Which of the following is true?

The disulfide bridges formed by oxidation of the sulfhydryl groups on cysteine stabilizes protein tertiary structure

The correct answer is 2. The disulfide bridges formed by oxidation of the sulfhydryl groups on cysteine stabilizes protein tertiary structure.

Disulfide bridges, also known as disulfide bonds, are covalent bonds that play a significant role in the stabilization of protein structures. These bonds form between the sulfhydryl (thiol) groups of two cysteine residues within a protein. Here's a detailed explanation of the correct statement and why the other statements are incorrect:

Correct Statement: The disulfide bridges formed by oxidation of the sulfhydryl groups on cysteine stabilizes protein tertiary structure

Formation of Disulfide Bridges: Disulfide bridges are created through an oxidation reaction where two cysteine residues are covalently linked by their sulfhydryl groups (-SH) to form a disulfide bond (-S-S-). This reaction can be represented as:

\(2 \text{R-SH} \rightarrow \text{R-S-S-R} + 2 \text{H}^+\)

where R represents the rest of the cysteine molecule.

Stabilization of Protein Structure: The formation of disulfide bridges introduces covalent cross-links within a protein. These cross-links can occur within the same polypeptide chain (intramolecular) or between different polypeptide chains (intermolecular), contributing significantly to the protein's overall stability. Disulfide bridges help maintain the protein's three-dimensional shape (tertiary structure) by restricting the flexibility of the polypeptide chain, thereby increasing the protein's resilience to denaturation.

Incorrect Statements:

1. The disulfide bridges formed by reduction of the sulfhydryl groups on cysteine destabilizes protein tertiary structure

Reduction of Disulfide Bridges: Reduction reverses the formation of disulfide bonds, converting the disulfide bond back into two sulfhydryl groups. This can be represented as:

\(\text{R-S-S-R} + 2 \text{H}^+ \rightarrow 2 \text{R-SH}\)

Destabilization: The reduction of disulfide bridges breaks the covalent links, potentially leading to the destabilization of the protein's tertiary structure. The absence of these stabilizing covalent bonds makes the protein more flexible and less resistant to denaturation.

3. The disulfide bridges formed by oxidation of the sulfhydryl groups on cysteine destabilizes protein tertiary structure

Incorrect Mechanism: As mentioned earlier, the oxidation of sulfhydryl groups to form disulfide bridges actually stabilizes the protein structure. This statement is incorrect because it suggests the opposite effect.

4. The disulfide bridges formed by reduction of the sulfhydryl groups on cysteine stabilizes protein tertiary structure

Incorrect Process: This statement is incorrect because reduction breaks disulfide bonds rather than forming them. The breaking of these bonds leads to destabilization, not stabilization, of the protein structure.

Summary:

Disulfide bridges are crucial for the structural integrity of many proteins. Their formation through the oxidation of cysteine residues results in a more stable tertiary structure by creating covalent cross-links within or between polypeptide chains. Conversely, the reduction of disulfide bridges breaks these bonds, leading to potential destabilization of the protein structure. Therefore, the correct statement is that the disulfide bridges formed by oxidation of the sulfhydryl groups on cysteine stabilize the protein tertiary structure.