Questions: Where does calcium attach to the contractile protein complex? myosin actin troponin tropomyosin

The answer is the third one: troponin.

Explanation for each option:

• Myosin: Myosin is a motor protein that interacts with actin to cause muscle contraction. Calcium does not directly attach to myosin.

• Actin: Actin is a protein that forms thin filaments in muscle fibers. Calcium does not directly bind to actin.

• Troponin: Troponin is a regulatory protein complex associated with the thin filaments of muscle tissue. Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing muscle contraction to occur.

• Tropomyosin: Tropomyosin is a protein that blocks the myosin-binding sites on actin filaments. Calcium does not bind to tropomyosin directly; instead, it binds to troponin, which then affects tropomyosin's position.

In summary, calcium attaches to troponin in the contractile protein complex.