Caption：The ribbon diagrams represent examples of the 4 levels of protein structure. (a) The linear sequence of amino acid residues defines the primary structure. (b) Secondary structure consists of regions of regularly repeating conformations of the peptide chain, such as alpha helices and beta sheets. (c) Tertiary structure describes the shape of the fully folded polypeptide chain. The example shown has two domains. (d) Quaternary structer refers to the arrangement of two or more polypeptide chains into a multisubunit molecule. The example shown consists of two polypeptide chains.
Notes：Each level of protein structure can be seen to be based on the "lower" levels of organization. In these examples, each polypeptide in the quaternary structure drawing are identical to the folded tertiary structure. The primary structure (a) is a portion of the underlying covalent structure shown in the cartoon model (b) .