Final answer:
The association of two or more polypeptide chains in a protein corresponds to its quaternary structure, which is held together by interactions like hydrogen bonds and hydrophobic interactions, as exemplified by insulin and hemoglobin.
Step-by-step explanation:
For a protein, an association of two or more polypeptide chains represents its quaternary structure. Proteins can have up to four levels of structure, with the quaternary structure being the highest level when it is present. This level is characterized by the assembly of multiple polypeptide chains, often referred to as subunits, which come together to form a complex and functioning protein. The quaternary structure of proteins like hemoglobin, which consists of four polypeptide chains, is held together by various interactions including hydrogen bonds, ionic bonds, and hydrophobic interactions, similar to those that maintain the tertiary structure.
These interactions help stabilize the complex structure, allowing the protein to be properly functional in biological processes. Examples of proteins with quaternary structure include insulin, which forms disulfide linkages, and hemoglobin with its specific arrangement of chains providing its unique properties.