If we observe the structure of proteins, disulphide bonds are formed between

Options

A. Cysteine residues that are close together
B. Cystine residues that are close together
C. Proline residues that are close together
D. Histidine residues that are close together

Answer

To solve the question regarding the formation of disulfide bonds in proteins, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding Protein Structure**:
- Proteins are made up of long chains of amino acids, which can be classified as polypeptides.

**Hint**: Remember that proteins are polymers of amino acids.

2. **Identifying Sulfur-Containing Amino Acids**:
- Among the 20 standard amino acids, only a few contain sulfur. The two primary sulfur-containing amino acids are methionine and cysteine.

**Hint**: Focus on the amino acids that have sulfur in their side chains.

3. **Formation of Disulfide Bonds**:
- Disulfide bonds (or bridges) are formed specifically between the thiol (-SH) groups of cysteine residues. When two cysteine molecules come close together, they can undergo oxidation to form a covalent bond known as a disulfide bond (–S–S–).

**Hint**: Recall that disulfide bonds are formed between the side chains of cysteine.

4. **Examples of Disulfide Bonds in Proteins**:
- Disulfide bonds play a crucial role in stabilizing the three-dimensional structure of proteins. For example, they are found in antibodies and insulin, linking different chains or loops within the protein structure.

**Hint**: Think about how disulfide bonds contribute to the stability of protein structures.

5. **Conclusion**:
- Since disulfide bonds are formed between cysteine residues, the correct answer to the question is that disulfide bonds are formed between cysteine residues that are close together.

**Hint**: Always check the options provided in a question to identify the correct amino acids involved.

### Final Answer:
Disulfide bonds are formed between cysteine residues that are close together.