Quantum Calculation of Proton and Other Charge Transfer Steps in Voltage Sensing in the Kv1.2 Channel

Ion channels in cell membranes control entry and exit of ions; their gating (opening and closing) is key to their functioning. It is known that protons can pass through the voltage-sensing domain (VSD) of channels such as Kv1.2. Quantum calculations for a section of the VSD show the steps protons take in responding to voltage and show no major displacement of the protein backbone with voltage change; 70 amino acids are included, 42 with side chains (9 directly in the proton path), 28 as backbone only, and 24 water molecules. Protons provide much of the gating current, the capacitative current immediately preceding channel opening with significant additional contributions from charge transfer to other groups. Most gating models, in contrast, require major protein displacement during gating. Energy terms without classical analogues (exchange plus correlation energy, which are greater than thermal energy) show that quantum calculations are required. Energy as a function of voltage for a key proton transfer leads to, approximately, the correct voltage for channel opening. Calculated total charge transfer (not only protons) for gating is reasonable compared to experimental values. We are also able to account, at least qualitatively, for two mutations, one with the gating current curve left-shifted and one right-shifted, and show the alternate proton paths that are required to account for these.