In skeletal muscle contraction, what binding event causes the displacement of tropomyosin from the myosin-binding sites on actin filaments?

Answer and explanation

Correct answer: B. Binding of Calcium ions () to Troponin C

  1. Step 1: In resting muscle fibers, filament sliding is inhibited because tropomyosin sterically covers the myosin cross-bridge attachment sites on actin monomers.
  2. Step 2: Upon depolarization, calcium ions () released from the sarcoplasmic reticulum bind specifically to the Troponin C subunit of the troponin complex.
  3. Step 3: This binding triggers a conformational rearrangement that shifts tropomyosin deeper into the actin groove, exposing the active sites for myosin head binding.
Answer
Binding of Calcium ions () to Troponin C.
Why not the others: Direct binding of ATP to actin monomers is biochemically incorrect because ATP binds to the globular heads of myosin to break actomyosin cross-bridges; Hydrolysis of acetylcholine at the motor end plate terminates synaptic transmission rather than initiating contractile filament sliding; Phosphorylation of the sarcoplasmic reticulum membrane regulates pump kinetics (phospholamban) without directly causing conformational shifting of tropomyosin.

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