What is meant by a 'subunit' in the context of quaternary structure?

In the context of quaternary structure, a 'subunit' refers to a distinct polypeptide chain that comes together with other polypeptide chains to form a functional protein complex. These subunits can be identical or different, and they interact with one another through non-covalent interactions such as hydrogen bonds, ionic interactions, and hydrophobic effects to create the overall architecture and functionality of the protein.

Quaternary structure is characterized by the arrangement and interaction of these multiple subunits, which can be essential for the protein's activity. Each subunit may contain its own secondary, tertiary, or even primary structure, but collectively, they work together to fulfill the protein's biological role.

In contrast, a single polypeptide chain refers to a primary structure and does not constitute quaternary structure unless it is interacting with other chains. A small protein fold and a domain of a larger protein pertain more to the tertiary structure or specific functional regions within a single polypeptide rather than to quaternary interactions. Lastly, secondary structure refers to local folding patterns like alpha-helices and beta-sheets within a single polypeptide, which is distinct from the interactions occurring in quaternary structure.