Figure 3.

Four-state reaction scheme for the troponin regulatory sites in which Mg²⁺ competes with Ca²⁺ for binding to the metal-free state. The values of k₊₁, k_−1, k₊₂, and k₋₂ are 1.77 × 10⁸ M⁻¹s⁻¹, 1,544 s⁻¹, 0.885 × 10⁸ M⁻¹s⁻¹, and 17.1 s⁻¹, respectively (16°C; Baylor and Hollingworth, 2007). In the simulations without Mg²⁺ competition (Figs. 5–7, 9, 10 A, and 11 A), the troponin reaction included only the Mg²⁺-free states; with resting [Ca²⁺] = 50 nM, the resting fractional occupancies are 0.993 (Trop), 0.006 (CaTrop), and 0.001 (Ca₂Trop). In the simulations of Figs. 8, 10 B, and 11 B, the values of k₊₃ and k₋₃ were 6 × 10⁴ M⁻¹s⁻¹ and 120 s⁻¹, respectively (K_D,Mg = 2 mM). With resting [Mg²⁺] = 1 mM, the resting fractional occupancies of the four states are 0.663 (Trop), 0.004 (CaTrop), 0.001 (Ca₂Trop), and 0.332 (MgTrop). Table 1 gives the (spatially averaged) troponin concentration.