What is the role of Ca2+ in contraction at sufficient intracellular concentration?

• A. Ca2+ binds to troponin; troponin changes shape and moves tropomyosin away from myosin-binding sites; myosin heads bind to actin; contraction occurs
• B. Tropomyosin reinforces blocking sites on actin
• C. Ca2+ binds directly to actin to initiate contraction
• D. Calcium uptake into SR ends contraction

Answer: (A)

When intracellular calcium is high enough, Ca2+ binds to a component of the thin filament’s regulatory system, troponin C. This binding causes a shape change in the troponin complex, which in turn moves tropomyosin away from the myosin-binding sites on actin. With those sites exposed, energized myosin heads can attach to actin and undergo cross-bridge cycling powered by ATP. As the myosin heads pull, the sarcomere shortens, producing contraction.

In the resting state, without sufficient Ca2+, tropomyosin remains positioned over the binding sites, blocking myosin and preventing contraction. So the key step is Ca2+ triggering the troponin-tropomyosin switch to reveal actin’s myosin-binding sites. Ca2+ binding to actin itself is not the mechanism, and contraction ends when Ca2+ is pumped back into the sarcoplasmic reticulum, lowering cytosolic Ca2+ and allowing the blocking position to be restored.