FIGURE 1.

Schematic showing the parallel pathway model for muscle regulation. Myosin must be bound to actin in the active state (A_a) for rapid hydrolysis of ATP. Actomyosin in the inactive state (A_i) does not hydrolyze ATP at a sufficient rate to drive muscle contraction and the enzymatic activity of unbound myosin is very low. Ca²⁺ partially shifts the equilibrium between states to favor the active state but cannot fully activate the filament. Tight binding forms of myosin or S1 (rigor myosin, myosin-ADP, NEM-S1) shift the equilibrium to the fully active state. Thus, alterations in troponin I could potentially alter the equilibrium between states, reduce myosin-ATP binding, or alter a step within the active pathway.