Which of the following reactions is not enzyme-mediated in biological system ?

Options

A. Dissolving `CO_(2)` in water
B. Unwinding the two strands of DNA
C. Hydrolysis of sucrose
D. Formation of peptide bond

Explanation

To determine which of the following reactions is not enzyme-mediated in a biological system, we will analyze each option provided.

### Step-by-Step Solution:

1. **Identify the first reaction: Dissolving of carbon dioxide in water.**
- In this reaction, carbon dioxide (CO2) dissolves in water to form carbonic acid (H2CO3). This reaction is facilitated by the enzyme carbonic anhydrase.
- **Conclusion:** This reaction is enzyme-mediated.

2. **Identify the second reaction: Unwinding the two strands of DNA.**
- During DNA replication, the enzyme DNA helicase unwinds the two strands of DNA. This is crucial for the replication process.
- **Conclusion:** This reaction is also enzyme-mediated.

3. **Identify the third reaction: Hydrolysis of sucrose.**
- Sucrose is hydrolyzed into glucose and fructose in the presence of the enzyme invertase. This enzymatic reaction is essential for breaking down sucrose.
- **Conclusion:** This reaction is enzyme-mediated as well.

4. **Identify the fourth reaction: Formation of peptide bond.**
- The formation of a peptide bond occurs when two amino acids combine, resulting in the release of water (a condensation reaction). This process does not require an enzyme.
- **Conclusion:** This reaction is not enzyme-mediated.

### Final Answer:
The reaction that is not enzyme-mediated in a biological system is the **formation of peptide bond** (Option D).

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