Figure 19.4 Hydrolysis of a peptide bond by chymotrypsin. (a) The polypeptide en

Question

Figure 19.4 Hydrolysis of a peptide bond by chymotrypsin.

(a) The polypeptide enters the enzyme active site with its hydrophobic side chain (the aromatic ring) in the hydrophobic pocket and the peptide bond to be broken (red) opposite serine and histidine residues. (b) H transfer from serine to histidine allows formation of a strained intermediate in which the serine side chain bonds to the peptide bond carbon (green). (c) The peptide bond is broken and the segment with the new terminal NH2 group leaves the active site. (d) In subsequent steps, a water molecule enters the active site; its H atom restores the serine side chain and its OH bonds to the other piece of the substrate protein to give a new terminal COOH group so that this piece can also leave the active site.

The ability to change a selected amino acid residue to another amino acid is referred to as “point mutation” by biochemists. Referring to the reaction for peptide bond hydrolysis in Figure 19.4, speculate on the effects that the following point mutations might have on the chymotrypsin mechanism shown in Figure 19.4: serine to valine; aspartate to glutamate.

Explanation / Answer

Chymotrypsin:

it is a serine protease forms with 245 amino acids and its molecular weight is 27KD.

Hydrolysis of peptide bond:

Chymotrypsin cleaves the peptide bond on the carboxyl side of a large hydrophobic amino acid (Phe, Tyr, Trp, Met) via hydrolysis.

The reaction mechanism of chymotrypsin is a nucleophilic substitution. In which RNH group replaces by OH turning amide into acid and amine

Point mutation: It is a changes in a single nucleotide pair of a gene. Generally a point mutations occur at a single point in the DNA sequence.

Point mutations are classified into 3 types, they are substitutions, insertions, and deletion.

Generally chymotrypsin cleaves the peptide bond on hydrophobic amino acid but due to the point mutation peptide bond Hydrolysis ability can changes to a selected amino acid residue to another amino acid.

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