Chymotrypsin: Videos & Practice Problems

Chymotrypsin, a key digestive enzyme, catalyzes protein breakdown by cleaving peptide bonds, particularly at aromatic amino acids like phenylalanine, tyrosine, and tryptophan. Its active site features a catalytic triad of aspartate 102, histidine 57, and serine 195, essential for enhancing serine's nucleophilicity through hydrogen bonding. This mechanism stabilizes the transition state, facilitating efficient catalysis. Understanding chymotrypsin's function is crucial for grasping broader enzymatic processes and kinetics, including Michaelis-Menten dynamics.

Chymotrypsin

Protein breakdown, also known as proteolysis, is essential for various biological processes. It allows for the recycling of amino acids from proteins and enzymes that are no longer needed within cells, facilitating the synthesis of new proteins and enzymes. Additionally, dietary proteins must be broken down into smaller peptides and amino acids to be absorbed by the intestines and utilized by the body.

Peptidases are a specific class of enzymes that catalyze the breakdown of proteins, and chymotrypsin is a notable example of a peptidase. As a digestive hydrolyase, chymotrypsin preferentially cleaves peptide bonds at the carboxyl end of aromatic amino acids, specifically phenylalanine, tyrosine, and tryptophan, with a lesser preference for leucine and methionine. This specificity is crucial for its function in protein digestion.

Chymotrypsin is secreted by the pancreas in an inactive form known as chymotrypsinogen, which is a zymogen. Upon the ingestion of food, chymotrypsinogen is activated through proteolytic cleavage, allowing chymotrypsin to begin its role in degrading dietary proteins. For instance, when chymotrypsin acts on a peptide containing phenylalanine and tyrosine, it cleaves the peptide bonds at the C-terminal end of these aromatic residues, resulting in multiple peptide fragments. This process exemplifies the enzyme's activity and its importance in digestion.

Understanding the mechanism of chymotrypsin and its specificity for aromatic amino acids is vital for comprehending how proteins are processed in the digestive system, highlighting the intricate relationship between enzyme function and nutrient absorption.

Chymotrypsin

Chymotrypsin is a well-studied enzyme that exemplifies Michaelis-Menten kinetics and employs multiple catalytic mechanisms. Its active site plays a crucial role in stabilizing the transition state through both covalent and general acid-base catalysis. A key feature of chymotrypsin's active site is the catalytic triad, which consists of three essential amino acid residues: aspartate 102, histidine 57, and serine 195. These residues are vital for the enzyme's catalytic function; the removal of any one of them results in a loss of catalytic activity.

The numbers associated with the amino acids indicate their specific positions in chymotrypsin's primary sequence, but it is more important to remember the amino acids themselves. The active site structure, represented visually, highlights the arrangement of these residues. Histidine 57 is central to the catalytic process, forming hydrogen bonds with both serine 195 and aspartate 102. This interaction enhances serine 195's nucleophilicity, making it a stronger nucleophile.

Specifically, histidine 57 acts as a base, accepting a hydrogen atom from serine 195, which results in serine acquiring a negative charge. This transformation is crucial for the nucleophilic attack that occurs during catalysis. Aspartate 102 is essential for positioning histidine 57 appropriately, allowing it to effectively acquire the hydrogen. Without aspartate 102, histidine's pKa would not be suitable for this role, thereby inhibiting the activation of serine 195.

In summary, the interactions among aspartate 102, histidine 57, and serine 195 are fundamental to enhancing the nucleophilic ability of serine 195, setting the stage for chymotrypsin's catalytic mechanism. Understanding these relationships is vital for grasping the enzyme's function and will be further explored in subsequent discussions.

Which amino acids in chymotrypsin's active site are critical participants in the cleavage of the substrate?

• A. His, Ser, Asn.
• B. His, Ser, Asp.
• C. His, Ser.
• D. Asp, Lys.

