Large proteins are often formed of complex molecules in a quaternary structure where two or more polypeptide chains may join together. As a result of the diverse nature of roles proteins carry out, how the polypeptide chains are joined relates specifically to its function. For example, fibrous proteins, which carry out a structural role, have a different quaternary structure to globular proteins such as enzymes that have a metabolic function. An example of a fibrous protein is collagen, which is formed by the joining of three fibrous polypeptide chains through cross linkages between the chains’ amino acids. At the point where one collagen molecule ends and the other begins is spread throughout the fibre, the strength and stability of the complete molecule is increased. This is important as collagen form tendons which join muscle to bone. Alternatively a prosthetic group (non protein) may become attached to the protein. An example of this is a Haem group (containing a ferrous ion) in haemoglobin. This group allows for the loading and unloading of oxygen into the haemoglobin molecule. This feature enables haemoglobin, formed of four polypeptide chains linked in a spherical molecule each containing a haem group, to carry out its function of carrying oxygen around the body.