FIGURE 3-2 Four levels of protein hierarchy. (a) The linear sequence of amino acids linked together by peptide bonds is the primary structure. (b) Folding of the polypeptide chain into local α helices or β sheets represents secondary structure. (c) Secondary structural elements, together with various loops and turns in a single polypeptide chain, pack into a larger, independently stable tertiary structure, which may include distinct domains. (d) Some proteins consist of more than one polypeptide associated together in a quaternary structure.