HA models for four concatamers containing one disabled binding site. The concatamers build, together with the two models in Fig. 1 and the models in Figs. S3 and S4, the 16 models of the CHA model. Blue circles represent a ligand, white circles represent an empty binding site, and a red cross on a white circle represents a disabled binding site. Equilibrium association constants for ligand binding, Kxxxx (x = 0 for an empty subunit, x = 1 for a subunit to be occupied, and x = 1 for a preoccupied subunit) are indicated in black for a WT and red for a disabled subunit. One HA model contains 32 Kxxxxs, either black or red. Shown is only the network for ligand binding. Each state model has to be complemented by the scheme of the closed–open isomerizations shown at the bottom of Fig. 1.