How is quaternary structure best described?

Quaternary structure refers to the arrangement and interaction of multiple polypeptide chains (subunits) in a protein complex. This structure is critical for the biological activity of many proteins, as the interaction between different chains can lead to functional diversity and regulatory mechanisms.

In this context, the correct answer highlights that quaternary structure involves a large protein complex composed of two or more polypeptide chains. These chains can be identical or different and come together through various interactions, including hydrogen bonds, ionic interactions, hydrophobic interactions, and van der Waals forces. The correct quaternary structure is essential for the functionality of multimeric proteins, such as hemoglobin, where the cooperative binding of oxygen occurs due to changes in the conformation of the subunits upon binding.

The other descriptions do not accurately capture the essence of quaternary structure: a single polypeptide chain signifies primary or tertiary structure, while a folded section typically refers to secondary or tertiary structure. A simple protein fold also does not encompass the complexity of interactions present in quaternary structures. Thus, the correct choice underscores the importance of multiple interacting polypeptide chains and their collective function within a protein complex.