Which of the following accurately describes the tertiary structure level of a protein?

• A. The association of multiple polypeptide subunits to form a functional protein complex.
• B. The linear sequence of amino acids in a polypeptide chain.
• C. The regular arrangement of amino acids into α-helices and β-sheets stabilized by hydrogen bonds.
• D. The overall three-dimensional folding of a single polypeptide chain stabilized by interactions among side chains (R groups).

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the hierarchical levels of protein structure. Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. Each level describes a different aspect of the protein's organization.
2. Step 2: Recall the definition of tertiary structure. The tertiary structure refers to the overall three-dimensional folding of a single polypeptide chain. This folding is stabilized by interactions among the side chains (R groups) of the amino acids.
3. Step 3: Differentiate tertiary structure from other levels. The primary structure is the linear sequence of amino acids, the secondary structure involves regular arrangements like α-helices and β-sheets stabilized by hydrogen bonds, and the quaternary structure describes the association of multiple polypeptide subunits.
4. Step 4: Identify the correct description of tertiary structure from the options provided. The correct answer is the one that mentions the three-dimensional folding of a single polypeptide chain stabilized by interactions among side chains (R groups).
5. Step 5: Recognize the types of interactions stabilizing tertiary structure. These include hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges, which contribute to the protein's overall shape and function.