Q.Which subunit of the troponin complex binds calcium ions and thereby brings about the conformational change that exposes the myosin-binding sites on actin during muscle contraction?

[!TLDR]

Troponin C is the subunit of the troponin complex that binds calcium ions.

[!ANSWER]

(c) Troponin C (TnC)

The troponin complex attached to tropomyosin along the actin thin filament is made of three distinct subunits. Troponin C is the calcium-binding subunit: when sarcoplasmic calcium rises after a nerve impulse, calcium ions bind troponin C, and this binding produces the conformational change in the whole troponin-tropomyosin complex that shifts tropomyosin away from the myosin-binding site on actin, allowing a cross-bridge to form. Troponin I helps hold tropomyosin over the binding site in the resting state, and troponin T anchors the troponin complex to tropomyosin -- neither of these two subunits itself binds calcium.

[!ANSWER]

(c) Troponin C (TnC)

Recall the three troponin subunits by their defining role: C for calcium-binding, I for inhibitory (blocks the site at rest), T for tropomyosin-binding (anchors the complex).

Confusing troponin C's calcium-binding role with tropomyosin, which is a separate, larger rod-shaped protein that physically covers the binding site but does not itself bind calcium.