In addition to primary and secondary structure, parts of proteins may fold and bond to other parts giving the protein a tertiary structure. Such proteins are referred to as globular proteins. Many of the proteins that function as enzymes are of this sort. At a still higher level, several polypeptides may come together to form a larger molecule. When this occurs the structure is referred to as quaternary structure. In the figure shown here two polypeptides have come together to form a larger unit which has quaternary structure. Hemoglobin, the molecule in red blood cells that carries oxygen, consists of 4 polypeptides joined together.