Final answer:
The quaternary structure is the assembly of protein subunits into a specific geometric arrangement that forms a functional protein complex, like hemoglobin or insulin. This structure is stabilized by various interactions among the subunits.
Step-by-step explanation:
Quaternary structure is the term used to describe the association of multiple polypeptide subunits in a specific geometric arrangement, forming a larger functional protein complex. This level of protein structure is beyond the primary, secondary, and tertiary structures and involves the unique combination of subunits through various interactions. The nature of these interactions can include hydrogen bonds, ionic bonds, hydrophobic interactions, and even disulfide bridges that contribute to the stability and function of the protein.
For instance, the protein hemoglobin, which is vital for oxygen transport in the blood, is composed of four distinct polypeptide chains held together by interactions that constitute its quaternary structure. Another example is insulin, a hormone critical for glucose regulation, which has a specific quaternary arrangement held together by hydrogen bonds and disulfide linkages. The quaternary structure is essential for the function of these proteins, as each subunit often plays a distinct role in the protein's overall activity.