Three-state, two-transition model of voltage- and [ATP]-dependent gating. (A) Simple three-state model consisting of an ATP-binding step and a gating step. C, the closed state with no bound ATP; C_A, the closed state after ATP is bound; O_A, the open state. k_bind and k_unbind represent the binding/unbinding rates for ATP binding to the channel. k_on and k_off represent the rates of the gating step. (B) k_on and k_off were calculated as described in Materials and methods from the data in Fig. 5 (B and C). Membrane potentials are shown in the figure.