What characterizes the quaternary structure of proteins?

The quaternary structure of proteins is characterized by the complex formation of multiple polypeptide chains, which can also be referred to as subunits. This higher level of organization allows for the assembly of two or more polypeptide chains that can function together as a single protein complex. These complexes can have different shapes and functions compared to individual polypeptide chains.

In contrast, other structural levels such as primary, secondary, and tertiary structures refer to different arrangements and interactions within or between a single polypeptide chain. The primary structure is the sequence of amino acids, the secondary structure includes local structures like alpha-helices and beta-sheets, and the tertiary structure is the three-dimensional shape of a single polypeptide. Quaternary structure emerges only when multiple polypeptides associate, thus highlighting the cooperative interactions between them and enabling complex functionalities in biological systems.