Glutathione (G-SH), one of the most common tripeptides in animals, plants, and bacteria, is a scavenger of oxidizing agents. In reacting with oxidizing agents, glutathione is converted to G-S-S-G. (a) Name the amino acids in this tripeptide. (b) What is unusual about the peptide bond formed by the \(N\)-terminal amino acid? (c) Write a balanced half-reaction for the reaction of two molecules of glutathione to form a disulfide bond. Is glutathione a biological oxidizing agent or a biological reducing agent?(d) Write a balanced equation for reaction of glutathione with molecular oxygen, \(\mathrm{O}_{2}\), to form G-S- S-G and \(\mathrm{H}_{2} \mathrm{O}\). Is molecular oxygen oxidized or reduced in this process?

Step by step solution

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(a) Naming the amino acids in glutathione

Glutathione (G-SH) is a tripeptide, meaning it is composed of three amino acids. Its structure is \(\gamma\)-glutamylcysteinylglycine or, in short, Glu-Cys-Gly. Therefore, the amino acids in glutathione are: 1. Glutamic acid (Glu) 2. Cysteine (Cys) 3. Glycine (Gly)

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(b) Unusual peptide bond in glutathione

In most peptides, the peptide bond is formed between the amino group (\(\mathrm{NH}_{2}\)) of one amino acid and the carboxyl group (\(\mathrm{COOH}\)) of another amino acid. However, in glutathione, the peptide bond between the N-terminal amino acid (glutamic acid) and the second amino acid (cysteine) is unusual because it is not formed between these functional groups. Instead, the peptide bond is formed between the carboxyl side chain (\(\mathrm{-COOH}\)) of glutamic acid and the amino group (\(\mathrm{-NH}_{2}\)) of cysteine, leading to a gamma (\(\gamma\)) linkage in the tripeptide.

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(c) Balanced half-reaction of glutathione

When two molecules of glutathione react to form a disulfide bond, a molecule of G-S-S-G is produced. The balanced half-reaction can be represented as follows: 2 G-SH ⟶ G-S-S-G + 2H+ + 2e- In this reaction, two glutathione molecules are reduced to form G-S-S-G, releasing two protons (H+) and two electrons (e-). Since glutathione loses electrons in this reaction, it acts as a biological reducing agent.

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(d) Reaction of glutathione with molecular oxygen

When glutathione reacts with molecular oxygen (O2), G-S-S-G and H2O are produced. To balance the equation, we must ensure that the number of atoms is equal on both sides of the reaction. The balanced equation can be represented as follows: 4 G-SH + O2 ⟶ 2 G-S-S-G + 2H2O In this reaction, glutathione (G-SH) is reduced to G-S-S-G, and molecular oxygen (O2) is reduced to water (H2O). Since oxygen gains electrons in this process, it is reduced.

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