Describe how haemoglobin binds to oxygen:

The binding curve of haemoglobin to its substrate is sigmoidal ('s' -shaped). Haemoglobin has four subunits, each of which will bind one molecule of oxygen. The reason for the sigmoidal shape is the process of cooperative binding. Initially, the binding of each successive molecule of oxygen causes the quaternary structure of haemoglobin to 'open up', making it easier for the next one to bind. However, as oxygen (substrate) concentration increases further, the reverse process occurs so that it is then harder for successive molecules to bind. This produces the characteristic binding curve. The shape of the curve is essential for facilitating oxygen loading and unloading by haemoglobin, allowing oxygen to be delivered to respiring tissues and taken up at the pulmonary capillaries.