Biological Systems MCAT Practice Exam

What is the function of tropomyosin in muscle contraction?

1. Enhances myosin-binding

2. Facilitates ATP binding

3. Blocks myosin binding sites on actin

4. Stimulates calcium release

The correct answer is: Blocks myosin binding sites on actin

Tropomyosin plays a critical role in regulating muscle contraction by interacting with actin filaments. Its primary function is to block myosin-binding sites on actin when the muscle is in a relaxed state. By doing so, tropomyosin prevents the myosin heads from attaching to actin, which is essential for muscle contraction to occur. When calcium ions are released during muscle contraction, they bind to troponin, a protein that is associated with tropomyosin on the actin filament. This binding causes a conformational change that moves tropomyosin away from the myosin-binding sites, allowing myosin heads to attach to actin. Consequently, muscle contraction can take place. This regulatory mechanism ensures that contraction only occurs when the calcium concentration is sufficient, thus playing a vital role in the control of muscle function. Understanding this relationship between tropomyosin, troponin, and the actin-myosin interaction is essential for grasping the complexities of muscle physiology.