Cyclic reaction scheme for AChR binding and gating. A is the agonist, R is the resting conformation of the AChR (low affinity, closed channel), and R* is the active conformation (high affinity, open channel). K_d and J_d are the equilibrium dissociation constants of R and R*. E₀, E₁, and E₂ are the gating equilibrium constants with zero, one, and two bound agonist molecules. The logarithm of E₀ is the intrinsic energy for the protein isomerization in the absence of bound agonists. In wt AChRs, the unboxed states are rarely visited and only the boxed states determine physiological and pharmacological responses. The two transmitter binding sites are assumed to be functionally equivalent. Without an external energy source, the product of the equilibrium constants for the outer paths connecting R with A₂R* is equal (E₂/K_d² = E₀/J_d²), so E₂ = E₀λ² where λ = K_d/J_d. A mutation that changes E₂ can do so by changing E₀, λ, or both.