Illustration of the scheme for charge movement coupled to polyamine binding at the entry to the SF. (A) K⁺ ions (purple spheres) occupy the canonical S1 and S3 sites within the SF, coordinated by carbonyl oxygen atoms (from Y62-G61, and V60-T59), as well as the S5 site, in which a partially hydrated K⁺ ion can be coordinated by side chain oxygens from T59. A hydrated polyamine (blue) can occupy the cavity, electrostatically repelled from the SF by the K⁺ ion at S5. (B) K⁺ ions occupying more extracellular S2 and S4 positions, coordinated by oxygen atoms (carbonyls from G61-V60, and carbonyl/side chain from T59-T59OH), as well as the S0 site, in which a partially hydrated K⁺ ion can be coordinated by carbonyl oxygens from Y62. The polyamine remains electrostatically repelled from the SF by the K⁺ ion at S4. (C) Further extracellular movement of K⁺ (to S1/S3) permits coordination of the polyamine by T59-OH, with dissociation of K⁺ to the bulk solution. Thus, polyamine blockade is coupled to net translocation of one electronic charge through the SF.