Question

Mention the catalytic triad (along with diagram) which contributes to the enzymatic activity of native subtilisin. Why was this enzyme improved by protein engineering? Write the method used in this process.

Hint:

Site-directed mutagenesis allows scientists to make single, targeted amino acid changes in a protein, which has revolutionized the design of enzymes for industrial and consumer products.

Answer 1

Step 1: The Catalytic Triad
Subtilisin is an alkaline serine protease isolated from Bacillus subtilis. Its catalytic activity relies on a conserved charge-relay system called the catalytic triad, which consists of three amino acid residues: Aspartate-32 (Asp32), Histidine-64 (His64), and Serine-221 (Ser221).

Step 2: Structural Arrangement representation
The physical layout of the triad allows His64 to act as a general base that abstracts a proton from Ser221, activating its oxygen atom to perform a nucleophilic attack on the carbonyl carbon of the substrate's peptide bond.

Step 3: Why the Enzyme was Improved
Subtilisin is widely used as a bio-additive in laundry detergents to break down protein-based stains. However, wild-type subtilisin is highly sensitive to chemical oxidation because it contains a methionine residue at position 222 (Met222) adjacent to the catalytic Ser221. - Bleaches and oxidizing agents in laundry detergents oxidize the sulfur atom in Met222 to methionine sulfoxide.
- This oxidized residue sterically blocks the adjacent active site, reducing the enzyme's catalytic activity by over $90%$.


Step 4: Method Used for Engineering
Scientists used Site-Directed Mutagenesis (SDM) to replace Met222 with other amino acids that are stable to oxidation, such as Alanine (Ala222) or Glutamine (Gln222). The engineered subtilisin mutant (Met222Ala) retained high catalytic activity and remained stable in bleach-containing detergents. Final Answer: The catalytic triad of subtilisin consists of Asp32, His64, and Ser221. The enzyme was improved because chemical bleaches oxidized the adjacent Met222, inactivating the active site. Site-Directed Mutagenesis was used to replace Met222 with oxidation-resistant residues like Alanine or Glutamine.