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. Which protein is the drug most likely targeting?
- AMyosin
- BTroponin
- CActin
- DTropomyosin
Show answer & explanationAnswer & explanation
Correct answer: B. Troponin
In muscle contraction, calcium ions bind to troponin, a regulatory protein. This binding causes a conformational change in troponin, which then moves tropomyosin away from the myosin-binding sites on actin, allowing myosin heads to bind and initiate the power stroke. If the drug interferes with calcium binding to its regulatory protein, it is directly targeting troponin.
Why the other options are wrong
- A. Myosin is the motor protein that forms thick filaments and binds to actin, but it's not the primary calcium-binding regulatory protein.
- C. Actin forms thin filaments and has myosin-binding sites, but it does not directly bind calcium for regulation.
- D. Tropomyosin blocks myosin-binding sites on actin, but its movement is regulated by troponin's calcium binding, it doesn't directly bind calcium.
Calcium-Troponin Interaction
In muscle contraction, calcium ions bind to the troponin complex, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin, thereby allowing cross-bridge formation.
- Calcium binds to the TnC subunit of troponin
- Binding causes troponin to change shape
- Troponin-tropomyosin complex moves
- Exposes myosin-binding sites on actin
Memory trick: CaT-ACT: Calcium to Troponin, Actin to Myosin.