The structure of the protein that is absolutely necessary for its biological activity is

Options

A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

Answer

To determine which structure of a protein is absolutely necessary for its biological activity, we need to analyze the different levels of protein structure:

### Step-by-Step Solution:

1. **Understanding Protein Structure**:
- Proteins are made up of long chains of amino acids, which are linked together by peptide bonds. The sequence of these amino acids determines the protein's structure and function.

2. **Levels of Protein Structure**:
- Proteins have four levels of structure: primary, secondary, tertiary, and quaternary.

3. **Primary Structure**:
- This is the linear sequence of amino acids in a polypeptide chain. While it provides the basic information about the protein, it does not determine the protein's biological activity.

4. **Secondary Structure**:
- This involves the local folding of the polypeptide chain into structures such as alpha-helices and beta-sheets. Although important for the overall shape, it is not sufficient alone for biological activity.

5. **Tertiary Structure**:
- This is the three-dimensional shape formed by the entire polypeptide chain, including the interactions between secondary structures. The tertiary structure is crucial because it determines the active site of enzymes and the overall functionality of the protein.

6. **Quaternary Structure**:
- This structure refers to the arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein. While it is important for some proteins, it is not universally necessary for all proteins.

7. **Conclusion**:
- The tertiary structure is the most critical for the biological activity of proteins, as it dictates how the protein interacts with other molecules and performs its function.

### Final Answer:
The structure of the protein that is absolutely necessary for its biological activity is the **tertiary structure**.

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