Problem 92 - Chemistry: The Central Science in SI Units, Global Edition

Question

Glutathione is a tripeptide found in most living cells. Partial hydrolysis yields Cys-Gly and Glu-Cys. What structures are possible for glutathione?

Step-by-Step Solution

Answer

The possible structures for glutathione based on the given partial hydrolysis products are Glu-Cys-Gly and Gly-Cys-Glu, with Glu-Cys-Gly being the most commonly found structure in living cells.

Step 1: Identify the amino acids present in glutathione

Based on the partial hydrolysis products, we can determine that glutathione is composed of three amino acids: cysteine (Cys), glycine (Gly), and glutamic acid (Glu).

Step 2: Determine the order of amino acids in glutathione

To determine the sequence of amino acids in glutathione, we can consider the partial hydrolysis products: Cys-Gly and Glu-Cys. Since both dipeptides have cysteine (Cys) in common, we can infer that Cys forms a link between Glu and Gly in the tripeptide. Thus, the possible sequences for glutathione are: 1. Glu-Cys-Gly 2. Gly-Cys-Glu

Step 3: Deduce the structure of glutathione

Using the possible sequences derived in step 2, we can now generate the structures for glutathione. Please note that for simplicity, we are only showing peptide bonds between amino acids and not the full amino acid structure. 1. Glu-Cys-Gly: Glu(-NH-CHR-C(=O)) - Cys(-NH-CHR1-C(=O)) - Gly(-NH-CHR2-C(=O))-OH 2. Gly-Cys-Glu: Gly(-NH-CHR1-C(=O)) - Cys(-NH-CHR-C(=O)) - Glu(-NH-CHR2-C(=O))-OH These are the possible structures for glutathione based on the partial hydrolysis information provided. The most common structure found in living cells is Glu-Cys-Gly.

Key Concepts

Peptide Bonds Explained

Peptide bonds are the links between amino acids in proteins and peptides. These bonds form during a reaction called a condensation reaction. During this reaction, the carboxyl group of one amino acid reacts with the amino group of another. This results in the release of a water molecule (H₂O).
Peptide bonds are essential because they form the backbone of proteins.

• They are strong and stable under physiological conditions.
• They support the various functions that proteins perform in living organisms.

The sequence of the amino acids also determines the protein's properties and function. For example, in glutathione, the peptide bonds link cysteine, glycine, and glutamic acid to form a tripeptide chain. This chain can connect in different sequences, like Glu-Cys-Gly or Gly-Cys-Glu, held together by these crucial peptide bonds.

Understanding Protein Hydrolysis

Protein hydrolysis is the process through which proteins are broken down into smaller peptides or even individual amino acids. This occurs when water interacts with the peptide bonds, leading to their cleavage.
In a laboratory or biological context, this process can be accelerated by enzymes or acids.
Hydrolysis is fundamental for several reasons:

• It helps in digesting proteins we eat into absorbable amino acids.
• It plays a role in recycling damaged or old proteins in the body.

For glutathione, partial hydrolysis provides smaller peptides such as Cys-Gly and Glu-Cys. These fragments help in determining the possible sequence of amino acids and the complete structure of the original protein.

Insights from Sequence Analysis

Sequence analysis involves determining the order of amino acids in a peptide or protein. Here, based on the products of hydrolysis, we understand the original sequence of the tripeptide.
This analysis is crucial for the following reasons:

• It helps in understanding the protein’s functionality.
• It reveals potential sites of activity or binding to other molecules.

In the case of glutathione, the presence of Cys in both Cys-Gly and Glu-Cys hints at a shared connection, suggesting Cys is located between Glu and Gly. Thus, sequence analysis allows for the deduction of possible configurations such as Glu-Cys-Gly or Gly-Cys-Glu. The most prevalent and biologically active sequence for glutathione is Glu-Cys-Gly, underscoring the significance of correctly analyzing amino acid sequences.