Final answer:
Active sites on actin in skeletal muscle become available for binding when Ca2+ binds to troponin, allowing the myosin heads to engage and muscle contraction to occur, with ATP powering the cycle of attachment and release.
Step-by-step explanation:
The active sites on actin become available for binding when calcium ions (Ca2+) bind to the troponin complex, causing a conformational change that shifts tropomyosin away from the binding sites. This process is critical for muscle contraction and is part of what is known as the sliding filament theory. The sequence begins with calcium release from the sarcoplasmic reticulum (SR) into the sarcoplasm.
Once the calcium binds to troponin, tropomyosin moves, exposing the active sites on actin, allowing the myosin heads to attach and perform the power stroke necessary for contraction. This sequence relies on a continuous supply of ATP to subsequently release the myosin heads and prepare for the next contraction cycle. The active sites remain available as long as there is sufficient calcium and ATP. As the calcium levels drop, tropomyosin re-covers the binding sites on actin, and the muscle relaxes.