The Structure and Function of Haemoglobin

 by Saanvi Ganesh

Haemoglobin is the red pigment that gives red blood cells their colour and is a protein found in many different organisms. Adult human haemoglobin is made of four polypeptide chains, each associated with a haem group that together create a quaternary structure that transports oxygen efficiently. 

In order to transport oxygen around the body efficiently, haemoglobin needs to readily dissociate with oxygen molecules at tissues requiring oxygen and readily associate with oxygen at gas exchange surfaces like the alveoli in lungs. This means that haemoglobin is required to change its affinity for oxygen in different regions of the body. Therefore the quaternary structure of haemoglobin changes in response to different environmental stimuli like oxygen and carbon dioxide. 

The structure of a protein is determined by its gene, transcripted to mRNA and translated to a linear polypeptide sequence. This is its primary structure which then folds into alpha helices and/or beta-pleated sheets by the formation of hydrogen bonds between R groups in the polypeptide chain. This is the protein’s secondary structure; it folds again by hydrogen, ionic and disulfide bonds to form a tertiary structure and many polypeptide chains (four) join along with prosthetic groups (haem group - Fe2+) to form the quaternary structure. The nature of the bonds or intermolecular forces that hold proteins in their folded structures means that the proteins can change shape when placed in different environments.

A respiring tissue, for example muscle tissue, produces carbon dioxide in large concentrations. The greater the rate of respiration, the more carbon dioxide produced. This carbon dioxide is then removed from the muscle tissue into tissue fluid and eventually the bloodstream. Carbon dioxide dissolves in the blood plasma, decreasing the pH of the blood (so it is more acidic). A lower pH means more H+ ions are dissociated into the blood plasma. These ions disrupt the ionic and hydrogen bonds that hold the structure of the protein haemoglobin, causing the protein to change its quaternary structure to one that lowers its affinity for oxygen. This means that oxygen is dissociated easily for use in the respiring muscle tissue.

At a gas exchange surface like alveoli, carbon dioxide is readily removed from the blood plasma. This increases the pH of blood and decreases the H+ ion concentration. This means that ionic and hydrogen bonds in haemoglobin’s structure are less disrupted and the protein changes its quaternary structure to increase its affinity for oxygen. This ensures that oxygen readily associates with haem groups in haemoglobin, in order to be transported to respiring tissues. Haemoglobin’s affinity for oxygen differs with exposure to different partial pressures of oxygen. This can be plotted on an oxygen dissociation curve. The curve shows that blood oxygen saturation rises exponentially with increasing oxygen partial pressure (until a maximum saturation is reached - around 97%). This is due to positive cooperativity and is again affected by protein structure. 

The quaternary structure of haemoglobin makes it difficult for the first oxygen molecule to bind to a haem group because the subunits are closely united. This means that in low oxygen partial pressures, very few molecules of oxygen bind to haemoglobin. As the partial pressure increases, there are higher chances of association of the first oxygen molecule to a haem group. After the first oxygen molecule has associated, the quaternary structure of the haemoglobin protein changes, exposing more haem groups and increasing the protein’s affinity for oxygen. This means that the second oxygen molecule binds to a haem group more easily and the third even more easily. This is shown as an exponential rise in blood oxygen saturation as partial pressure increases on the oxygen dissociation curve and is known as positive cooperativity. 

However, the graph plateaus and reaches maximum saturation below 100% saturation. This is despite increased affinity for oxygen for the fourth oxygen binding site on the protein. The chances of a successful collision of the fourth binding site to an oxygen molecule is significantly reduced, meaning that the fourth binding site may never be occupied. This results in a maximum blood oxygen saturation that is below 100%.