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Structure of Hemoglobin, a Hetero-tetramer Protein
Hemoglobin is a protein containing non-protein prosthetic group ‘heme’ and protein parts, globulin proteins. Heme is an iron-containing prosthetic group composed of a porphyrin ring structure on which one iron atom is located centrally. Hemoglobin is a heterotetramer protein with four subunits of two types. These are four polypeptides, two β-chains, and two α-chains. Each globulin is attached to a heme prosthetic group so there are four globulin chains and four heme groups.
The quaternary structure of hemoglobin is a biologically active and functional biomolecule that is found in the red blood cells involved in the transport of oxygen from lungs to different tissues and carbon dioxide from different tissues to the lungs. This is the protein responsible for the red color of red blood cells due to the presence of an iron-containing heme prosthetic group.
This iron atom is the site (binding site) of attachment of oxygen. Thus, one molecule of hemoglobin can hold four molecules of oxygen. Hemoglobin, when bound with oxygen is called as oxygenated hemoglobin or oxyhemoglobin. Hemoglobin without oxygen is called as deoxygenated hemoglobin.
Transport of Gases By Hemoglobin
Carbon dioxide binding occurs through an amide linkage. The terminal amino group of each globulin chain can be attached to one molecule of carbon dioxide (in the form of bicarbonate ion) and carbon dioxide bound HB is called as carbaminohemoglobin. Carbon monoxide is a potent competitor of oxygen binding to the hemoglobin.
Analog of oxygen, carbon monoxide binds to the hemoglobin in a similar way to the oxygen. Carbon monoxide binds to the iron atom in heme prosthetic group with 200 times more affinity than that of oxygen and is not readily released from hemoglobin as easy as oxygen is released. The carbon monoxide bound hemoglobin is called as carboxyhemoglobin.
Diseases Associated With Hemoglobin
Structure and function of hemoglobin are determined by the amino acids present in the globulin proteins which are encoded by different genetic codons present in their respective genes in the chromosome. Thus, any defect in the gene may lead to the formation of defective globulin production and ultimately there will be the defective hemoglobin formation. There are many types of abnormalities resulting from the defective formation of hemoglobin.
Sickle Cell Anemia
Sickle cell anemia occurs when there is a defective production of β globulin. This is a genetic disease in which β-globulin is defective in which valine residue at the position 6 is replaced by the glutamic acid. Thus, defective beta globulin protein leads to the defective assembly of subunits and thus assembled hemoglobin molecules are aggregated to form fiber-like insoluble products that distort the normal shape of RBCs causing rupture of RBCs to release hemoglobin in the blood. However, people with Sickle Cell Anemia are resistant to malaria because once malarial parasite invades RBCs they eventually die upon rupture of RBCs.
This is another type of abnormality related defect in hemoglobin. This is also a genetic defect in which the biosynthesis of beta globulin is absent or defective and, therefore, there will be no sufficient beta globulin to form a quaternary structure of hemoglobin. The gene HBB is involved in the synthesis of beta chain and is defective one unable to produce functional β globulin.
This is similar to β-Thalassemia. But in this case, genes responsible for the production of the alpha chain of hemoglobin are defective leading to the defective alpha globulin chain production. There are two genes HBA1 and HBA2 which synthesize the alpha 1 and alpha 2 globulin chain of hemoglobin.
There will be a defect in either HBA1 or in HBA2 and the result is a defective alpha globulin production. Again, in this case, similar to β-Thalassemia the biologically active and functional hemoglobin is not formed.