Which structure of a protein involves multiple polypeptide chains?

The quaternary structure of a protein is defined by the arrangement and interactions of multiple polypeptide chains, also known as subunits. This level of protein structure emerges when two or more polypeptide chains come together to form a functional protein complex. The interactions between these chains can be due to various types of bonding, including hydrogen bonds, ionic bonds, and hydrophobic interactions, leading to a stable structure that is critical for the protein's function. Examples of proteins that exhibit quaternary structure include hemoglobin and immunoglobulins, where the individual polypeptides contribute to the overall functionality and stability of the protein. Understanding this structure is essential to recognizing how proteins can have complex functionalities beyond that of a single polypeptide chain.