Figure 2. Model accounting for results of functional studies.

The TnI^sp is modeled as binding either TnC or actin and TnC is modeled as being in either the closed or open conformation. The model consists of two subsystems that partially overlap. The states of Tm (blue) include C, M, and B_i (i = 1, 2), corresponding to interactions with actin (green) in positions central, myosin dependent, and blocking, respectively. Steady-state constant, $K_0^{\prime }$, governs the ensemble dynamic interactions of myosin with Tm that sustain M. The states of Tn (TnT, yellow; TnI, magenta, TnC, black-white) include T_i and B_i, corresponding to energetically coupled and uncoupled in the Ca²⁺-free state (i = 1) and Ca²⁺-bound state (i = 2), respectively. Most probable occupancy of the N-terminal domain regulatory site of TnC is shown as Ca²⁺-bound (black) or Ca²⁺-free (white). Equilibrium constants govern spontaneous interactions that occur between Tn and actin (K₁ and K₃), and Ca²⁺ and Tn (K₂ and K₄); K₁ and K₃ also include the equilibrium that favors Tm in the C position. For the Tn states, representative distributions of closed and open conformations of TnC are diagramed to show the ratios 9/1, 1/3, 3/1, and 1/9 for B₁, B₂, T₁, T₂, respectively, which are intended for relative comparison only.