The naturally occurring amino acid that contains only one basic functional group in its chemical structure is

Options

A. arginine
B. histidine
C. lysine
D. asparagine

Answer

To solve the question, we need to identify which of the given amino acids contains only one basic functional group in its structure. The options provided are arginine, histidine, lysine, and isoleucine.

### Step-by-Step Solution:

1. **Understand the Structure of Amino Acids**:
- Amino acids generally have a basic amino group (-NH2), an acidic carboxyl group (-COOH), and a variable side chain (R group).
- The basic functional group is typically the amino group.

2. **Analyze Each Option**:
- **Arginine**: Contains a guanidinium group, which has three nitrogen atoms that can act as basic sites. Therefore, it has more than one basic functional group.
- **Histidine**: Contains an imidazole side chain, which has two nitrogen atoms that can also act as basic sites. Thus, it has more than one basic functional group.
- **Lysine**: Contains an additional amino group in its side chain (-NH2), which means it has two basic functional groups.
- **Isoleucine**: This amino acid has a simple alkyl side chain and contains only one amino group (-NH2) and one carboxyl group (-COOH). Thus, it has only one basic functional group.

3. **Conclusion**:
- Among the options, isoleucine is the only amino acid that contains only one basic functional group. Therefore, the correct answer is **Isoleucine**.

### Final Answer:
The naturally occurring amino acid that contains only one basic functional group in its chemical structure is **Isoleucine**.