The tertiary structure of a protein is best described as which of the following?

• A. Linear sequence of amino acids in a polypeptide chain linked by peptide bonds
• B. Overall three-dimensional folding of a single polypeptide chain driven by interactions among side chains (R groups)
• C. Local regular structures such as $\alpha$-helices and $\beta$-sheets formed primarily by backbone hydrogen bonding
• D. Association of multiple polypeptide subunits into a functional protein complex

Answer: (B)