Movie 4. Probability [Low affinity] = 0.1  This simulation illustrates the behavior of an affinity-switching selectivity filter with a single K-binding site. During the simulation, the binding site can be seen to switch stochastically between a high affinity state (red circle, 90% of the time)- and low affinity state (green square, 10% of the time). When a K ion approaches the filter while it is green (high affinity), it is captured and trapped. The next time the filter turns red (low affinity) the bound K ion is released, either to the site it came from, or to the bottom compartment (determined by chance and with equal probability). In the movie, about half of the bound ions can be seen to return to the top compartment. If the K ion crosses, it counts as a permeated ion. Na ions are able to permeate when they approach the filter while it is in the low-affinity state. They bind very briefly and dissociate to the upper or lower compartment (with equal probability). They also get counted when they cross the membrane. At the end of the short simulation (5,000 steps), a total of 8 K ions and 1 Na ion have crossed the membrane.      (Movie 5)    (Back)