Question

1. in chymotrypsin, a mutant was constructed with ser 189, which is in the bottom of the substrate - specificity pocket, changed to asp. what effect would you predict for this mutation? √4

Explanation:

Chymotrypsin's Ser 189 in its substrate pocket interacts with the positive charge of the aromatic/ large hydrophobic side chain of its target amino acids (like phenylalanine, tryptophan) in substrates. Mutating Ser (neutral, polar) to Asp (negatively charged, acidic) introduces a negative charge in the pocket. This will repel negatively charged amino acid side chains and alter the affinity for the usual positively/ neutrally charged large hydrophobic substrates, shifting the enzyme's substrate specificity to favor positively charged amino acid residues (e.g., lysine, arginine) instead of the typical aromatic ones.

Answer:

The mutation would alter chymotrypsin's substrate specificity: the negatively charged Asp 189 will repel negatively charged substrate side chains and favor binding of substrates with positively charged amino acid residues (such as lysine or arginine), instead of the aromatic/large hydrophobic residues that wild-type chymotrypsin targets.