A Study on Haemoglobin

Hemoglobin, (haemoglobin) (from the Greek word haîma ‘blood’ + Latin globus ‘ball, sphere’ + -in), abbreviated Hb or Hgb, is an iron-containing oxygen-transport metalloprotein found in almost all vertebrates’ red blood cells (erythrocytes) (with the exception of the fish family Channichthyidae), as well as some invertebrates’ tissues. Blood delivers oxygen from the respiratory organs (such as gills or lungs) to the rest of the body (i.e. tissues). It then releases the oxygen, allowing aerobic respiration to supply energy to an organism’s tasks in the process known as metabolism. In every 100 mL of blood, a healthy human possesses 12 to 20 gms of haemoglobin.

The chromoprotein makes up roughly 96 percent of the dry content (by weight) of red blood cells in mammals, and around 35 percent of the overall content (including water). Hemoglobin has a 1.34 mL O2 per gms oxygen-binding capacity, which boosts total blood oxygen capacity seventy-fold when compared to dissolved oxygen in blood. Up to four oxygen molecules can be bonded (carried) by the mammalian haemoglobin molecule. 

Hemoglobin also transports other gases: it transports some of the body’s respiratory carbon dioxide (approximately 20–25 percent of total) as carbaminohemoglobin, in which CO2 is linked to the heme protein. Nitric oxide, a key regulatory chemical, is linked to a thiol group in the globin protein and released at the same time as oxygen. Hemoglobinemia is a medical disorder in which the blood plasma contains too much haemoglobin. This is a side effect of intravascular hemolysis, a kind of anaemia in which haemoglobin separates from red blood cells.

Structure of Haemoglobin

Many multi-subunit globular proteins, together with haemoglobin, possess a quaternary structure. In haemoglobin, the majority of the amino acids form alpha helices, which are joined by short non-helical segments. The helical parts of this protein are stabilised by hydrogen bonds, which generate attractions within the molecule, causing each polypeptide chain to fold into a certain form. Hemoglobin’s quaternary structure is composed of four subunits that are arranged in an approximately tetrahedral pattern.

The haemoglobin molecule is composed of four globular protein subunits in the majority of vertebrates. A protein chain is closely linked to a non-protein prosthetic heme group in each subunit. In a globin fold arrangement, each protein chain is organised into a collection of alpha-helix structural segments that are joined together. This arrangement is the same folding motif found in other heme/globin proteins such as myoglobin, thus the name. This folding pattern has a pocket that links the heme group tightly.

A heme group is also composed of an iron (Fe) ion contained in a porphyrin, which is a heterocyclic ring. The iron ion is attached in the middle of this porphyrin ring, which is made up of four pyrrole molecules cyclically connected together (via methine bridges). The four nitrogen atoms in the middle of the ring, which all lie in one plane, coordinate with the iron ion, which is the location of oxygen binding. The N atoms of the imidazole ring of F8 histidine residue (also referred as the proximal histidine) below the porphyrin ring bind the heme tightly (covalently) to the globular protein. A coordinate covalent bond in the sixth position can reversibly bind oxygen, forming the octahedral group of six ligands. Hemoglobin’s reversible bonding along with oxygen is one of the important reasons it is so effective in transporting oxygen throughout the body. One oxygen atom attaches to Fe and the other protrudes at an angle, forming a “end-on bent” shape. When oxygen is not bound, the position is filled with a very weakly linked water molecule, resulting in a deformed octahedron.

The most prevalent haemoglobin type in adult humans is haemoglobin A, a tetramer (protein with four subunits) that consists of two non-covalently bonded subunits with 141 and 146 amino acid residues, respectively. This is symbolised by the number 22. The subunits have similar structural similarities and are roughly the same size. The tetramer has a total molecular weight of around 64,000 daltons (64,458 g/mol), with each member having a molecular weight of about 16,000 daltons. As a result, 1 g/dL equals 0.1551 mmol/L. Hemoglobin A is the haemoglobin molecule that has been studied the most. Hydrogen bonds, salt bridges and the hydrophobic effect bind the four polypeptide chains together.

Synthesis of Haemoglobin

Hemoglobin (Hb) is produced through a complicated process. In juvenile red blood cells, the heme component is made in a sequence of processes in the mitochondria and cytosol, whereas the globin protein parts are made by ribosomes in the cytosol. Hb production continues in the cell throughout its early development in the bone marrow, from the proerythroblast through the reticulocyte. The nucleus is lost in human red blood cells at this time, but not in birds or many other animals. In mammals, leftover ribosomal RNA allows Hb to be synthesised even after the nucleus is lost, until the reticulocyte loses its RNA shortly after entering the vasculature (this hemoglobin-synthetic RNA thus gives the reticulocyte its reticulated name and appearance).

Levels of Haemoglobin

The measurement of haemoglobin concentration is one of the most common blood tests, usually as part of a full blood count. It’s usually tested before or after a blood donation, for example. The results are given in g/L, g/dL, or mol/L units. Although the latter units are less commonly employed due to uncertainty about the polymeric state of the molecule, 1 g/dL equals around 0.6206 mmol/L.

This conversion factor is more frequent for haemoglobin concentration in blood, as it uses a single globin unit molecular weight of 16,000 Da. The conversion factor 0.155, which utilises the tetramer weight of 64,500 Da, is more commonly used for MCHC (mean corpuscular haemoglobin concentration). The following are the normal levels:

• Women: 12.1 to 15.1 g/dL (121 to 151 g/L, or 7.51 to 9.37 mmol/L)

• Men: 13.8 to 18.0 g/dL (138 to 180 g/L, or 8.56 to 11.17 mmol/L)

• Pregnant women: 11 to 14 g/dL (110 to 140 g/L, or 6.83 to 8.69 mmol/L) (9.5 to 15 usual value during pregnancy)

• Children: 11 to 16 g/dL (110 to 160 g/L, or 6.83 to 9.93 mmol/L)

Anemia occurs when the concentration falls below the usual range. The size of red blood cells, which carry haemoglobin in vertebrates, is used to classify anemias. If red cells are small, the anemia is called “microcytic,” “macrocytic” if they are large and “normocytic” if they are not. Hematocrit, or the percentage of blood volume taken up by red blood cells, is usually three times the haemoglobin concentration in g/dL. For example, a haemoglobin concentration of 17 g/dL corresponds to a hematocrit of 51 percent.

Conclusion

Hemoglobin deficiency can be produced by a decrease in the number of haemoglobin molecules in the blood, as in anaemia, or by a decrease in each molecule’s ability to bind oxygen at the same partial pressure of oxygen, as in hypoxia. 

Both can be caused by hemoglobinopathies (genetic abnormalities that result in aberrant haemoglobin molecule structure). In any case, haemoglobin shortage reduces the capacity of the blood to deliver oxygen. Although both are causes of hypoxia, haemoglobin deficiency is distinct from hypoxemia, which is defined as a reduction in the partial pressure of oxygen in the blood (insufficient oxygen supply to tissues). Loss of blood, nutritional deficiencies, bone marrow disorders, chemotherapy, kidney failure, or atypical haemoglobin are all major reasons for low haemoglobin (like those of sickle-cell disease).