Ch. 21 - Amino Acids, Peptides, and Proteins

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 21 - Amino Acids, Peptides, and Proteins

Problem 65a,b

Chapter 22, Problem 65a,b

After the polypeptide shown below was treated with maleic anhydride, it was hydrolyzed by trypsin. (After a polypeptide is treated with maleic anhy- dride, trypsin will cleave the polypeptide only on the C-side of arginine.)
Gly-Ala-Asp-Ala-Leu-Pro-Gly-Ile-Leu-Val-Arg-Asp-Val-Gly-Lys-Val-Glu-Val-Phe-Glu-Ala-Gly- Arg-Ala-Glu-Phe-Lys-Glu-Pro-Arg-Leu-Val-Met-Lys-Val-Glu-Gly-Arg-Pro-Val-Gly-Ala-Gly-Leu-Trp
a. After a polypeptide is treated with maleic anhydride, why does trypsin no longer cleave it on the C-side of lysine?
b. How many fragments are obtained from the polypeptide?

Verified step by step guidance

Step 1: Understand the role of maleic anhydride in the reaction. Maleic anhydride reacts with primary amines, such as the ε-amino group of lysine residues, forming a succinimide derivative. This modification blocks the ε-amino group of lysine, preventing trypsin from recognizing and cleaving at the C-side of lysine residues.

Step 2: Recall the specificity of trypsin. Trypsin is a proteolytic enzyme that cleaves peptide bonds specifically at the C-side of arginine and lysine residues. However, after treatment with maleic anhydride, lysine residues are no longer accessible for cleavage due to the chemical modification.

Step 3: Analyze the polypeptide sequence provided. Identify all the arginine and lysine residues in the sequence. Note that trypsin will only cleave at the C-side of arginine residues after maleic anhydride treatment.

Step 4: Count the number of arginine residues in the sequence. Each arginine residue represents a cleavage site for trypsin, resulting in fragments. Exclude lysine residues from cleavage consideration due to maleic anhydride treatment.

Step 5: Determine the number of fragments obtained. The number of fragments is equal to the number of cleavage sites (arginine residues) plus one, as cleavage at each site divides the polypeptide into separate fragments.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Trypsin Cleavage Specificity

Trypsin is a serine protease that specifically cleaves peptide bonds on the C-side of lysine (Lys) and arginine (Arg) residues. Understanding this specificity is crucial for predicting how trypsin will act on a polypeptide after it has been modified by other reagents, such as maleic anhydride, which can alter the reactivity of these amino acids.

Maleic Anhydride Modification

Maleic anhydride is a reagent that can react with amino acids, particularly those containing nucleophilic side chains like lysine and arginine. This modification can prevent trypsin from recognizing these residues for cleavage, thus affecting the overall fragmentation pattern of the polypeptide after treatment.

Polypeptide Hydrolysis and Fragmentation

Hydrolysis of a polypeptide involves breaking peptide bonds, resulting in smaller peptide fragments. The number of fragments produced after hydrolysis depends on the specific cleavage sites available, which can be influenced by prior modifications like those from maleic anhydride, thereby altering the expected fragmentation pattern.

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Related Practice

Textbook Question

α-Amino acids can be prepared by treating an aldehyde with ammonia/trace acid, followed by hydrogen cyanide, followed by acid-catalyzed hydrolysis.

a. Draw the structures of the two intermediates formed in this reaction.

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Textbook Question

α-Amino acids can be prepared by treating an aldehyde with ammonia/trace acid, followed by hydrogen cyanide, followed by acid-catalyzed hydrolysis.

b. What amino acid is formed when the aldehyde that is used is 3-methylbutanal?

c. What aldehyde is needed to prepare isoleucine?

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Textbook Question

Identify the location and type of charge on the hexapeptide Lys-Ser-Asp-Cys-His-Tyr at each of the following pH values:

c. pH=7

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Textbook Question

Reaction of a polypeptide with carboxypeptidase A releases Met. The polypeptide undergoes partial hydrolysis to give the following peptides. What is the sequence of the polypeptide?

1. Ser, Lys, Trp

2. Gly, His, Ala

3. Glu, Val, Ser

4. Leu, Glu, Ser

5. Met, Ala, Gly

6. Ser, Lys, Val

7. Glu, His

8. Leu, Lys, Trp

9. Lys, Ser

10. Glu, His, Val

11. Trp, Leu, Glu

12. Ala, Met