When some proteins form assemblies with other molecules

• A. Primary structure of proteins
• B. Secondary structure of proteins
• C. Tertiary structure of proteins
• D. Quaternary structure of proteins

Answer: (D)

When proteins form assemblies with other molecules, the key idea is how multiple polypeptide chains come together to create a functional complex. This organization is the quaternary structure, which describes the arrangement and interactions between subunits (and with other molecules or cofactors) in a protein complex. For example, hemoglobin is made of four subunits that work together to bind and release oxygen, a hallmark of quaternary structure. In contrast, the primary level is just the amino acid sequence, the secondary level is local folding patterns like helices and sheets, and the tertiary level is the overall 3D shape of a single chain.