Protein molecules in the red blood cells that carry oxygen molecules from the lungs to the body’s tissues and give carbon dioxide from the tissues back to the lungs are termed haemoglobin. It is made up of four protein molecules termed globin chains and all these are connected together.
A normal adult human body contains two alpha-globin and beta-globin chains whereas in the case of infants, beta-globin chains are not and haemoglobin in infants is made up of two alpha and two gamma globin chains.
With an increase in age, gamma globin changes in beta globin and forms an adult haemoglobin structure. Along with that globin also contains an important iron-containing porphyrin compound known as heme group and inside the heme compound, there is an iron atom that plays a crucial role in transporting oxygen and carbon dioxide in our blood. Iron in haemoglobin also imparts a red colour to blood.
Structure of Haemoglobin
The structure of haemoglobin was discovered by Max Perutz in the year 1959 and it was known to be a tetrameric protein. In adults, haemoglobin is made up of two subunits each of ‘𝜶’ and ‘𝝱’ polypeptide chains and each polypeptide chain is attached to a prosthetic group.
A. 𝜶 Subunit – This haemoglobin subunit is made up of an alpha polypeptide chain containing 141 amino acid residues.
B. 𝝱 Subunit – This haemoglobin subunit is made up of a beta polypeptide chain containing 146 amino acid residues.
C. Heme Group – This group contains iron molecules having a prosthetic group, which is attached to each polypeptide chain. Iron is situated in the centre of the porphyrin ring.
There is a strong interaction present between 𝜶 and 𝝱 subunits in the quaternary structure of haemoglobin. On treating haemoglobin with mild urea they undergo partial dissociation but 𝜶𝝱 dimers remain intact. Subunits in haemoglobin are mostly bounded with hydrophobic interactions, hydrogen bonding and a few ion pairs or salt bridges.
Representation of haemoglobin is mainly done in two conformations, i.e. R state and T state. More affinity is shown by oxygen towards the R state whereas deoxyhemoglobin is primarily present in the T state.
Functioning of Haemoglobin
One of the major functioning of haemoglobin molecules is to carry and transport oxygen to various tissues of body parts. When oxygen binds to haemoglobin then it is called a cooperative process. The binding of oxygen and release of oxygen from Hb in the lungs and tissue and this process occurs because of the transition between low oxygen affinity T state (Tense) and high oxygen affinity R state (Relaxed).
Transport of Oxygen
There are so many factors on which the affinity of oxygen to haemoglobin depends. Some of the important factors are listed below:
pH: Lower the pH higher will be, the 2,3-Bisphosphoglyceric acid and CO2 present in the tissue favour T- state, and further oxygen is released, whereas R-state is favourable for alveoli when pH value is higher, low CO2 and BPG concentration, which leads to the binding of oxygen to Hb.
Partial Pressure: Transportation of oxygen also depends on the partial pressure of oxygen. More the value of pCO2 then oxygen will bind with Hb and tissue and if pO2 is low then oxygen is released. For every 100 mi of oxygenated blood, it carries 5ml of O2 to tissues.
At first oxygen molecules binds with the heme unit of one subunit of the deoxyhaemoglobin i.e. T-state which leads to conformational changes and causes an increase in the affinity, further the second molecules bind more rapidly. Whereas binding of the fourth molecule occurs, when they are already present in the R state. When haemoglobin binds with the oxygen they show a sigmoid curve. This binding process is also known as allosteric binding apart from that when binding at one affects the binding at the remaining sites.
An oximeter is a tool that is used to measure the amount of oxygen present in the blood. All these facts tell us that oxyhemoglobin and deoxyhemoglobin have different absorption spectra. This is the main concept based on which doctors can check the oxygen saturation level of COVID -19 patients and also in those who come under a high-risk zone.
Diseases related to Haemoglobin
There are many health problems that occur because of haemoglobin deficiency and deficiency in haemoglobin causes a decrease in the oxygen-carrying capacity of the blood. This deficiency can be because of several reasons like nutritional deficiency, cancer, kidney failure or any genetic defects. In case if the level of haemoglobin is higher than normal haemoglobin, then this condition shows signs of various heart and pulmonary diseases.
Some of the common diseases which are caused by a decreased or increased level of haemoglobin are listed below:
A. Sickle Cell Anaemia:
This disease occurs because of a defect in haemoglobin. In this condition there occurs a point mutation in the 𝞫 globin chain. Because of this ‘GAG’ gets converted into ‘GTG’ which causes the replacement of glutamic acid by valine at the 6th position.
This disease occurs because of less production of haemoglobin. Further, it is categorised into two types they are 𝜶-thalassemia and 𝞫-thalassemia. It can also occur because of defective genes or because a number of genes are missing or defective.