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

(a) CO2 gets dissolved in water, a reaction which is not always catalysed by any enzyme.
Unwinding and winding of the two strands of DNA is catalysed by the enzyme topoisomerase.
Hydrolysis of sucrose is regulated by sucrase.
Peptide bonds are formed by the action of enzyme peptidyl transferase.

Page 155, XI NCERT