What defines the quaternary structure of a protein?

The quaternary structure of a protein is defined by the interaction of multiple polypeptide chains. This level of structure occurs when two or more polypeptide chains, each with their own primary, secondary, and tertiary structures, come together to form a functional protein complex. The polypeptide chains can be identical or different and may be held together by various interactions, such as hydrogen bonds, ionic bonds, or hydrophobic interactions.

Understanding quaternary structure is essential, as it often plays a critical role in the protein's overall function and stability. For example, hemoglobin, which carries oxygen in the blood, consists of four polypeptide chains that collaborate to perform its function effectively.

The other provided options represent different aspects of protein structure. The sequence of amino acids pertains to the primary structure; the arrangement of alpha-helices and beta-pleated sheets refers to the secondary structure; and the folding of a single polypeptide chain describes the tertiary structure. Each of these layers is crucial for protein function, but they do not encompass the interactions that define the quaternary structure.