Predictions for the ATP dependence of the burst ratio for tetrameric constructs based on different models. Relationship between the burst ratio (BR) and ATP concentration for connected K_ATP tetramers containing between one and four WT ATP-binding sites for channels containing four (A), one (B) or two (C) gating units. These correspond to the HH, MWC, and D models respectively. D depicts the relationship when the first ATP bound has a dominant effect on channel closure. The number of WT ATP-binding sites is indicated to the right of each trace. 2 SD, tetrameric construct with 2WT ATP-binding sites located on the same dimer subunit; 2 DD, tetrameric construct with two WT ATP-binding sites each located on a different dimer subunit. The lines are fit assuming mutant ATP-binding sites are nonfunctional (i.e., m = 1 or K_O = 0). The lines are drawn using Eqs. 16–23 given in the Appendix. The K_O and m parameters for the wild-type subunits of each model were obtained by fitting the data for unconnected four WT K_ATP channels (Fig. S12 B, open circles).