During skeletal muscle contraction, an action potential leads to the release of calcium ions into the sarcoplasm. These calcium ions play a critical role in initiating the cross-bridge cycle. What is the immediate and direct consequence of calcium binding to the troponin complex?
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The answer
Correct answer: D. Calcium binding to troponin C induces a conformational change that shifts tropomyosin away from the myosin-binding sites on actin filaments.
Tested concept: Calcium and troponin in muscle contraction
Explanation
The correct answer is that calcium binding to troponin C causes a conformational change. This change in troponin then moves tropomyosin, which normally covers the myosin-binding sites on actin. Once these sites are exposed, myosin heads can bind to actin, initiating the cross-bridge cycle and muscle contraction. Distractor 1 is incorrect because calcium binds to troponin, not directly to myosin. Distractor 2 is incorrect because myosin, not troponin, is the motor protein responsible for the power stroke. Distractor 3 is incorrect because calcium binding to troponin exposes the binding sites, allowing myosin to bind and then hydrolyze ATP; it does not directly activate the ATPase activity of myosin heads.
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