What describes the quaternary structure of proteins?

The quaternary structure of proteins refers to the arrangement and interaction of multiple polypeptide chains, or subunits, into a single functional protein complex. This organization can profoundly influence the protein's functionality, stability, and regulation. For example, hemoglobin, which carries oxygen in the blood, consists of four polypeptide subunits that work together to perform its biological role. Each subunit can contribute to the overall functionality by allowing cooperative binding of oxygen, showcasing how the interaction of these pieces creates a functional unit greater than the sum of its parts.

In contrast, other structures mentioned, like the linear sequence of amino acids, are associated with primary structure, while the unique three-dimensional shape of a single polypeptide pertains to secondary and tertiary structures, which focus on individual polypeptides rather than their interactions in a multi-subunit arrangement.