Advanced Placement (AP) Biology Practice Exam

What characterizes quaternary structure in proteins?

1. Single polypeptide chain functioning alone

2. More than one polypeptide chain bonded together

3. Random coiling of amino acids

4. All peptide bonds broken

The correct answer is: More than one polypeptide chain bonded together

Quaternary structure in proteins is characterized by the association of more than one polypeptide chain, which come together to form a functional unit. This level of structure is significant for proteins that require multiple subunits to perform their biological functions effectively. Each polypeptide chain, known as a subunit, may have its own tertiary structure, and the interactions between these subunits can include hydrogen bonds, ionic bonds, van der Waals forces, and disulfide links which contribute to the stability and functionality of the overall protein complex. In contrast, a single polypeptide chain functioning alone pertains to primary or tertiary structures, where the protein operates as an isolated unit rather than as a complex. Random coiling of amino acids does not accurately describe a structured organization but rather speaks to a lack of defined shape, which contradicts the highly organized nature of quaternary structures. Lastly, the breaking of all peptide bonds refers to denaturation or degradation of proteins rather than a characteristic of their structural organization. Therefore, the feature that defines quaternary structure is indeed the presence and interaction of multiple polypeptide chains.