BPS2026 – Quantum calculations on shaker mutants at the gate show ion dehydration as protein plus Cl− outcompete K+ for water, while the channel closes with protons

Experimentally, a P⋄D mutant at the gate of the Shaker channel is constitutively open, while the P⋄S mutant is closed at all physiological potentials (Sukhareva et al., 2003). The aspartate side chain is larger than the proline that it replaces, while serine is much smaller. If the gate is to be physically blocked in the closed state, while the open state should be wide open, this is paradoxical. We have carried out quantum calculations on the mutants, testing a mechanism by which the potassium ion is partially dehydrated at the gate to pass through the channel; since the ion arrives hydrated by about 6 water molecules, and leaves accompanied by at most 2, partial dehydration is required. Therefore, the open channel must not be open so wide as to allow the fully hydrated ion to pass through. The protein and Cl − outcompete the K + for water, when open. This leaves the question of the nature of the closed state. In an earlier work, we have shown that protons could move from the voltage sensing domain to the pore, thus constituting gating current. Here, we present calculations that show the consequence of having protons at the gate, which need not change conformation. With protons, plus K + and some Cl − to balance charge, we see that the K + joins a relatively immobile cluster containing water, protons, and both ions, thus blocking the gate; aspartate appears to remove protons. Natural Bond Order calculations show how the bonding, including hydrogen bonding, makes this happen. The P⋄T mutation opens about like wild type, much better than the always closed P⋄S mutation, consistent with this interpretation.