Covalent catalysis by serine proteases (e.g., trypsin, chymotrypsin) involves a catalytic triad. Which amino acid acts as a nucleophile, directly attacking the peptide bond of the substrate?
- A Serine ✓
- B Histidine
- C Aspartate
- D Cysteine
Correct answer: A. Serine
Explanation
In serine proteases, the catalytic triad consists of Ser, His, and Asp. The aspartate positions the histidine, which acts as a general base to abstract the proton from the serine hydroxyl, dramatically increasing the nucleophilicity of the serine oxygen. The activated serine oxygen directly attacks the carbonyl carbon of the target peptide bond, forming a covalent tetrahedral intermediate and then an acyl-enzyme intermediate, which is subsequently hydrolyzed to release the product and regenerate the free enzyme.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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