Nerve and Muscle Physiology — MCQs

Question 471: Which of the following proteins prevents contraction by covering binding sites on actin, thereby preventing its interaction with myosin?

• A. Thymosin
• B. Troponin
• C. Calmodulin
• D. Tropomyosin (Correct Answer)

Explanation: ***Tropomyosin*** - **Tropomyosin** is a regulatory protein that wraps around the **actin filament**, covering the **myosin-binding sites** in a resting muscle. This physically blocks the myosin heads from attaching to actin. - When **calcium ions** bind to troponin, it causes a conformational change that shifts tropomyosin away, exposing the binding sites and allowing muscle contraction. *Thymosin* - **Thymosin** is a protein involved in the regulation of **actin polymerization**, primarily by sequestering **G-actin monomers**. - It plays a role in the formation and regulation of the **actin cytoskeleton** but is not directly involved in blocking myosin binding sites on actin filaments during muscle contraction. *Troponin* - **Troponin** is a complex of three proteins (troponin C, I, and T) that is attached to **tropomyosin**. - Its primary role is to **bind calcium ions**, which then causes a conformational change that moves tropomyosin, *exposing* the myosin-binding sites on actin, rather than *covering* them. *Calmodulin* - **Calmodulin** is a ubiquitous calcium-binding messenger protein that plays a key role in various cellular processes, including smooth muscle contraction. - In smooth muscle, **calmodulin binds calcium** and activates myosin light chain kinase, but it does not directly cover myosin-binding sites on actin in skeletal muscle.