Question

The amino acid cysteine forms a disulfide when oxidized. What is the structure of the disulfide?

Short answer

Cysteine forms cystine with a disulfide bond \( -S-S- \).

Step-by-step solution

Understand Cysteine Structure

Cysteine is an amino acid with a thiol (-SH) group attached to its side chain, which is chemically represented as CH2SH.

Identify Oxidation Reaction

When two cysteine molecules undergo an oxidation reaction, the thiol groups (-SH) from each cysteine oxidize to form a disulfide bond (-S-S-).

Combine Two Cysteines

In this oxidation process, two cysteine molecules link together via the newly formed disulfide bond to create a molecule known as cystine.

Draw the Disulfide Structure

The structure of the disulfide should be represented as: \( ext{Cysteine-S-S-Cysteine} \), where two cysteine molecules are joined by a disulfide bond.

Key concepts

Cysteine

Cysteine is a unique amino acid that plays an essential role in protein structure. It is classified as a polar amino acid because of its side chain, which contains a thiol group. This makes cysteine stand out among other amino acids. The basic structure of cysteine includes a central carbon atom attached to:

• An amino group (-NH₂)
• A carboxyl group (-COOH)
• A hydrogen atom
• A side chain consisting of a methylene bridge (-CH₂) and a thiol group (-SH)

The presence of the thiol group is critically important for cysteine’s ability to form disulfide bonds. This small, sulfur-containing group gives cysteine its ability to stabilize protein structures and form complex protein networks. As such, cysteine not only contributes to protein integrity but also affects its function, making it a vital part of many biological processes.

Thiol Group

Thiol groups are functional groups characterized by a sulfur atom bonded to a hydrogen atom, denoted as -SH. In cysteine, the thiol group is attached to the methylene bridge of the side chain. This group is highly reactive, which makes cysteine particularly interesting for various biochemical reactions. The sulfur atom in the thiol group is larger and less electronegative compared to oxygen, which is why thiols tend to be more reactive than their oxygen analogs, alcohols (-OH). Some important properties of thiol groups include:

• Their ability to donate hydrogen ions easily
• Reactivity with electrophiles leading to diverse biochemical reactions
• Their role in redox balancing, as they can be oxidized to form disulfide linkages

Thiol groups are crucial in the formation of disulfide bonds, which occur when two thiol groups are oxidized, resulting in a bond between two sulfur atoms. This oxidative link can occur within the same molecule or between different molecules, playing key roles in maintaining the structural and functional stability of proteins.

Oxidation Reaction

Oxidation reactions involve the loss of electrons from a molecule, atom, or ion. In the context of cysteine, an oxidation reaction results in the formation of a disulfide bond between two cysteine molecules. This transformation is pivotal in biochemistry for stabilizing protein structures. When two cysteine molecules come into close proximity, their thiol groups undergo an oxidation reaction. The thiol (-SH) groups each lose a pair of electrons and join through their sulfur atoms to form a disulfide bond (-S-S-). This process is not just a simple chemical conversion, but a key reaction that has significant implications for protein folding and stability.

• Disulfide bonds can lock different parts of a protein together.
• They can link separate protein chains, enhancing multi-chain protein structures.
• This oxidative process helps maintain the three-dimensional shape of proteins, which is crucial for their proper function.

The complete oxidation of two cysteine molecules results in a new structure known as cystine. Understanding this reaction helps in comprehending how proteins maintain their integrity and functionality in biological systems.