During cocking of the myosin head, what occurs?

• A. ATP hydrolysis re-energizes the myosin head for the next cycle
• B. ATP binds to the myosin causing detachment
• C. Calcium is released from the SR
• D. Tropomyosin binds actin more tightly

Answer: (A)

The step being tested is how the myosin head is prepared for the next cycle by using energy from ATP. When ATP is hydrolyzed by the myosin ATPase, the head is re-energized and moves into a cocked, high-energy position with ADP and Pi bound. This stored energy allows the head to quickly form a strong bond with actin and then perform the power stroke when the binding site is ready.

Detachment happens when a new ATP binds to myosin, causing release from actin, which is not the cocking step. Calcium release from the sarcoplasmic reticulum regulates exposure of actin sites via troponin-tropomyosin, but it doesn’t energize the head. Tropomyosin binding to actin becomes tighter in the resting state and moves to expose sites with calcium, not during cocking. So the re-energizing by ATP hydrolysis to cock the head for the next cycle is the correct idea.