A quaternary structure is a protein structure composed of two or more Polypeptide chains. The quaternary structure is held together by a combination of Hydrogen bonds, disulphide bridges and Ionic bonds. They often also contain an inorganic group called a prosthetic group. An example of a quarternary protein that contains the prosthetic Haem group is Haemoglobin. Haemoglobin is composed of two Alpha polypeptide chains and two Beta polypeptide chains, the Haem group contains four Fe2+ ions which oxygen binds to in order for the protein to transport oxygen around the body. As it contains four Fe2+ ions, when it has four Oxygen molecules bound it is known as being saturated and is carrying.