Question

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 Ser 189 --> Asp 189 mutation? What sort of substrate specificity would you expect to see in the mutated version of the protein?

Answer 1

A mutant version of chymotrypsin was constructed by generating a point mutation to change Ser 189 to Asp 189. Chymotrypsin is a digestive enzyme that acts in the duodenum and aids the breakdown of proteins and polypeptides. Serine is a polar amino acid and is known to play an important role in preforming catalytic functions of various enzymes. Furthermore, serine is considered to be electronegative and contains a hydroxyl group.
Asparagine is one of the most common natural AA. Asparagine is also considered to be a polar amino acid, but contains a terminal carboxamide. The mutation of Ser 189 to Asp 189 alters the active site of chymotrypsin to mimic that of the active site of trypsin. Chymotrypsin with Ser 189 is less specific and can accommodate bulky aromatic R groups. However, when mutated to Asp 189, the active site of mutant chymotrypsin will resemble that of trypsin and will allow the binding of positively charged lysine or arginine.