Fig 4. Kinetic model employed to explain the V_m, Ca²⁺, and Cl⁻ contributions to TMEM16A gating.

Scheme I represents V_m-dependent transitions in the absence of intracellular Ca²⁺ and external Cl⁻. C and O are closed and open states, and α₁ and β₁ are V_m-dependent rate constants. Scheme II(A) represents the transitions in the presence of one external Cl⁻ when [Ca²⁺]_i =0. [Cl⁻]_ok_CCl1 and [Cl⁻]_ok_OCl1 are rate constants controlling Cl⁻ association to C and O configurations, while k_CCl2, k_OCl2 rate constants control Cl⁻ dissociation. Scheme III(A) results after binding of one Ca²⁺ to states represented in Scheme II(A). The [Ca²⁺]_ik_O1, [Ca²⁺]_ik_O1Cl, [Ca²⁺]_ik_C1Cl and [Ca²⁺]_ik_C1 are Ca²⁺ association rate constants while k_O2, k_O2Cl, k_C2Cl and k_C1 are Ca²⁺ dissociation rate constants. The subscript Cl indicates rate constants associated with Cl⁻-occupied transitions. Scheme II(B) represents transitions resulting after binding of one intracellular Ca²⁺ to C and O states depicted in Scheme I. [Ca²⁺]_ik_C1, [Ca²⁺]_ik_O1 are rate constants controlling Ca²⁺ association to C and O configurations, while k_C2, k_O2 rate constants control Ca²⁺ dissociation. Scheme III(B) represents the transitions resulting after binding of an additional Ca²⁺ to states shown in Scheme II(B). Association and dissociation of the second Ca²⁺ was assumed to be same as for the first Ca²⁺ binding reaction. Scheme IV results after binding of an additional Ca²⁺ to states shown in Scheme III(A). The association and dissociation rate constants were the same as that for the first Ca²⁺ binding (left pathway). Alternatively, Scheme IV could be achieved after binding of one external Cl⁻ to states shown in Scheme III(B) with forward rate constants [Cl⁻]_ok_CCl5, [Cl⁻]_o k_CCl3, [Cl⁻]_ok_CCl1, [Cl⁻]_ok_OCl1, [Cl⁻]_ok_OCl3 and [Cl⁻]_ok_OCl5 and backward rate constants k_CCl6, k_CCl4, k_CCl2, k_OCl2, k_OCl4 and k_OCl6. Concentric kinetic cycles with states connected by wider arrows indicate cycles that did not satisfy microscopic reversibility. Table 2A shows the Equations for each rate constant.