Describe the structure and function of haemoglobin

* Haemoglobin is the oxygen carrying pigment found in red blood cells. It transports oxygen from the lungs in the blood to the respiring tissues. * Haemoglobin is a globular protein and is made up of four polypeptide chains two of which are alpha chains and the other two are identical beta chains. Each of the beta chains has a tertiary structure * The four polypeptide chains pack closely together into a spherical shape with their hydrophobic R groups pointing inwards towards the centre of the molecule and their hydrophilic ones pointing outwards * The interactions between the hydrophobic R groups make the correct 3-D shape * The hydrophilic R groups are essential to maintain the solubility * The tertiary structure of haemoglobin makes it soluble. The hydrophilic chains are therefore able to make hydrogen bonds with the water molecules * Also the haemoglobin has the ability to bind with oxygen. The haem group contained within each polypeptide chain enables the haemoglobin molecule to combine with the oxygen. Oxygen molecules combine with the iron ion Fe2+ in the haem group. One oxygen molecule (with two oxygen atoms) combine with each haem group so one haemoglobin molecule can combine with four oxygen molecules (eight oxygen atoms). * Also the haemoglobin has the ability to pick up and release oxygen. The overall shape of the haemoglobin enables it to pick up oxygen when the relative concentration is high and release the oxygen when the relative concentration is low. Small changes in oxygen concentration have a large affect on how much oxygen the haemoglobin molecule can hold. Once one oxygen molecule has combined with one haem group, the whole molecule changes it’s shape in such a way that it is easier for oxygen to combine with the other three haem groups * It is the haem group that is responsible for the colour of haemoglobin. This colour depends on if the iron ions have combined with the oxygen. If they are the molecule is named oxyhaemoglobin and is bright red in colour. If not the colour is purplish.