Which type of protein structure may incorporate multiple subunits?

The correct answer refers to quaternary structure, which is a level of protein structure that involves the assembly of multiple polypeptide chains, known as subunits, into a single functional complex. These subunits can be identical or different and come together to perform a specific biological function.

Quaternary structure is crucial for the functionality of many proteins, such as hemoglobin, which is composed of four subunits. The interactions between these subunits can affect the protein's stability and activity, often resulting in cooperative behavior, where the binding of a molecule to one subunit influences the behavior of others.

In contrast, primary structure refers to the linear sequence of amino acids in a polypeptide chain, whereas tertiary structure describes the overall three-dimensional shape of a single polypeptide, determined by the interactions between the side chains of the amino acids. The term "pyrimidine" does not represent a level of protein structure but rather refers to a type of nitrogenous base found in nucleic acids. Thus, it is clear why quaternary structure is the only choice that pertains directly to the incorporation of multiple subunits in protein architecture.