Step 1: Contraction Cycle Begins
Begins with the arrival of calcium ions within the zone of overlap
Step 2: Active-Site Exposure
Calcium ions bind to troponin, weakening the bond between actin and the troponin-tropomyosin complex.
This reaction leads to the exposure of the active sites on the actin molecules of the thin filaments.
Step 3: Cross-Bridge Formation
Once the active sites are exposed, the energized myosin heads bind to them, forming cross bridges.
Step 4: Myosin Head Pivoting
After cross-bridge formation, the stored energy is used to pivot the myosin head toward the M line. This action is called the power stroke, when it occurs, the bound ADP and phosphate group are released.
Step 5: Cross-Bridge Detachment
When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken.
The active site in now exposed and able to form another cross bridge.
Step 6: Myosin Reactivation
Myosin reactivation occurs when the free myosin head splits ATP into ADP and P.
What is contraction cycle?
The muscle contracts in a pattern of repeated joining and releasing between the two thin and thick strands of the sarcomere. ATP is important for preparing myosin for binding and 'loading' of myosin.
ATP first binds to myosin, putting it in a high energy state. ATP is hydrolyzed to ADP and inorganic phosphate (Pi) by the enzyme ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head to a 'tilted' position, ready to bind to actin when the site becomes available. ADP and Pi remain connected. Myosin exists in a high energy configuration.
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