In: Chemistry
Biefly describe the roles of three amino acids, Asp, His, and Ser in the Chymotrypsin-catalyzed reaction.
Roles of three amino acids, Asp, His, and Ser in the Chymotrypsin-catalyzed reaction.
Chymotrypsin, a protease, is an enzyme that cleaves the carbonyl side of certain peptide bonds primarily by covalent catalysis.
In this mechanism, a nucleophile becomes covalently attached to a substrate in a transition state with an acyl-enzyme. The protease cleaves proteins by a hydrolysis reaction, an addition of a water molecule.
The double bond between the carbon and nitrogen strengthens its bond
In chymotrypsin, the carboxylate R-group of Asp102 forms a hydrogen bond with R group of His 57.
When this happens, it compresses this hydrogen bond and shifts electron density to the other nitrogen atom (not involved in the H-bond) in the R-goup of His57 becomes a very strong base
. This allows His 57 to deprotonate Ser195 and turn it into a strong nucleophile that can attack the substrate
Oxygen develops a partially negative charge in the oxyanion hole.
Instability of the negative charge on the substrate carbonyl oxygen when will leads to collapse of the tetrahedral intermediate, re-formation of a double bond with carbon which breaks the peptide bond between the carbon and amino acid group.
The amino leaving group is protonated by His57, facilitating its displacement.
Once the oxyanion hole stabilizes the negative charge, the bond breaks because the proton from Histidine is binding to nitrogen to make it less likely to carbon.
The leaving group is stabilized and the acyl-enzyme is formed.