represented by the green (inside activating pH) and red (inside inactivating pH) circular areas. In contrast the grey square indicates that activity is independent of the pH of the outer solution.