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 69

Chapter 22, Problem 69

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

Verified step by step guidance

Step 1: Understand the role of carboxypeptidase A. This enzyme cleaves the C-terminal amino acid of a polypeptide unless the terminal residue is Pro, Arg, or Lys. Since the reaction releases Met, this indicates that Met is the C-terminal residue of the polypeptide.

Step 2: Analyze the peptide fragments provided. These fragments are the result of partial hydrolysis, meaning the polypeptide was broken into smaller pieces. The goal is to reconstruct the original sequence by identifying overlaps between the fragments.

Step 3: Start with the fragment containing Met (Met, Ala, Gly). Since Met is the C-terminal residue, this fragment must represent the end of the polypeptide. Look for another fragment that overlaps with this sequence, such as Ala, Gly, His.

Step 4: Continue building the sequence by identifying overlaps between fragments. For example, if Gly, His, Ala overlaps with another fragment like Glu, His, then you can connect these fragments to extend the sequence.

Step 5: Verify the final sequence by ensuring all fragments are accounted for and that the sequence aligns with the rules of peptide bond formation. Additionally, confirm that the C-terminal residue is Met, as determined in Step 1.

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Key Concepts

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

Polypeptide Structure

A polypeptide is a chain of amino acids linked by peptide bonds, forming the primary structure of proteins. The sequence of amino acids determines the polypeptide's properties and functions. Understanding the specific order of amino acids is crucial for predicting how the polypeptide will interact with enzymes like carboxypeptidase A.

Carboxypeptidase A Function

Carboxypeptidase A is an enzyme that catalyzes the hydrolysis of peptide bonds at the carboxyl-terminal end of polypeptides. It releases free amino acids, which can help identify the sequence of the original polypeptide. Knowing how this enzyme operates is essential for interpreting the results of the polypeptide's partial hydrolysis.

Peptide Hydrolysis

Peptide hydrolysis is the process by which peptide bonds are broken down, resulting in the formation of smaller peptides or free amino acids. This reaction is typically facilitated by enzymes and is important for protein digestion and metabolism. Understanding the products of hydrolysis helps in deducing the original polypeptide sequence from the resulting fragments.

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Peptide Sequencing: Partial Hydrolysis Concept 1

Related Practice

Textbook Question

Write the mechanism for the reaction of an amino acid with di-tert-butyl dicarbonate.

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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.

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

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Textbook Question

Glycine has pK_a values of 2.3 and 9.6. Do you expect the pK_a values of glycylglycine to be higher or lower than these values?

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Textbook Question

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?

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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

Dithiothreitol reacts with disulfide bridges in the same way that 2-mercaptoethanol does. With dithiothreitol, however, the equilibrium lies much more to the right. Explain.

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