MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A research team is investigating the effects of a novel drug on cardiac muscle contraction. They observe that the drug significantly reduces the force of contraction by interfering with the binding of calcium ions to their regulatory protein, thereby preventing the exposure of myosin-binding sites on actin. Which protein is the drug most likely targeting?
- ATropomyosin
- BTroponin
- CActin
- DMyosin
Show answer & explanationAnswer & explanation
Correct answer: B. Troponin
In muscle contraction, calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin, allowing cross-bridge formation. If the drug interferes with calcium binding to its regulatory protein and prevents exposure of myosin-binding sites, it is targeting troponin.
Why the other options are wrong
- A. Tropomyosin is the protein that covers the myosin-binding sites on actin in the absence of calcium.
- C. Actin is the thin filament that myosin binds to, but calcium does not directly bind to actin to initiate contraction.
- D. Myosin is the motor protein that forms cross-bridges and pulls actin filaments.
Calcium-Troponin Interaction
In muscle contraction, calcium ions bind to the troponin complex (specifically troponin C), leading to a conformational change that shifts tropomyosin away from the myosin-binding sites on the actin filament, allowing myosin to bind and initiate contraction.
- Calcium is the trigger for muscle contraction.
- Troponin is a complex of three proteins (TnC, TnI, TnT).
- Calcium binds to Troponin C (TnC).
- This binding causes tropomyosin to move, exposing actin's binding sites for myosin.
Memory trick: Calcium hits Troponin, Tropomyosin moves, Myosin grabs Actin.