Haematology
Hemoglobin
Hemoglobin (Hb) is the red blood pigment, exclusively found in erythrocytes (Greek: erythrose—red; kytos—a hollow vessel). The normal concentration of Hb in blood in males is 14–16 g/dl, and in females 13–15 g/dl. Hemoglobin performs two important biological functions concerned with respiration
1. Delivery of O2 from the lungs to the tissues.
2. Transport of CO2 and protons from tissues to lungs for excretion.
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| Figure 01. Hemoglobin |
Structure of hemoglobin
Hemoglobin (mol. wt. 64,450) is a conjugated protein, containing globin—the apoprotein part—and the heme—the nonprotein part (prosthetic group). Hemoglobin is a tetrameric allosteric protein.
- Structure of globin
Globin consists of four polypeptide chains of two different primary structures (monomeric units). The common form of adult hemoglobin (HbA1) is made up of two α-chains and two β-chains (α2β2). Some authors consider hemoglobin consisting of two identical dimers—(αβ) 1 and (αβ) 2. Each α-chain contains 141 amino acids while β-chain contains 146 amino acids. Thus HbA1 has a total of 574 amino acid residues. The four subunits of hemoglobin are held together by non-covalent interactions primarily hydrophobic, ionic and hydrogen bonds. Each subunit contains a heme group.
- Structure of heme
The characteristic red colour of hemoglobin (ultimately blood) is due to heme. Heme contains a porphyrin molecule namely protoporphyrin IX, with iron at its center. Protoporphyrin IX consists of four pyrrole rings to which four methyl, two propionyl and two vinyl groups are attached.
Heme is common prosthetic group present in cytochromes, in certain enzymes such as catalase, tryptophan pyrolase, and chlorophyll (Mg2+). In case of cytochromes, oxidation and reduction of iron is essential for electron transport chain.
Functions of hemoglobin
Hemoglobin is largely responsible for the transport of O2 from lungs to tissues. It also helps to transport CO2 from the tissues to the lungs.
Binding of O2 to hemoglobin
One molecule of hemoglobin (with four hemes) can bind with four molecules of O2. This is in contrast to myoglobin (with one heme) which can bind with only one molecule of oxygen. In other words, each heme moiety can bind with one O2.
Oxygen dissociation curve
The binding ability of hemoglobin with O2 at different partial pressures of oxygen (pO2) can be measured by a graphic representation known as O2 dissociation curve.
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| Oxygen dissociation curve |
Four levels structure of Hemoglobin
• Primary Structure
At its simplest level, hemoglobin is made up of amino acids in chains. These chains are polypeptides that are also stuck to a heme molecule, which is where the oxygen will eventually stick. Hemoglobin is different than other proteins because its individual polypeptides, of which there are four, are called globins instead of simply protein subunits.
• Secondary Structure
The 7 alpha helices and short non helical randomly coiled segments.
• Tertiary Structure
The heme molecule is important for the tertiary bending structure of hemoglobin, as it helps twist the globins into shape by connecting to histidine residues on them. Non polar residues are located deep inside the coils, polar residues are at the outside. The hem entity is buried into the hydrophobic pocket.
• Quaternary Structure
Of the four globins that make up hemoglobin, two are identical and called alpha chains, and the other two are called beta chains and are also identical. They can also be called alpha-globins and beta-globins. The four peptides packed tightly together in a tetrahedral array to form an overall spherically shaped structure that is held together by hydrophobic interaction, some hydrogen bonds and salt bridges.
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