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 ability of calcium ions to bind to troponin. Which of the following events would be directly impaired by this drug?
- AFormation of cross-bridges between actin and myosin
- BRelease of acetylcholine at the neuromuscular junction
- CRepolarization of the cardiac muscle cell membrane
- DATP hydrolysis by myosin heads
Show answer & explanationAnswer & explanation
Correct answer: A. Formation of cross-bridges between actin and myosin
Calcium binding to troponin causes a conformational change in tropomyosin, exposing myosin-binding sites on actin. If calcium cannot bind to troponin, these sites remain blocked, preventing the formation of cross-bridges and muscle contraction.
Why the other options are wrong
- B. Acetylcholine release occurs prior to muscle cell excitation and is not directly affected by calcium-troponin interaction.
- C. Repolarization involves ion channel activity and is a later event in the action potential, not directly linked to calcium-troponin binding.
- D. ATP hydrolysis by myosin heads is the energy source for the power stroke, but it cannot occur if cross-bridges haven't formed.
Calcium-Troponin Interaction
In muscle contraction, calcium ions bind to troponin, causing a conformational change that moves tropomyosin and exposes myosin-binding sites on actin filaments.
- Calcium is released from sarcoplasmic reticulum.
- Binds to troponin C subunit.
- Moves tropomyosin away from actin active sites.
- Allows myosin heads to bind to actin.
Memory trick: Calcium Connects Troponin, Tropomyosin Moves, Myosin Meets Actin