Chymotrypsin has three residues in the active His-57, Asp-102, and Ser 195. To evaluate the importance of Ser-195 for the enzymatic activity, this amino acid residue is replaced by another amino acid. Predict what changes you expect in the activity of the enzyme in the following three different mutant enzymes. In every case, explain your answer (5 p each).

The question given is incomplete, below is a complete questiongotten from google:

Chymotrypsin has three residues in the active site: His-57, Asp-102, and Ser 195. To evaluate the importance of Ser-195 for the enzymatic activity, this amino acid residue is replaced by another amino acid. Predict what changes you expect in the activity of the enzyme in the following three different mutant enzymes. In every case, explain your answer (5 p each).

A) Ser-195 to Thr

B) Ser-195 to Cys

C) Ser-195 to Ala

Answer:

Explanation:

A serine protease that breaks the peptide bond in the protein at the carboxy terminal of aromatic amino acids tyrosine, tryptophan and phenylalanine is known as Chymotrypsin.

Three amino acids form a catalytic triad: aspartate, histidine and serine.

Histidine acts as a general acid base catalyst, and it is stabilized by Aspartate, while serine provides the nucleophile in form of oxygen that attacks on the carbonyl carbon of the peptide bond to form a tetrahedral intermediate.

Since serine has a hydroxyl (-OH) group and the bond between oxygen and hydrogen is removed by histidine which acts a general base catalyst and accepts the proton. This makes the oxygen in the hydroxyl group nucleophilic and it binds with carbonyl carbon of the peptide bond. This function can be replaced by any other amino acid which can produce a nucleophile.

A) Threonine has a hydroxyl group. Just like serine, the bond between oxygen and hydrogen is broken due to the acceptance of protons by histidine. Here, also oxygen in the hydroxyl group become negatively charged species, hence a nucleophile that can attack on the carbonyl carbon of the peptide bond.

B) Cysteine has thiol (-SH) group. Here, similar to serine and threonine, when histidine, acting as a general base, will accept the protons, the Sulphur (S) in cysteine will become negatively charged and act as a nucleophile. It can then attack the carbonyl carbon of the peptide bond to be cleaved.

Similar to serine, threonine and cysteine also have polar side chain so they can produce nucleophile and can replace the serine residue in the active site.

C) Alanine has a non polar side chain containing a methyl group. This methyl group being non polar is not able to produce a nucleophile specie when histidine acting as a general base catalyst tries to extract proton from methyl. No nucleophile is produced and it fails to attack the carbonyl carbon of the peptide bond, therefore no peptide bond is broken.