When more than one polypeptide chains of tertiary structure are assembled together then called_____

Answer

**Step-by-Step Solution:**

1. **Understanding Protein Structure**: Proteins have different levels of structure: primary, secondary, tertiary, and quaternary. Each level represents a different arrangement of amino acids.

2. **Identifying Tertiary Structure**: The tertiary structure of a protein refers to the three-dimensional shape formed by a single polypeptide chain. This structure is stabilized by various interactions, including hydrogen bonds, ionic bonds, and hydrophobic interactions.

3. **Assembling Multiple Polypeptide Chains**: When more than one polypeptide chain (each having its own tertiary structure) comes together, they form a larger, more complex protein structure.

4. **Defining Quaternary Structure**: The assembly of multiple polypeptide chains is referred to as the quaternary structure of a protein. This structure involves the interaction of these subunits, which can be identical or different.

5. **Examples and Importance**: An example of a protein that exhibits quaternary structure is hemoglobin, which consists of four polypeptide chains. The quaternary structure is crucial for the protein's function, as it allows for cooperative binding of oxygen.

6. **Conclusion**: Therefore, when more than one polypeptide chain of tertiary structure is assembled together, it is called the **quaternary structure**.

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