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Chapter 18: Problem 5

Glutathione ( \(\mathrm{Y}\) -glutamylcysteinylglycine) is a tripeptide found in all cells of higher animals. It contains glutamic acid joined in an unusual peptide linkage involving the carboxyl group of the \(\mathrm{R}\) group (known as y-carboxyl group), rather than the usual carboxyl group (the \(\alpha\) -carboxyl group). Draw the structure of glutathione.

Short Answer

Expert verified
Draw glutamic acid using the \(\gamma\)-carboxyl, link it to cysteine, and then to glycine with correct peptide bonds.

Step by step solution

01

Identify Components

Glutathione is a tripeptide composed of three amino acids: glutamic acid, cysteine, and glycine. It is important to identify where each amino acid fits into the structure.
02

Understand Unusual Bonding

Note that glutamic acid forms a peptide bond using the \(\gamma\)-carboxyl group from its side chain, rather than the usual \(\alpha\)-carboxyl group. The rest of the peptide bonds are formed in the usual manner for cysteine and glycine.
03

Arrange the Amino Acids

Place glutamic acid at the start, linked through the \(\gamma\)-carboxyl group to the amino group of cysteine. Then, link cysteine's carboxyl group to the amino group of glycine using a typical peptide bond.
04

Draw the Structure

1. Start with Glutamic Acid: Draw the \(\alpha\)-amino group connected to the \(\alpha\)-carbon, which is connected to an R-group (side chain: CH2-CH2-COOH). Instead of using the \(\alpha\)-carboxyl group, use the \(\gamma\)-carboxyl to connect to Cysteine. 2. Connect the Cysteine: Its \(\alpha\)-amino group connects to Glutamic Acid's \(\gamma\)-carboxyl, and it has a side chain containing an -SH group. 3. Conclude with Glycine: Connect its \(\alpha\)-amino to Cysteine's \(\alpha\)-carboxyl and end with a simple hydrogen as its R-group. 4. Ensure all amino acid backbones (N-C-C) are correctly drawn, with peptide bonds clearly shown as CONH between each.
05

Verify Peptide Connectivity

Review the entire structure to ensure that glutamic acid is linked via its \(\gamma\)-carboxyl group, cysteine connects properly, and glycine is correctly added to the sequence. Verify that peptide bonds and side chains are accurate as per the amino acids involved.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Unusual Peptide Linkage
In the world of peptides, glutathione has an unusual feature. This tripeptide includes an unexpected bond involving glutamic acid. Typically, amino acids form bonds with the help of their central (alpha) carboxyl groups. However, in glutathione, glutamic acid links using its side chain's carboxyl group, known as the gamma-carboxyl group.
This unusual peptide linkage creates a unique structure for glutathione, setting it apart from other tripeptides.
  • Most peptides use alpha-carboxyl groups for bonding, but glutathione uses a gamma-carboxyl group in glutamic acid.
  • This results in a different bonding pattern and structure.
Understanding this unusual linkage helps explain glutathione's special role in biological processes.
Glutamic Acid
Glutamic acid is a key player in the structure of glutathione and many other proteins. It is an amino acid with a unique side chain that consists of two additional carbon atoms and a carboxyl group (-COOH). Normally, the alpha-carboxyl group is used to form peptide bonds.
However, in glutathione, the gamma-carboxyl group is involved in bonding. This flexibility in bonding changes how glutamic acid can connect with other amino acids, such as cysteine.
  • Glutamic acid has the ability to form bonds with either the alpha or gamma carboxyl group.
  • This characteristic enables the unusual structure of glutathione.
Furthermore, glutamic acid provides a distinct chemistry that is critical for the stability and function of proteins it resides in.
Tripeptide Amino Acids
A tripeptide is a molecule formed by three amino acids linked together. Glutathione is a prime example, made from glutamic acid, cysteine, and glycine. The order and linkage of these amino acids determine the properties and function of the tripeptide.
For glutathione, the sequence starts with glutamic acid using its gamma-carboxyl group, followed by cysteine, and ending with glycine.
  • Tripeptides, like glutathione, have diverse functions based on their sequence and linkages.
  • Changing the sequence can alter the properties of the tripeptide.
This makes understanding the structure and specific bond formations crucial for comprehending its biological role.
Amino Acid Bonding
Amino acids bond together to form peptides through a process facilitated by the carboxyl and amino groups. In a typical peptide bond, the carboxyl group of one amino acid reacts with the amino group of another.
In glutathione, however, the initial bond is unusual because it uses the gamma-carboxyl group of glutamic acid. The following bonds between cysteine and glycine are typical, using the alpha-carboxyl groups. This combination of typical and unusual bonding creates a distinct tripeptide structure.
  • Typical bonding involves alpha carboxyl and amino groups.
  • Glutathione features both typical and atypical bonding methods.
These bond formations play vital roles in determining the physical and chemical properties of the peptide.
Peptide Bond Formation
Peptide bond formation is an essential process in creating peptides and proteins. When forming a peptide bond, a carboxyl group of one amino acid links to the amino group of another, releasing a water molecule.
In glutathione, this general mechanism operates differently due to the unique linkage of glutamic acid.
  • Normal peptide bonds form between the alpha carboxyl and amino groups.
  • In glutathione, the gamma-carboxyl group of glutamic acid starts the link, resulting in unusual peptide bonding.
Understanding peptide bond formation within glutathione helps clarify not only its unique structure but also its special functional properties in biological systems.

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