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 the structure of a protein that is absolutely necessary for its biological activity, we need to analyze the different levels of protein structure: primary, secondary, tertiary, and quaternary.

### Step-by-Step Solution:

1. **Understanding Protein Structure**:
- Proteins are made up of amino acids linked by peptide bonds, forming a polypeptide chain. The structure of proteins can be categorized into four levels: primary, secondary, tertiary, and quaternary.

2. **Primary Structure**:
- The primary structure refers to the linear sequence of amino acids in the polypeptide chain. While it is essential for determining the overall structure of the protein, it does not dictate the protein's biological activity directly. Thus, it is not the correct answer.

3. **Secondary Structure**:
- The secondary structure involves the folding of the polypeptide chain into structures such as alpha-helices and beta-sheets. This level of structure contributes to the overall shape of the protein but does not define its biological activity. Therefore, this option is also incorrect.

4. **Tertiary Structure**:
- The tertiary structure is formed when the secondary structure folds further into a three-dimensional shape. This configuration is crucial because it creates active sites where biochemical reactions can occur. The specific shape and arrangement of the tertiary structure are vital for the protein's biological function, making this the correct answer.

5. **Quaternary Structure**:
- The quaternary structure involves the assembly of multiple polypeptide chains into a single functional protein complex. While this structure is important for some proteins, it is not universally necessary for biological activity. Thus, this option is also incorrect.

### Conclusion:
The structure of the protein that is absolutely necessary for its biological activity is the **tertiary structure** (Option 3).

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