What is Haemoglobin?

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What is Haemoglobin?

Haemoglobin is a type of globular protein, which carries oxygen in erythrocytes. It is a tetrameric protein and contains the heme prosthetic group attached to each subunit. It is a respiratory pigment and helps in transporting oxygen as oxyhaemoglobin from the lungs to different parts of the body. Some amount of carbon dioxide is also transported back via haemoglobin as carbaminohaemoglobin.

Other oxygen binding proteins are myoglobin in muscles, haemocyanin in arthropods and molluscs, leghaemoglobin in legumes, etc.

Haemoglobin A present in humans is coded by HBA1, HBA2 and HBB genes. The sequence of amino acids in polypeptide chains of Hb varies in different species.

Heme part of Hb is synthesized in mitochondria and cytoplasm of immature RBCs. Globin protein is synthesized in the cytoplasm by ribosomes. Even after losing nucleus in mature mammalian RBCs, the residual rRNA continues to synthesize Hb until the reticulocytes enter the vasculature.

The haemoglobin level is measured in g/dL of the blood. In a healthy individual, the level ranges from 12 to 20 g/dL. Generally Hb level in males is greater compared to females. The normal level in males is 13.5 to 17.5 g/dL and in females, it is 12 to 15.5 g/dL.

Let us see the structure of haemoglobin.

Haemoglobin Structure

Max Perutz described the molecular structure of haemoglobin in 1959. Haemoglobin is a tetrameric protein, made up of two subunits each of β€˜πœΆβ€™ and β€˜π±β€™ polypeptide chains. Each polypeptide chain is linked to a heme prosthetic group.

  • 𝜢 subunit – It is made up of alpha polypeptide chain having 141 amino acid residues.
  • 𝝱 subunit – It is made up of beta polypeptide chain having 146 amino acid residues.
  • Heme group – It is an iron-containing prosthetic group, which is attached to each polypeptide chain. It contains iron in the centre of the porphyrin ring.

In the quaternary structure, there is a strong interaction between 𝜢 and 𝝱 subunits. On mild treatment with urea, haemoglobin partially dissociates but 𝜢𝝱 dimers remain intact. The subunits are bound together by mostly hydrophobic interactions, hydrogen bonding and a few ion pairs or salt bridges.

In infants, there are 2 alpha and 2 gamma chains, which get replaced by beta chains.

Haemoglobin is present in two conformations, i.e. R state and T state. Oxygen has more affinity to R state and deoxyhaemoglobin is primarily present in T state.

πŸ’‘ Did you know? Chlorophyll pigment contains an Mg porphyrin ring.

Haemoglobin Function

The main function of Hb is to carry and transport oxygen to various tissues. The binding of oxygen to Hb is cooperative binding. The binding and release of oxygen from Hb in the lungs and tissues respectively is due to the transition between low oxygen affinity T state (Tense) and high oxygen affinity R state (Relaxed).

Transport of oxygen

The affinity of oxygen to Hb is affected by pH, 2,3 BPG (2,3-Bisphosphoglyceric acid). Low pH, high BPG and CO2 present in tissues favour T-state and oxygen are released, whereas R-state is favoured in the alveoli due to high pH, low CO2 and BPG concentration, which leads to the binding of oxygen to Hb.

Binding of oxygen is also regulated by the partial pressure of oxygen. In the lungs where pO2 is high, oxygen binds with Hb and in tissues, where pO2 is low, oxygen is released.

Every 100 ml of oxygenated blood carries 5 ml of O2 to the tissues.

Binding of the first oxygen molecule to the heme unit of one subunit of the deoxyhaemoglobin (T-state) causes conformational changes leading to an increase in the affinity, thereby the second molecule binds more rapidly. The binding of the fourth molecule occurs, when it is already in the R state. The binding of oxygen to Hb shows a sigmoid curve.

This type of binding is known as allosteric binding, where binding at one site affects the affinities of the remaining binding sites.

The pulse oximeter measures the amount of oxygen present in the blood. It is used to diagnose hypoxia. It is based on the fact that oxyhemoglobin and deoxyhemoglobin have different absorption spectra. This is a major tool that doctors are using to check the oxygen saturation level of COVID-19 patients and also in those who are at risk.

Also Check: MCQs on Blood Group

Transport of Carbon dioxide

Around 20-25% of CO2 is transported bound to haemoglobin as carbaminohaemglobin. In tissues where pCO2 is more and pO2 is less, binding of carbon dioxide is favoured and in the alveoli dissociation of carbaminohaemoglobin takes place due to high pO2 and low pCO2. Rest of the CO2 is transported as bicarbonate, which is facilitated by an enzyme called carbonic anhydrase.

Every 100 ml of deoxygenated blood carries 4 ml of CO2 to the alveoli.

Haemoglobin also transports nitric oxide bound to the globin protein. It binds to the thiol groups present in the globin chains.

Carbon monoxide can also bind to haemoglobin and forms the carboxyhaemoglobin complex. Haemoglobin has 250 times higher affinity for carbon monoxide than oxygen. So even the slightest concentration of CO can affect the binding of oxygen. So, inspiring air rich in CO can cause headache, nausea or even unconsciousness. It can block 20% of active binding sites of oxygen in heavy smokers.

Diseases related to Haemoglobin

There can be various reasons for haemoglobin deficiency. Haemoglobin deficiency leads to the lower oxygen-carrying capacity of the blood. It can be due to nutritional deficiency, cancer, kidney failure or any genetic defects.

Higher than normal haemoglobin level is associated with various heart and pulmonary diseases.

Sickle cell anaemia – It is due to a defect in the haemoglobin gene. There is a single nucleotide or point mutation in the 𝞫 globin chain. β€˜GAG’ gets converted into β€˜GTG’ leading to the replacement of glutamic acid by valine at the 6th position.

Thalassemia – It is caused due to less production of haemoglobin. There are two types of thalassemia, 𝜢-thalassemia and 𝞫-thalassemia. It is also caused due to defective genes and severity depends on how many genes are missing or defective.

Haemoglobin level is commonly used as a diagnostic tool. The HbA1c level, i.e. glycosylated Hb or Hb linked with sugar is a marker for average glucose level in the blood of a diabetic patient.

To sum up, haemoglobin is an essential pigment, which is required for oxygen transport and to carry out normal body functions.

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