Not all proteins have a

Options

A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure.

Answer

### Step-by-Step Solution:

1. **Understanding Protein Structures**:
Proteins are made up of amino acids and have different levels of structure: primary, secondary, tertiary, and quaternary.

2. **Primary Structure**:
The primary structure of a protein is the sequence of amino acids in a polypeptide chain. This structure is present in all proteins, as it is determined by the genetic code in DNA.

3. **Secondary Structure**:
The secondary structure refers to the local folding of the polypeptide chain into structures such as alpha helices or beta pleated sheets. All proteins exhibit some form of secondary structure.

4. **Tertiary Structure**:
The tertiary structure is the overall three-dimensional shape of a single polypeptide chain. This structure is also present in all proteins, especially in globular proteins and enzymes.

5. **Quaternary Structure**:
The quaternary structure occurs when two or more polypeptide chains come together to form a functional protein. Not all proteins have this structure; it is only present in proteins that consist of multiple polypeptide chains, such as hemoglobin.

6. **Conclusion**:
Therefore, while all proteins have primary, secondary, and tertiary structures, not all proteins exhibit quaternary structure. The correct answer to the question is that not all proteins have a quaternary structure.