Haemoglobin – Structure and Function

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  • Haemoglobin is found in red blood cells and carries oxygen efficiently from the lungs to the tissues in body. It also aids in transporting hydrogen ions and carbon dioxide back to the lungs. Haemoglobin is capable of binding to oxygen (O2) and gaseous nitric oxide (NO). When red blood cells pass along the capillary beds of the gills (in fish), lungs or any other type of respiratory organ, diffusion of oxygen into the erythrocytes occur and haemoglobin ends up binding to NO and O2.
  • Diffusion of oxygen into the body cells occur when haemoglobin releases its content in the capillaries. The capillary walls are relaxed by NO to allow expansion thus facilitating the transport of oxygen to the cells.
  • Haemoglobin is composed of four subunits each containing a cofactor known as a haem group that includes an iron atom center. The main component that binds with oxygen is the iron. Hence each molecule of haemoglobin is capable of carrying four oxygen molecules.
  • In order to transport oxygen efficiently, corporation between the four subunits in a haemoglobin molecule becomes mandatory. In truth, all four subunits bind with oxygen collectively. When oxygen gets bound to one site among the four subunits, it also increases the chances of other remaining sites being bound to oxygen. Haemoglobin’s role in carrying oxygen through the blood from the lungs to the tissues is considered crucial for survival.
  • When the concentration of oxygen becomes low, haemoglobin shows a low affinity towards oxygen. This has a lot of impact in our body and has helped us in adapting efficiently. The lower concentrations and lower affinity means that when we work out, our body’s oxygen supply becomes less. Since haemoglobin shows less affinity towards oxygen, they can drop off the oxygen easily on human tissues. This ensures a good supply of oxygen when faced with an oxygen dependent situation.
  • Haemoglobin displays a higher affinity towards oxygen when the oxygen concentration is high and hence will not release oxygen when not required. This is a smart and a complex system evolved to maintain haemoglobin as a significant biological molecule throughout a long period of time.
  • Myoglobin, also a protein, is used for storing oxygen in muscles. Myoglobin’s affinity for oxygen is slightly higher than the haemoglobin, specifically at lower levels. This is due to the fact that myoglobin has a simpler job than haemoglobin which is to store and release oxygen to the muscles whereas, haemoglobin is also responsible for carrying and releasing the oxygen at the right places.
  • Similar to all the other proteins, haemoglobin is coded for by DNA. Mutations and changes in haemoglobin result in several blood related diseases like sickle cell anemia, a disease where the structure of the cell gets distorted and do not have the capacity to carry much oxygen in the required manner like a normal blood cell would. This signifies the underlying concept related to structural biochemistry: structure determines function.
  • The condition of sickle cell anemia shows us why and how diseases tend to develop. The gene for sickle cell anemia protects one against malaria. In countries plagued by malaria, more than the average number of people carry the gene for sickle cell anemia. The heterozygous form is the safest because it will prevent the development of sickle cell anemia while warding off malaria. The homozygous form would make a person sick with either malaria or sickle cell anemia. This is the reason why there are people higher than the average number with heterozygous form of sickle cell anemia in areas ridden with malaria. It is also the reason why the disease still lives on. The nature selects the carrier state.