Answer: (B)

The following polypeptide N-G-I-F-D-A-Y-G-N-T-W-R-A-P-C-F-V-A is cleaved by chymotrypsin to produce multiple peptide fragments. Based on the specificity of chymotrypsin cleavage, how many peptide fragments are produced?

• A. 2 peptide fragments.
• B. 3 peptides fragments.
• C. 4 peptide fragments.
• D. 5 peptide fragments.
• E. 6 peptide fragments.
• F. No cleavage event occurs

Answer: (D)

The activity of chymotrypsin drastically changes as the pH fluctuates in the relatively small range of pH 5-9. What amino acid residue in chymotrypsin's active site is most likely to be responsible for this effect of pH?

• A. Asp-102.
• B. His-157.
• C. Ser-195.
• D. Tyr-23.
• E. Cys-198.
• F. Asn-122.

Answer: (B)

Here’s what students ask on this topic:

What is the function of chymotrypsin in the digestive system?

Chymotrypsin is a digestive enzyme that plays a crucial role in protein breakdown within the digestive system. It is secreted by the pancreas in response to eating a meal, initially in its inactive form, chymotrypsinogen. Once activated to chymotrypsin, it cleaves peptide bonds specifically at the C-terminal end of aromatic amino acids such as phenylalanine, tyrosine, and tryptophan. This enzymatic activity helps degrade dietary proteins into smaller peptides and amino acids, which can then be absorbed by the intestines and utilized by the body's cells for various functions, including the synthesis of new proteins and enzymes.

What is the catalytic triad in chymotrypsin and why is it important?

The catalytic triad in chymotrypsin consists of three amino acid residues: aspartate 102, histidine 57, and serine 195. These residues are essential for the enzyme's catalytic activity. Histidine 57 forms hydrogen bonds with both aspartate 102 and serine 195, positioning serine 195 as a stronger nucleophile. Aspartate 102 helps position histidine 57 to act as a better base, facilitating the transfer of a hydrogen ion from serine 195 to histidine 57. This interaction enhances the nucleophilicity of serine 195, enabling it to effectively participate in the cleavage of peptide bonds. Without any one of these residues, chymotrypsin's catalytic function would be impaired.

How does chymotrypsin exhibit specificity in its enzymatic activity?

Chymotrypsin exhibits specificity in its enzymatic activity by preferentially cleaving peptide bonds at the C-terminal end of aromatic amino acids such as phenylalanine, tyrosine, and tryptophan. This specificity is due to the enzyme's active site, which is structured to recognize and bind these aromatic residues effectively. The enzyme also has a lower preference for cleaving bonds adjacent to leucine and methionine. This selective cleavage ensures that chymotrypsin efficiently breaks down proteins into smaller peptides and amino acids, facilitating their absorption and utilization by the body.

What role does the pancreas play in the activation of chymotrypsin?

The pancreas plays a crucial role in the activation of chymotrypsin. It secretes chymotrypsin in its inactive form, known as chymotrypsinogen. This zymogen is then activated in the small intestine through proteolytic cleavage, typically by the enzyme trypsin. The activation process involves the removal of specific peptide segments from chymotrypsinogen, converting it into its active form, chymotrypsin. Once activated, chymotrypsin can then participate in the breakdown of dietary proteins into smaller peptides and amino acids, aiding in digestion and nutrient absorption.

How does chymotrypsin stabilize the transition state during catalysis?

Chymotrypsin stabilizes the transition state during catalysis through a combination of covalent and general acid-base catalysis. The enzyme's active site contains the catalytic triad of aspartate 102, histidine 57, and serine 195. Histidine 57 acts as a base, accepting a proton from serine 195, which enhances serine's nucleophilicity. Aspartate 102 helps position histidine 57 to be a more effective base. This arrangement allows serine 195 to form a covalent bond with the substrate, stabilizing the transition state and lowering the activation energy required for the reaction. This stabilization is crucial for efficient catalysis and peptide bond cleavage.