Advanced Placement (AP) Biology Practice Exam

What allows proteins to fold into their functional 3D shape?

1. Weak bonds between polypeptide chains

2. Peptide bonds between amino acids

3. Interactions between side chains of amino acids

4. Hydrogen bonds only

The correct answer is: Interactions between side chains of amino acids

Proteins achieve their functional three-dimensional shape primarily through interactions between the side chains, or R groups, of the amino acids that make up the polypeptide chain. These interactions include a variety of forces such as hydrogen bonds, ionic bonds, hydrophobic interactions, and Van der Waals forces. The specific sequence of amino acids in a protein determines how the side chains interact with one another, resulting in the unique folding patterns essential for the protein's function. While weak bonds between polypeptide chains can influence the stability of protein structure, it is the interactions at the side chain level that enable the initial folding to form the secondary, tertiary, and quaternary structures. Peptide bonds link individual amino acids together to form a polypeptide chain, but they do not determine the folding; rather, they create the backbone that undergoes folding. As for hydrogen bonds, while they play a significant role in stabilizing the structure, focusing solely on them overlooks the importance of side chain interactions that drive the folding process. Thus, it is the comprehensive interplay of side chain interactions that is fundamental to the formation of a protein's functional three-dimensional shape.