Globular protein molecule which masks the active sites on F-actin is:

Options

A. Troponin
B. Tropomyosin
C. Myosin
D. Light meromyosin

Answer

To answer the question about which globular protein molecule masks the active sites on F-actin, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding F-actin**: F-actin, or filamentous actin, is a polymerized form of globular actin (G-actin) and is a crucial component of the muscle contraction mechanism.

2. **Identifying the Proteins Involved**: In muscle contraction, several proteins interact with actin, including:
- **Troponin**: A globular protein that is part of the troponin complex.
- **Tropomyosin**: A protein that runs along the grooves of the actin filament.
- **Myosin**: The motor protein that interacts with actin for muscle contraction.
- **Light Meromyosin**: A part of the myosin molecule.

3. **Role of Troponin and Tropomyosin**:
- Troponin is associated with tropomyosin and plays a critical role in regulating muscle contraction.
- In a relaxed muscle, tropomyosin covers the active sites on F-actin, preventing myosin from binding.

4. **Troponin's Function**: When calcium ions bind to troponin, it causes a conformational change that moves tropomyosin away from the active sites on F-actin, allowing myosin to bind and initiate contraction.

5. **Conclusion**: The globular protein that masks the active sites on F-actin is **Troponin**.

### Final Answer:
The globular protein molecule which masks the active sites on F-actin is **Troponin**.

---