Which of the following best describes quaternary structure?

Quaternary structure refers specifically to the arrangement and interaction of multiple polypeptide chains, or subunits, to form a larger and more complex protein entity. This level of protein structure is crucial for understanding how proteins function, as many biological activities depend on the interactions between these subunits.

In the context of larger proteins, quaternary structure can lead to new functional properties that arise from the combination of different polypeptides. For instance, hemoglobin, which carries oxygen in red blood cells, is composed of four polypeptide chains that work together. The cooperative interaction among these chains enhances the overall functionality of the protein.

The other options describe different aspects of protein structure. A single polypeptide chain characterizes primary structure, while a helix or sheet formation pertains to secondary structure. The linear sequence of amino acids is also reflective of primary structure. Understanding these differences is critical for grasping the overall hierarchy of protein structure, which culminates in the quaternary level where multiple polypeptides interact.