Movie 6. Probability [Low affinity] = 0.01   For this simulation, the probability of being in the low-affinity state  is reduced to 1%. The simulation is run for 200,000 steps, displaying every 200th frame. Because the K-binding site is almost always in the high-affinity state, a K ion is occupying the filter almost continuously. This very efficiently prevents Na ions from permeating, resulting in high ion selectivity (K/Na flux ratio = 100). Whenever the binding site switches to the low-affinity state it releases the bound ion, which is almost always a K ion that was trapped. Despite the 100-fold gain in K selectivity, the relative flux rate for K ions has been reduced only 5-fold compared to free diffusion: the gating filter allows high K selectivity with minimal penalties on permeation rate. The reason that the permeation rate does not get seriously degraded is that the channel spends most of its time waiting for a K ion to present itself at its entrance, where it is available to cross the membrane. As long as the average life time of the high-affinity state is not much longer than this waiting time, permeation will be relatively efficient.  (Back)