Final answer:
ATP binds to the myosin head of the myosin II molecule, causing it to detach from actin and, after ATP hydrolysis, enter a high-energy, cocked position. The ATPase enzyme at the myosin head catalyzes this reaction.
Step-by-step explanation:
ATP binds to the myosin head of the myosin II molecule. This binding is crucial for muscle contraction because once ATP is bound, it causes the myosin head to detach from the actin filament. Subsequently, the ATP is hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate (Pi), which powers the myosin head into a "cocked" position. After the power stroke, if actin binding sites are available and not covered, a cross-bridge will form, allowing the myosin head to attach to the actin filament, pulling it towards the center of the sarcomere, which results in muscle contraction.
However, if no ATP binds or if ATP is not present, the myosin head will not detach from actin, leading to a state known as rigor.
It is also worth noting that the enzyme at the binding site on myosin known as ATPase facilitates the hydrolysis of ATP, making it a key player in the muscle contraction cycle.