Answer:
The main substrate of chymotrypsin includes tryptophan, tyrosine, phenylalanine and methionine
Explanation:
1.
Histidine yields a proton to aspartate and recovers it from serine.
Seen in another way: aspartate captures a proton from the serine through histidine.
2.
a) The serine (deprotonated) is thus capable of attacking the peptide bond (nucleophilic carbonyl attack) and forms a tetrahedral intermediate; The substrate is thus covalently bound to the enzyme (now it is a transition state).
b) The peptide bond is broken and the released amino terminus (R) recovers a proton from histidine.
c) Histidine, in turn, recovers it from aspartic.
3.
a) Aspartate captures a proton of histidine again, so that it can capture it in turn from water.
b) This generates a hydroxide anion that attacks the ester intermediate between the serine and the carboxyl part (R ′) of the substrate peptide.
c) A new tetrahedral intermediate bound to the enzyme is formed (via serine residue).
4.
a) The carboxyl group of the peptide is regenerated, the serine being separated and the other peptide fragment being free (the R ′ part with a free carboxyl end)
b) The serine recovers the proton at the expense of histidine, which in turn captures it from aspartic acid.
c) The catalytic triad (Asp, His, Ser) has been regenerated in its original state.
The net reaction is:
R–NH – CO –R ′ + H2O ⟶ R– NH2 + HOOC –R ′ ⟶ R – NH3 + + −OOC –R ′
The active site or catalytic center of chymotrypsin is formed by several amino acid residues, among which the essential role corresponds to the "catalytic triad".
