Rigor is a transient state. Figure 20.5 A single cross-bridge cycle uses one molecule of ATP and moves the actin filament about 10 nm The myosin ATPase hydrolizes ATP to ADP, and energy is transferred to the cross-bridge. ADP and Pi remain bound to myosin. The myosin head moves to the cocked position and loosely binds a G-actin. When Ca2+ is present, the cross-bridge attaches tightly to the G-actin and goes through another cycle. The myosin head releases Pi as it swivels in the power stroke. It moves the thin lament 10 nm toward the center of the sarcomere. Myosin unbinds ADP after the power stroke and stays attached to actin in rigor. Rigor is a transient state. ATP binding dissociates myosin from actin. ADP ADP Pi ATP ADP Pi ADP Pi Myosin Myosin-binding sites ATP-binding site Actin-binding site Figure 20.5 A single cross-bridge cycle uses one molecule of ATP and moves the actin filament about 10 nm Each cross-bridge goes through several cycles during a single contraction. The two myosin heads function independently, and only one binds to actin at a time. Structural studies suggest that no more than four myosin heads can attach over a span of seven G-actin monomers. 10nm G-actin