Quaternary structure refers to the highest level of protein organization, where multiple polypeptide chains, known as subunits, come together to form a functional protein complex. These subunits can be identical or different, and their arrangement is crucial for the protein's overall function. Examples of proteins with quaternary structure include hemoglobin, which carries oxygen in the blood, and antibodies, which play a key role in the immune response. The interactions between subunits in quaternary structure can involve various types of bonds, such as hydrogen bonds, ionic bonds, and hydrophobic interactions. This level of structure is essential for the stability and activity of many proteins, as it allows for cooperative behavior and regulation. Understanding quaternary structure is important in fields like biochemistry and molecular biology, as it helps explain how proteins perform their diverse functions in living organisms.