Actin binding sites are located over

Options

A. troponin
B. tropomyosin
C. meromyosin
D. both troponin and tropomysoin

Answer

To determine where actin binding sites are located, we need to analyze the options provided: troponin, tropomyosin, meromyosin, or both troponin and tropomyosin.

### Step-by-Step Solution:

1. **Understanding the Components**:
- **Troponin**: This is a complex of three regulatory proteins (troponin C, I, and T) that play a crucial role in muscle contraction. However, troponin itself does not have actin binding sites; instead, it regulates the position of tropomyosin on actin.
- **Tropomyosin**: This protein runs along the length of actin filaments and blocks the myosin binding sites on actin, preventing muscle contraction in the absence of calcium ions. Therefore, tropomyosin does not have actin binding sites either.
- **Meromyosin**: This is a part of the myosin molecule and contains both an actin binding site and an ATP binding site. It is involved in the interaction with actin during muscle contraction.
- **Both Troponin and Tropomyosin**: Since neither of these proteins has actin binding sites, this option can also be ruled out.

2. **Identifying the Correct Answer**:
- Since both troponin and tropomyosin do not have actin binding sites and only meromyosin has an actin binding site, the correct answer is **meromyosin**.

### Final Answer:
The actin binding sites are located over **meromyosin**.

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