USA Biology Olympiad (USABO) Practice Exam

The quaternary structure of a protein is determined by what?

1. Hydrogen Bonds between Amino Acids

2. Interactions between Different Polypeptide Chains

3. Peptide Bonds within a Single Chain

4. Disulfide Bridges Only

The correct answer is: Interactions between Different Polypeptide Chains

The quaternary structure of a protein refers to the complex that forms when two or more polypeptide chains (subunits) come together. This level of structure is characterized by the spatial arrangement and interactions among these multiple subunits, which can be identical or different. The interactions between the polypeptide chains may include hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges, all contributing to the stability and functionality of the protein as a whole. In contrast, the other choices focus on different aspects of protein structure. Hydrogen bonds between amino acids primarily relate to the secondary and tertiary structures, where they help stabilize alpha helices and beta sheets within a single polypeptide chain. Peptide bonds form the primary structure of proteins, linking amino acids in a linear sequence. Disulfide bridges, while important for stabilizing the tertiary and quaternary structures, are just one type of interaction and do not encompass the full range of interactions that determine quaternary structure. Thus, the correct answer highlights the critical role of inter-subunit interactions in forming the quaternary structure of the protein.