Figure 2.

Mckillop-Geeves (McK-G) three-state model scheme.⁸ In the McK-G model the structural unit, actin₇·TmTn, is schematically shown as seven open circles representing the actin monomers connected via a line representing the tropomyosin. This unit exists in a dynamic equilibrium between the three states as represented by the different positions of tropomyosin: the blocked state, A^B_, in which no myosin-S1 binding can occur, the closed or calcium induced state, A^C_, in which only weak binding of S1 can occur, and the open or myosin induced state, A^M_, which allows isomerization of the myosin-S1 to the rigor-like state. The ratio of the three states in the absence of myosin-S1 is defined by the equilibrium constants K_B (between the closed and blocked states) and K_T (between the closed and open states). The blocked state is only present in the absence of Ca²⁺ when the Tn complex is tightly bound to the thin filament; in the presence of Ca²⁺, there is little or no occupancy of the blocked state. Weakly bound myosin states are denoted as A-states (A^MM_A) and rigor-like states are denoted as R-states (A^MM_R). The rate of myosin binding is defined by equilibrium constant K₁= k₁c/k₋₁, and the rate of isomeration of S1 into R state is defined by equilibrium constant K₂ = k₂/k₋₂. Backward rate constants used in all simulations are taken to be k_−B = 100 s⁻¹, k_−T = 3000 s⁻¹, k₋₁ = 10 s⁻¹ and k₋₂ =5 s⁻¹. Also the equilibrium constants were fixed over all simulations, unless is denoted differently in the figure legends are: K₁ = 0.22 μM⁻¹ and K₂ = 200.