Enzymes are biocatalysts. They catalyze biochemical reactions. In general, they reduce the activation energy of reactions. Many physicochemical processes are enzyme-mediated. Which of the following reactions is not enzyme-mediated in the biological system?

1.  Dissolving CO₂ in water

2.  Untwining the two strands of DNA

3.  Hydrolysis of sucrose

4.  Formation of peptide bond