Final answer:
A protein subunit is the building block for many complex proteins. These subunits come together to form a protein's quaternary structure, but not all proteins have such a structure. Hemoglobin, with four subunits, is an example of a protein with a quaternary structure.
Step-by-step explanation:
A protein subunit is the modular unit from which many larger single-chain proteins are constructed. Proteins often contain more than one polypeptide chain, with each chain referred to as a subunit. The arrangement of these multiple subunits is known as the quaternary structure of a protein.
For instance, hemoglobin has a quaternary structure composed of four polypeptide subunits. Moreover, not all proteins have a quaternary structure; some proteins function as a single polypeptide with primary, secondary, and tertiary structures. It is the weak interactions between these subunits, such as hydrogen bonds and disulfide bonds, that help to stabilize the overall structure of the protein.