Question

Explain why the mutation of Asp-52 to Asn-52 decreases enzyme activity of lysozymes by 10,000 fold.

Answer 1

The active site of lysozyme has Asp52 and Glu35 as the catalytic residues. During catalysis, glutamic acid residue transfers a proton to the substrate to form an oxonium ion intermediate. Simultaneously with the formation of intermediate, the negatively charged aspartate residue stabilizes the positively charged oxonium ion intermediate. Moreover, stabilization of ES complex also reduces the activation energy of the reaction and forms transition state at much lower energy level. Catalysis proceeds further with the stabilized intermediate (ES complex).

Mutation leading to Asp52 replaced by Asn52 does not contribute much to stabilize the positively charged oxonium ion intermediate. A less stable transition state ES complex it at higher energy level and requires greater activation energy to process catalysis. Thus, formation of less stable intermediate in presence of Asn52 drastically reduces the rate of enzyme catalysis.