Define “quaternary structure” of a protein.

Quaternary structure refers to the arrangement and interaction of multiple polypeptide chains, or subunits, to form a single functional protein complex. Many proteins are composed of two or more polypeptide chains that come together, and the way these chains fit together contributes significantly to the protein's overall functionality.

For example, hemoglobin is a classic example of a protein with quaternary structure, as it consists of four polypeptide chains that work together to transport oxygen throughout the body. The specific interactions—such as hydrogen bonds, ionic bonds, and hydrophobic interactions—between these subunits are crucial for the proper functioning of the protein.

This concept is separate from the primary, secondary, and tertiary structures of proteins, which relate to the linear sequence of amino acids, local folding patterns such as alpha helices and beta sheets, and the overall three-dimensional shape of a single polypeptide chain, respectively. While the relationship between a protein and its ligands is essential for its function, this pertains more to functionality rather than structural classification. Thus, focusing on the collaborative arrangement of multiple chains underscores the essence of quaternary structure.