Top image: Primary structure is the sequence of amino acids in a polypeptide chain. Each blue knot represents an amino acid linked to the next by a peptide bond.
Middle image: Secondary structure is the local folding of the polypeptide chain into regular structures such as the alpha helix illustrated by the twirling dark pink yarn and the beta-pleated sheet illustrated by the zig-zagging light pink yarn. In both cases, hydrogen bonding (in white) stabilizes these local structures.
Bottom left image: Tertiary structure is the overall three-dimensional shape of a single polypeptide chain shown here with dark pink alpha helices, dark blue sheets, and cyan random coils.
Bottom right image: Quaternary structure describes how multiple polypeptides interact to form a single protein. The example shown here is hemoglobin with four subunits (folded chains) each given a different color. Hemoglobin carries oxygen in our red blood cells from the lungs to the tissues - one oxygen molecule for each subunit.
