In relaxed muscle, the myosin-binding site on actin is blocked by

Out of the following given choices;

a.       titin b. troponin c. myoglobin d. tropomyosin

The answer is B. Troponin is attached to tropomyosin in the actin filament. It sits in the myosin-binding site in the actin hence preventing the formation of the cross-bridge required for contraction of the muscle. When calcium ion bind to the troponin, its conformation changes and moves from the myosin-binding site. This allows the formation of the cross-bridge between actin and myosin in preparation for contraction.