The main structural feature of proteins is:

Options

A. Peptide linkage
B. Glycoside linkage
C. Ether linkage
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the main structural feature of proteins. Proteins are large biomolecules made up of smaller units called amino acids.

2. **Identifying the Linkages**: The options provided include:
- Peptide linkage (amide linkage)
- Glycosidic linkage
- Ether linkage
- All of these

3. **Defining Peptide Linkage**: Peptide linkage is a specific type of bond that forms between two amino acids. It occurs when the carboxyl group of one amino acid reacts with the amino group of another, resulting in the release of a water molecule (condensation reaction).

4. **Illustrating with an Example**:
- Consider two amino acids: Glycine (NH2-CH2-COOH) and Alanine (NH2-CH(CH3)-COOH).
- When these two amino acids react, the hydroxyl group (OH) from the carboxyl group of Glycine and a hydrogen atom (H) from the amino group of Alanine are removed, leading to the formation of water (H2O).
- This reaction results in the formation of a peptide bond (C=O and N-H), linking the two amino acids together.

5. **Forming the Protein Structure**: The peptide bonds can link multiple amino acids together to form long chains, which fold into specific three-dimensional structures, ultimately forming proteins.

6. **Conclusion**: The main structural feature of proteins is indeed the peptide linkage (amide linkage), which is responsible for connecting amino acids in a protein chain.

### Final Answer:
The main structural feature of proteins is **peptide linkage (option A)**.

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