A scientist is studying a protein that consists of two polypeptide chains. She finds that the protein loses its function when the two chains are separated. What is the highest level of structure that is directly responsible for this protein's functionality?

• A. Protein structure.
• B. Secondary structure.
• C. Tertiary structure.
• D. Quaternary structure.
• E. Impossible to tell.

Answer: (D)

Verified step by step guidance

1. Understand the levels of protein structure: Proteins have four levels of structure—primary, secondary, tertiary, and quaternary. Each level contributes to the protein's overall shape and function.
2. Recognize that the quaternary structure refers to the arrangement of multiple polypeptide chains (subunits) in a protein. This level of structure is responsible for the interaction between these chains.
3. Analyze the problem: The scientist observes that the protein loses its function when the two polypeptide chains are separated. This indicates that the protein's functionality depends on the interaction between these chains.
4. Conclude that the highest level of structure directly responsible for the protein's functionality is the quaternary structure, as it involves the arrangement and interaction of multiple polypeptide chains.
5. Note that the other levels of structure (primary, secondary, and tertiary) are important for the individual chains' folding and stability, but they do not explain the loss of function when the chains are separated.