Which of the following is incorrect?

Options

A. Quaternary structure refers to the spatial relations between individual polypeptide chains in a multichained protein
B. The tertiary structure is absolutely necessary for many biological activities of protein
C. Biologists describe the protein structures at 3 levels only
D. Protein structure is correlated with protein function

Answer

To determine which statement is incorrect regarding protein structure, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Quaternary Structure**:
- Quaternary structure refers to the arrangement of multiple polypeptide chains in a protein.
- These polypeptide chains, or subunits, interact through non-covalent interactions to form a functional protein.
- **Conclusion**: Option 1 is correct.

2. **Understanding Tertiary Structure**:
- The tertiary structure is the three-dimensional shape of a single polypeptide chain.
- This structure is crucial for the biological activity of many proteins, including enzymes, as it determines how the protein interacts with other molecules.
- **Conclusion**: Option 2 is correct.

3. **Levels of Protein Structure**:
- Proteins are described at four levels:
- Primary structure (sequence of amino acids)
- Secondary structure (local folding, such as alpha helices and beta sheets)
- Tertiary structure (overall 3D shape of a single polypeptide)
- Quaternary structure (assembly of multiple polypeptides)
- The statement that biologists describe protein structure at three levels is incorrect.
- **Conclusion**: Option 3 is incorrect.

4. **Correlation of Structure and Function**:
- The structure of a protein is closely related to its function. The specific shape of a protein allows it to perform its biological role effectively.
- **Conclusion**: Option 4 is correct.

### Final Answer:
The incorrect statement is **Option 3**: "Biologists describe the protein structure at three levels only."

---