Globular And Fibrous.docx

Globular proteins consist of molecules that curl up into a ‘ball’ like shape. They are non- polar, with hydrophobic R groups in the centre, and hydrophilic R groups on the outside of the molecule. Because of this they are soluble molecules, with an example of this being haemoglobin. Fibrous molecules are long strands of protein molecules. Because of this they are usually insoluble meaning they are used more as structural proteins such as collagen. Haemoglobin is an example of a globular protein, and is the oxygen carrying pigment in red blood cells. It consists of 4 polypeptide chains so has a quaternary structure. Each chain of the protein is called a globin, with both alpha and beta globins being found in haemoglobin. Sickle cell anaemia is a disease which is caused by a mutation in haemoglobin. The mutation is a substitution of one of the amino acids in the surface of a beta globin, with a glutamic acid being swapped for a valine. Glutamic acid is polar whereas the valine is non-polar which makes the haemoglobin less soluble than it should be, causing dangerous and unpleasant symptoms. Each polypeptide chain has one haem group, which is not made of amino acids meaning it is called a prosthetic group. Furthermore, each haem group contains one iron atom. This means that with 4 polypeptide chains, each with a haem group, one haemoglobin molecule can carry 4 oxygen molecules. Oxyhaemoglobin I haemoglobin with oxygen and is a bright red colour, whereas haemoglobin is of a purple colour. Collagen is the most common protein in animals. It is an insoluble fibrous protein so is used more as a structural protein than haemoglobin. It consists of 3 polypeptide chains and takes up a triple helix shape, with polypeptide chains being wound around each other. These strands are held together by hydrogen bonds and some covalent bonds. Furthermore, every third amino acid is glycine which is the smallest amino acid, this allows the 3 strands to lie together- any other amino acid would be too big. The 3 strand molecules lie parallel with their R groups being covalently bonded together and these crosslinks forming fibrils. In addition to this the ends of these parallel strands are staggered meaning there is no week spot, increasing strength. Collagen is very flexible but also has high tensile strength, for example the Achilles can withstand up to a force of 300 N per mm squared. To allow for this the fibres line up in the direction of the tension. For example, in tendons they line up in parallel bundles along the length of the tendon, the direction of tension.