Final answer:
Tropomyosin blocks the myosin-binding site on actin in a relaxed muscle, which is part of the troponin-tropomyosin complex that regulates muscle contraction.
Step-by-step explanation:
In a relaxed muscle, the myosin-binding site on actin is blocked by tropomyosin. This protein winds around the chains of the actin filament and effectively covers the myosin-binding sites which prevents the formation of cross-bridges between actin and myosin, thus preventing muscle contraction. Tropomyosin is part of the troponin-tropomyosin complex; troponin helps to position tropomyosin on the actin molecule. Troponin is also responsible for binding calcium ions (Ca++), which when bound, cause a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing muscle contraction to proceed.
Tropomyosin is a protein that winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin. Tropomyosin binds to troponin to form a troponin-tropomyosin complex. The troponin-tropomyosin complex prevents the myosin "heads" from binding to the active sites on the actin microfilaments. Troponin also has a binding site for Ca++ ions.