Fig. 1

The four orders of protein structures. A protein has (1): a primary structure (the sequence of AAs along the polypeptide chain; (2) a secondary structure (the conformation of the polypeptide backbone); (3) a tertiary structure (the three-dimensional arrangement of protein); and (4) a quaternary structure (the spatial arrangement of polypeptide subunits). The primary sequence of AAs in a protein determines its secondary, tertiary, and quaternary structures, as well as its biological functions