What is the role of the troponin-tropomyosin complex in skeletal muscle contraction?

• A. It binds calcium to supply energy for contraction.
• B. It forms the cross-bridge with myosin.
• C. It positions tropomyosin to block actin-myosin binding in the absence of Ca2+ and moves to expose binding sites when Ca2+ is present.
• D. It anchors thick filaments to the Z-disc.

Answer: (C)

The troponin-tropomyosin complex acts as the calcium-regulated switch that controls access to the myosin-binding sites on actin. In resting skeletal muscle, tropomyosin blocks those sites, kept in place by the troponin complex. When Ca2+ is released, Ca2+ binds to troponin C, causing a conformational change that moves tropomyosin away from the actin binding sites. This exposure lets myosin heads bind to actin and perform the power stroke using energy from ATP, leading to contraction. When Ca2+ levels fall, the complex returns to its blocking position and relaxation occurs. The other options aren’t correct because calcium isn’t an energy source, cross-bridges form when binding sites are exposed (not directly by the complex), and thick filament anchoring is not a function of the troponin-tropomyosin complex.