Figure 3.

Four levels of protein structures. (A) The primary structure is shown as 3-letter codes, unlike Figure 2. The sequence is randomly written as a demonstration. (B–D) The secondary structure shows alpha helices as an example. Secondary, tertiary, and quaternary structures are visualized in PyMOL [80], a visualization tool for molecules, and macromolecules like proteins. The PDB ID used is 7SF8 [78].