Advanced Placement (AP) Biology Practice Exam

Which type of bond is NOT considered an R group interaction in proteins?

1. Covalent disulfide bonds

2. Ionic bonds

3. Hydrogen bonds

4. Peptide bonds

The correct answer is: Peptide bonds

In protein structure, R group interactions are critical for stabilizing the three-dimensional conformation of proteins, which ultimately determines their function. These interactions typically occur between the side chains, or R groups, of amino acids. Peptide bonds form between the carboxyl group of one amino acid and the amino group of another during the process of protein synthesis, linking amino acids together to form a polypeptide chain. This linkage is vital in establishing the primary structure of a protein. Unlike R group interactions, which can vary based on the nature of the R groups involved and are important for the tertiary and quaternary structures, peptide bonds do not involve the R groups themselves but rather connect the backbone of the protein. On the other hand, covalent disulfide bonds, ionic bonds, and hydrogen bonds arise from interactions between the R groups of different amino acids. Covalent disulfide bonds occur between the sulfur atoms of cysteine residues, ionic bonds involve the attraction between positively and negatively charged R groups, and hydrogen bonds form due to the polar nature of certain side chains. Therefore, while all the other options relate to the interactions and stabilization of protein structures through R group interactions, peptide bonds are fundamentally different in that they link the building blocks of