FIGURE 10.

A hypothetical four-state model for a simple binding interaction between amines and toxin derivatives. U is the unoccupied state, B_amine is the amine-bound state, B_toxin is the toxin-bound state, B_amine/toxin is the doubly occupied, blocked state. The symbol, Δ, represents the amine (TPrA or DEA) bound from internal side; (*) represents the toxin (R13E or R13Q) bound from external side. A₁ is the equilibrium dissociation constant for amine binding to the unoccupied channel; A₂ is the equilibrium dissociation constant for amine binding to the toxin-occupied channel; T₁ is the equilibrium dissociation constant for toxin binding to the unoccupied channel; T₂ is the equilibrium dissociation constant for toxin binding to the amine-occupied channel.