Final answer:
Insulin does not exhibit a quaternary structure as it is made from two polypeptide chains forming a single molecule stabilized by disulfide bonds. Quaternary structure is characterized by the assembly of multiple distinct protein subunits, which is not the case with insulin.
Step-by-step explanation:
Understanding Insulin Structure
Insulin is not an example of quaternary structure because it consists of two polypeptide chains (A and B) which come together to form the active insulin molecule. However, these two chains alone do not define a quaternary structure, which typically involves multiple, distinct polypeptide subunits that interact to form a larger complex. Insulin undergoes post-translational modification where it starts as a single polypeptide and, after certain internal sequences are lost, the remaining chains are held together by disulfide bonds. These disulfide bonds play a critical role in stabilizing the insulin molecule, but they do not qualify as quaternary structure because they do not involve the association of separate folded proteins. Instead, the complex of insulin's two chains (A and B) is a representation of its tertiary structure.
X-ray crystallography significantly contributed to understanding the structure of insulin, revealing the importance of disulfide bridges in stabilizing the insulin molecule. While some proteins in nature exhibit quaternary structure, insulin's organization is limited to its primary, secondary, and tertiary levels, where the tertiary structure is achieved through the interaction and folding of its own polypeptide chains without forming a larger protein complex.