The active site of an enzyme is situated in its amino acids :

Options

A. amino groups
B. carboxyl group
C. R-groups
D. all of these

Explanation

To solve the question about the location of the active site of an enzyme in relation to its amino acids, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Active Site**: The active site of an enzyme is the specific region where substrate molecules bind and undergo a chemical reaction. This site is crucial for the enzyme's function.

2. **Identify the Components of Amino Acids**: Amino acids, the building blocks of proteins, consist of an amino group, a carboxyl group, a hydrogen atom, and a variable side chain known as the R group.

3. **Evaluate Each Option**:
- **Option 1: Amino Groups**: While amino groups are part of the structure of amino acids, they primarily contribute to the overall shape and specificity of the enzyme rather than being the active site itself.
- **Option 2: Carboxyl Groups**: Carboxyl groups can play a role in catalysis but are not the primary site where substrate binding occurs.
- **Option 3: R Groups**: The R groups (side chains) of amino acids are unique to each amino acid and are responsible for the specific interactions with the substrate. The active site is formed by the arrangement of these R groups, which facilitate the binding and catalysis of the substrate.
- **Option 4: All of These**: This option suggests that all components contribute to the active site, but we have established that the R groups are the primary contributors.

4. **Conclusion**: Based on the evaluation, the correct answer is **Option 3: R groups**. The active site of an enzyme is primarily situated in the R groups of its amino acids.

### Final Answer:
The active site of an enzyme is situated in its amino acids: **R groups (Option 3)**.