The long protein mulecule, which masks the active sites on the f-actin is

Options

A. myosin
B. troponin
C. tropomyosin
D. light meromyosin

Answer

To solve the question "The long protein molecule, which masks the active sites on the F-actin is," we need to identify the protein that performs this function in muscle contraction.

### Step-by-Step Solution:

1. **Understand the Structure of Muscle Fibers**:
- Muscle fibers consist of thick and thin filaments. The thick filaments are primarily made of myosin, while the thin filaments are primarily made of actin.

2. **Identify the Components of Thin Filaments**:
- The thin filaments are composed of actin (F-actin) and regulatory proteins, including tropomyosin and troponin.

3. **Function of Tropomyosin**:
- Tropomyosin is a rod-shaped fibrous protein that wraps around F-actin. It plays a crucial role in muscle contraction by covering the active sites on the actin filaments.

4. **Role of Troponin**:
- Troponin is a globular protein that is associated with tropomyosin. It binds to tropomyosin and helps regulate the position of tropomyosin on the actin filaments.

5. **Active Sites on F-actin**:
- The active sites on F-actin are the sites where myosin heads bind during muscle contraction. When tropomyosin is in place, it blocks these active sites.

6. **Conclusion**:
- The protein that masks the active sites on F-actin is **troponin**. Therefore, the correct answer is **troponin**.

### Final Answer:
The long protein molecule, which masks the active sites on the F-actin is **troponin**.

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