Structure of the MWC model. The empty binding site (white circle) of each subunit can bind one ligand (blue circle) in either the resting (gray squares) or activated (gray circles) state. The equilibrium association constant is equal for all binding steps in the resting protein (K; mol⁻¹), and in the activated protein (fK; mol⁻¹), resulting in the indicated stoichiometric factors. f is a constant allosteric factor. A joint allosteric step with the equilibrium isomerization constant fⁿE₀ (f > 1, n = 0…4) leads to progressive activation. For the double-liganded channel, the ligands are drawn below the channel cartoon to illustrate that there are two options for binding, adjacent or diagonal.