Fits with flip models containing three binding sites (Schemes 3a and 3b). (A) Scheme 3a (top) differs from Scheme 2 (Fig. 4 A) in the presence of a third agonist binding site. Scheme 3b (bottom) is a variant that lacks the partially liganded AF state. Both models have only one open state that can be accessed after flipping, when the three binding sites are occupied by glycine. (B) Plots that show open-period and shut-time distributions fitted with Scheme 3a. Identical fits were obtained with Scheme 3b. Note also the similarity with predictions from Scheme 2 (Fig. 4 B). Fits with models 3a and 3b were performed with constraints for non-interacting binding sites (resolution, 20 µs). (C) Simulation of a single-channel record (at 20 µM glycine) using the fitted rate constants of the set shown in B. Similar simulations are obtained for Scheme 3b. The prediction from both models is similar to those from Scheme 2 and in a very good agreement with our cell-attached data (compare with Figs. 2 A and 4, C and D).