Hemoglobin: Structure, Function and its Properties
- Marcello Malpighi described the RBCs in 1665.
- Felix Hope Seyler in 1862 isolated pure Hemoglobin.
- Christian Bohr in 1904 discovered that hemoglobin is the transporter of oxygen.
- In 1912 Kutster established the structure of hemoglobin.
- Hans Fischer synthesized heme in laboratory in 1920 (Nobel prize, 1930).
- In 1945, Linus Pauling (Nobel prize, 1954) described abnormal hemoglobins.
- Max Perutz (Nobel Prize, 1962) studied the 3D structure of Hemoglobin.
What is Hemoglobin?
Hemoglobin is a globular heme protein in vertebrate red blood cells and in the plasma of many invertebrates that carries oxygen and carbon dioxide; heme group binds oxygen and carbon dioxide and as well as imparts red color to the blood; also spelt as hemoglobin.
- Why Proteins are Very Important? How to Explain?
- Peptide bonds : Backbone of the Proteins
- What is Amino acid and its Structural Chemistry?
Introduction of Hemoglobin Protein:
- Red colored conjugated protein (made up of heme and Globin) present inside the RBC
- Normal Hb% in adult male is 14 to 16 gm.
- Approximately 6.25 gm of Hb are synthesized and destroyed every day.
- Heme structure does not vary from species to species.
- It is the basic protein globin that varies in amino acid composition and sequence in different species.
- Globin is rich in Histidine and lysine.
- Hb binds O2 transports O2 and delivers the same to tissues.
- Hb binds CO2, a waste product of metabolism.
- 2-3 BPG,produced in RBC by Rapport-Leubering shunt stabilizes Hb conformation at quaternary level and enhances dissociation of O2 from Hb at tissue site.
- Cyanide combines with methamoglobin to form cyanomethemoglobin which is non-toxic.
- Study of Hb structure gives an insight into the molecular basis of hemoglobinopathies.
Structure of Hemoglobin:
- Heme is iron porphyrin compound. Porphyrin is a tetrapyrrole structure.
- Ferrous iron occupies the center of the porphyrin ring and establishes linkages with all the four nitrogens of all the pyrrole rings.
- It is also linked to nitrogen of imidazole ring of histidine present in globin part.
- Globin part is made of four polypeptide chains, to identical α-chains and two identical β-chains in normal adult hemoglobin.
- Each chain contains a “heme” in the so called ‘heme pocket’. So one Hb molecule possess four heme units.
- Hb molecule contains hydrophobic amino acids inside and hydrophilic ones on the surface.
- Heme pockets of α-subunits are of just adequate size to give entry to an O2 molecule. Entry f O2 into heme pockets of β-subunits is blocked by a valine residue.
- Varieties of normal human Hb are
- Hb-A1 (two α-chains and β-chains)
- HbF (two α-chains and ¥-chains)
- Hb-A2 (two α-chains and delta-chains)
- Embyonic Hb (two α-chains and €-chains)
- Hb-A3 (Altered from Hb-A found in old red cells)
- HbA1C (Glycosylated Hb, present in concentration of 3-5% of total Hb). In diabetes mellitus it is increased to 6 to 15%.
- Hemin (Hematin hydrochloride)
- Methemoglobin (Oxidation product of Hb; produced by drugs like nitrites, phenacetin, sulphonamide drugs; lack of enzymes like methamoglobin reductase, diaphorase-I, HbM.
- Toxic effects of Met Hb are : cyanosis, fatigue, tachycardia, tachypnea, depression.
- Methemalbumin: (Combination of hematin with albumin),not present in normal adult blood, when present indicates intravascular hemolysis, detected by Schumm’s test.
Combination of Hb with gases:
- Oxyhemoglobin: (loose and reversible combination with O2); 1.34ml O2 combines with each gm of Hb.
- One mole of Hb can maximally combine with four mols of O2. Partial pressure of O2 favors oxygenation.
- Partial pressure of CO2 favors dissociation. Acidosis favors liberation of O2.
- Oxygenated Hb is in relaxed state i.e, ‘R’ state. R state characterized by removal of valine residue from heme pocket of β-subunit; broken salt bridges; can not bind BPG; FFe++ comes in plane of porphyrin ring; Heme-Heme interaction increases te affinity to O2; Histidines ofβ-chains release protons (H+).
- Deoxygenated Hb: In “T” form i.e. Taut form, salt bridges plenty and intact, valine residue covers heme pocket of β-chain and does not allow entry of O2; can bind BPG; F++ out of the plane of porphyrin ring; low affinity to O2; β-chain histidine residue protonated (H+ added).
- Carboxyhemoglobin (Hb+ carbo monoxide): Firmer combination, not reversible, affinity of Hb to CO in 210 times more than )2; inhibits cytochrome oxidase of electron transport chain.
- Carbamino Hb (Hb + CO2):
- Sulfhaemoglobin: Greenish pigment; formed when H2S reacts with Oxy-Hb, seen in severe constipation, certain types of bacteria.
More than 30 abnormal types descries, differentiated by their characteristic electrophoretic mobilities, generally transmitted; are due to single mutant gene; Two types –
- Due to mutation of structural gene. E.g: HbS, HbM, HbC, HbD (Punjab) etc.
- Due to mutation in regulator gene. E.g: Thalassemias.
- Detection by Finger Printing techniques and Hybridization
Effects of abnormal Hb:
- Changed Red cell morphology
- Haemolytic anemia, Jaundice
- High O2 affinity E.g:Hb chesapea ke, Hb-Rainier
- Interfere with mRNA formation e.g.: Hb constant spring
In both β-chains glutamic acid in 6th position is replaced by Valine. This results in increase of viscosity and precipitation of HbS. Hence the crescent or sickle shaped RBC of more fragile nature. However such RBCs show increased resistance to malaria but more vulnerable to salmonella infections.
- α-chain Thalassemia: Synthesis of α-chains are replaced. E.g.: HbH (β4) and Hb-Barts (¥4).
- β-chain Thalassemia (Thalassemia major): Synthesis of β-chain is repressed. As a result increased synthesis of HbA2 or HbF.