An active site of an enzyme is a crevice or pocket into which the substrate fits. These active sites are present on :-

Options

A. Primary structure of protein
B. Secondary structure of protein
C. Tertiary structure of protein
D. All of the above

Answer

To solve the question regarding the active site of an enzyme, we need to understand the structure of proteins and how they relate to enzyme function.

### Step-by-Step Solution:

1. **Understanding Active Sites**: The active site of an enzyme is a specific region where the substrate binds. This site is crucial for the enzyme's catalytic activity.

2. **Protein Structure Levels**: Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. Each level of structure contributes to the overall shape and function of the protein.

3. **Primary Structure**: This is the linear sequence of amino acids in a polypeptide chain. While it is essential for determining the protein's characteristics, it does not form the active site.

4. **Secondary Structure**: This refers to the local folding of the polypeptide chain into structures such as alpha-helices and beta-sheets. Although important for overall structure, the secondary structure alone does not create the active site.

5. **Tertiary Structure**: This is the three-dimensional shape of a protein that results from the interactions between the side chains (R groups) of the amino acids. The specific arrangement of these side chains creates the active site where substrate binding occurs.

6. **Conclusion**: The active sites of enzymes are formed by the tertiary structure of proteins. Thus, the correct answer to the question is that active sites are present on the tertiary structure of proteins.

### Final Answer:
The active sites of enzymes are present on the **tertiary structure of proteins**.

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