Respuesta :
Chemostrypsin is a protease- an important enzyme that play an important role in breaking down the proteins we eat, so that they get absorbed in our body. It is present in a part of small intestine- the duodenum.
Chemotrypsin specifically cuts cleaves the bonds of aromatic amino acids from the C-terminal of proteins. These amino acids are:
- Phenylalanine bonds
- Tyrosine bonds
- Tryptophan bonds
Mechanism:
When any aromatic aminoacid comes in the active site of Chemostrypsin, then the reaction of cleavage of peptide bond starts.
In the active site there are three amino acids which participate in the reaction: aspartic acid, histidine, and serine.
Step 1:
Substrate (a polypeptide) binds with aromatic group in active site and sets up peptide bond in such a position that can be cleaved.
Step 2:
In chymotrypsin, the carboxylate R-group of Aspartate forms a hydrogen bond with R group of Histidine. The histidine de-protonates Serine, which then become a strong nucleophile ready to attack substrate.
Oxygen creates a partial negative charge on oxyanion hole present in tetrahedral intermediate structure.
Step 3:
The negative charge on carbonyl group is instable and it collapses tetrahedral intermediate, reforms a double bond with carbon that breaks the peptide bond between the amino acid group and carbon. The amino leaving group is protonated by Histidine.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. This leads to the formation of acyl-enzyme when the leaving group is stabilized.
Step 4:
During the next step, amino part is removed (from enzyme) and is bonded to the serine part. This leads to the completion of enzyme acylation and formation of first product.
Step 5:
In the next step, water molecule is combined with N terminal. Then, Histidine removes the proton from water molecule and leads to the synthesis of hydroxyl ion which is attached wih carbon. This destabilizes the acyl intermediate and breaks the bond.
Step 6:
There is again a deprotonation of water molecule made through acid-base catalysis. This generates a strong ion-nucleophillic hydroxide . In next step, hydroxide ion attacks the ester linkage of acylenzyme and forms tetrahedral intermediate structure.
Step 7:
When the tetrahedral structure is collapsed, the proton moves from Histidine to the serine.
Step 8:
New substrate is ready to attach in active site.
Answer:
The steps are the following:
-His 57 catalyses removal of H from Ser 195 hydroxyl.
-Ser 195's nucleophilic O attacks carbonyl C of substrate.
-His 57 donates H to N of sissile peptide bond, tetrahedral intermediate decomposes.
-The portion (the C-terminal end) of original substrate with the new amino terminus diffuses away.
-Water donates H to His 57
-Resulting OH attacks carbonyl of ramining substrate
-His 57 donates H to Ser 195O, leading to collapse of tetrahedral intermediate
-The portion (N-terminal end) of original substrate with the new carboxylate terminus diffuses away .
Explanation:
It can be said that the primary structure of the compound called chymotrypsin is formed by disulfide bonds, in addition to the active site composed of Ser-195, His-57 and Asp-102. Ser-195 is linked to His-57 via a hydrogen bond and to Asp-102 via a hydrogen bond. The function of His-57 is to polarize the hydroxyl group, while Asp-102 targets His-57 and stabilizes it through hydrogen and electrostatic bonds.
