Which regulatory movement occurs when calcium is present, causing tropomyosin to uncover myosin-binding sites on actin?

• A. Tropomyosin shifts to uncover binding sites
• B. Troponin binds actin directly
• C. Myosin binds to actin without regulatory proteins
• D. Calcium binds directly to actin

Answer: (A)

Calcium’s role is to unlock the actin filament by moving the regulatory proteins aside. When calcium is present, it binds to troponin C, which causes the troponin-tropomyosin complex to shift the tropomyosin strand away from the myosin-binding sites on actin. That sliding exposes the binding sites, allowing myosin heads to attach and form cross-bridges, which is how contraction can proceed. Without calcium, tropomyosin remains blocking those sites, so cross-bridge formation can’t happen. The idea that calcium binds directly to actin, or that troponin binds actin directly, isn’t how this regulation works, and contraction requires this calcium-driven repositioning of tropomyosin to expose the binding sites.