Final answer:
The quaternary structure of a protein refers to the complex formed by the association of multiple polypeptide chains, each with its own tertiary structure, which comes together to form a functioning protein, as seen in the example of hemoglobin.
Step-by-step explanation:
Quaternary structure of a protein refers to the association of multiple polypeptide subunits to form a functional protein. It is a higher level of protein structure which occurs when two or more polypeptides join together, often by non-covalent interactions such as hydrogen bonds, ionic bonds, or van der Waals forces, and sometimes by covalent bonds such as disulfide bridges. For instance, hemoglobin, which carries oxygen in our blood, has a quaternary structure consisting of two alpha and two beta polypeptide chains. Each of these chains is folded into its own tertiary structure, and their coming together produces the functional protein.
Oligomeric proteins that have a quaternary structure can either be homomers if all the polypeptide units are identical, or heteromers if they are different. These subunits can be held together by various interactions, which are similar to those that maintain the tertiary structure of individual polypeptide chains. The correct function of many proteins, such as enzymes and receptors, is often dependent on their quaternary structure, linking structure intricately to function.
In conclusion, the correct answer to the student's question is option D) the association of multiple polypeptide subunits to form a functional protein. This level of structure is essential for the protein's biological activity and its ability to interact correctly within the cellular environment.