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 41

Chapter 22, Problem 41

Indicate the peptides produced from cleavage by the indicated reagent:
a. His-Lys-Leu-Val-Glu-Pro-Arg-Ala-Gly-Ala by trypsin
b. Leu-Gly-Ser-Met-Phe-Pro-Tyr-Gly-Val by chymotrypsin

Verified step by step guidance

Step 1: Understand the cleavage specificity of the reagents. Trypsin cleaves peptide bonds on the carboxyl side of lysine (Lys, K) and arginine (Arg, R), except when followed by proline (Pro, P). Chymotrypsin cleaves peptide bonds on the carboxyl side of aromatic amino acids: phenylalanine (Phe, F), tyrosine (Tyr, Y), and tryptophan (Trp, W), except when followed by proline (Pro, P).

Step 2: For part (a), identify the positions of lysine (Lys, K) and arginine (Arg, R) in the sequence His-Lys-Leu-Val-Glu-Pro-Arg-Ala-Gly-Ala. These are the cleavage sites for trypsin, provided they are not followed by proline (Pro, P).

Step 3: For part (a), split the peptide at the identified cleavage sites. Ensure that the cleavage occurs on the carboxyl side of the lysine (Lys, K) and arginine (Arg, R) residues, and list the resulting peptide fragments.

Step 4: For part (b), identify the positions of phenylalanine (Phe, F), tyrosine (Tyr, Y), and tryptophan (Trp, W) in the sequence Leu-Gly-Ser-Met-Phe-Pro-Tyr-Gly-Val. These are the cleavage sites for chymotrypsin, provided they are not followed by proline (Pro, P).

Step 5: For part (b), split the peptide at the identified cleavage sites. Ensure that the cleavage occurs on the carboxyl side of the aromatic residues, and list the resulting peptide fragments.

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Key Concepts

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

Proteolytic Enzymes

Proteolytic enzymes, or proteases, are enzymes that catalyze the breakdown of proteins into smaller peptides or amino acids by cleaving the peptide bonds. Different proteases have specific cleavage sites based on the amino acid sequence of the substrate. Understanding the specificity of these enzymes is crucial for predicting the products of protein digestion.

Trypsin Specificity

Trypsin is a serine protease that preferentially cleaves peptide bonds at the carboxyl side of lysine (Lys) and arginine (Arg) residues. This specificity is essential for determining the resulting peptide fragments when a protein is digested by trypsin. Recognizing the positions of these amino acids in a peptide sequence allows for accurate prediction of the cleavage products.

Chymotrypsin Specificity

Chymotrypsin is another serine protease that cleaves peptide bonds preferentially at the carboxyl side of aromatic amino acids such as phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr). This characteristic enables the identification of peptide fragments produced from proteins containing these residues. Understanding chymotrypsin's specificity is vital for analyzing peptide sequences and their cleavage patterns.

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Related Practice

Textbook Question

Show the steps in the synthesis of the tetrapeptide in Problem 34, using Merrifield's method.

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Textbook Question

Three peptides were obtained from a trypsin digestion of two different polypeptides. In each case, indicate the possible sequences from the given data and tell what further experiment should be carried out in order to determine the primary structure of the polypeptide.

a. polypeptide I:

1. Val-Gly-Asp-Lys

2. Leu-Glu-Pro-Ala-Arg

3. Ala-Leu-Gly-Asp

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Textbook Question

Glycine has pK_a values of 2.34 and 9.60. At what pH does glycine exist in the forms shown?

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Textbook Question

How would a protein that resides in the nonpolar interior of a membrane fold compared with the water-soluble protein just discussed?

A decapeptide undergoes partial hydrolysis to give peptides whose amino acid compositions are shown. Reaction of the intact decapeptide with Edman's reagent releases PTH-Gly. What is the sequence of the decapeptide?

1. Ala, Trp

2. Val, Pro, Asp

3. Pro, Val

4. Ala, Glu

5. Trp, Ala, Arg

6. Arg, Gly

7. Glu, Ala, Leu

8. Met, Pro, Leu, Glu

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Textbook Question

Write the mechanism for the reaction of a cysteine side chain with iodoacetic acid.