Question

multiple choice question

what happens when calcium ions bind to troponin?

tropomyosin moves off the active site on actin.
troponin binds to myosin.

Explanation:

Identify the role of troponin in muscle contraction

Using the Troponin Function knowledge point

• Troponin is a regulatory protein complex bound to tropomyosin on the thin (actin) filaments of skeletal muscle.
• In a relaxed muscle, tropomyosin physically blocks the active myosin-binding sites on the actin molecules, preventing cross-bridge formation.

Determine the effect of calcium binding

• When calcium ions (\(\text{Ca}^{2+}\)) are released from the sarcoplasmic reticulum, they bind directly to the troponin complex.
• This binding induces a conformational (shape) change in troponin.
• Because troponin is linked to tropomyosin, this shape change pulls the tropomyosin molecule away from the active sites on the actin filament.
• This exposes the active sites, allowing myosin heads to bind to actin and initiate contraction.

Evaluate the given options

• Option 1: "Tropomyosin moves off the active site on actin." This matches the physiological mechanism perfectly.
• Option 2: "Troponin binds to myosin." This is incorrect; troponin does not bind to myosin.

Answer:

• Tropomyosin moves off the active site on actin. (Correct answer)
• Troponin binds to myosin.