Which protein covers actin's myosin-binding sites when calcium is not present?

Which protein covers actin's myosin-binding sites when calcium is not present?

• A. Troponin
• B. Myosin
• C. Actin
• D. Tropomyosin

Answer: (D)

When calcium isn’t present, tropomyosin blocks the myosin-binding sites on actin, preventing cross-bridge formation. Tropomyosin sits along the grooves of the actin filament and covers those binding sites, keeping myosin from attaching. Troponin acts as the calcium sensor: without calcium, it holds tropomyosin in this blocking position. When calcium binds to troponin C, troponin changes shape and moves tropomyosin away from the sites, exposing them so myosin can bind and contraction can proceed. Myosin is the motor that would bind once the sites are exposed, and actin is the filament that presents those sites.

When calcium isn’t present, tropomyosin blocks the myosin-binding sites on actin, preventing cross-bridge formation. Tropomyosin sits along the grooves of the actin filament and covers those binding sites, keeping myosin from attaching. Troponin acts as the calcium sensor: without calcium, it holds tropomyosin in this blocking position. When calcium binds to troponin C, troponin changes shape and moves tropomyosin away from the sites, exposing them so myosin can bind and contraction can proceed. Myosin is the motor that would bind once the sites are exposed, and actin is the filament that presents those sites.