Question

Chymotrypsin is a member of the serine protease family of enzymes.

a) Describe the role of the active site serine residue in catalysis by serine proteases.

b) Predict the effect of mutating the serine at the active site of chymotrypsin. be very specific!

Answer 1

Answer:

a) : The active site of serine proteases contain a catalytic triad, a coordinated structure consisting of three amino acids Histidine, Serine, and Aspartic acid.

• In the primary structure of the protein, these three amioa cids ae located far away from oone another.
• However, in the 3-D conformation, they ae brought close together to form the catalytic site.
• Serine acts as a nucleophile, which means it can donate electron pairs. The nucleophilic property is due to the -OH group of serine.
• The nucleophilic group of serine attacks the carbonyl carbon of the peptide bond it cleaves, and donates electrons to carbonyl oxygen.
• which is an electrophile. This initial attack forms a tetrahedral intermediate.
• The tetrahedral intermediate is an acyl-enzyme intermediate. Another nucleophile (generally water acts as a second nucleophile) attacks the acyl-enzyme intermediate that results in hydrolysis.
• Depending on the second nucleophile present in the viscinity of serine proteases, either hydrolysis of the substrate occurs or it results in a group transfer.
• Group transfer occurs if the second nucleophile is an organic molecule.
• Histidine increases the reactivity of serine by polarizing it and facilitate its deprotonation.

b) :

• Aspartic acid is required to stabilize the positive charge on Histidine during the catalysis, and restricts its side chain rotation.
• The acid is not essential in the triad, and replacement of aspartic acid with asparagine does not alter the enzyme properties.
• It is required only for proper orientation of the base (histidine).

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