Structure and Function of 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 B/dl, and in females 13-15 B/dl. Hemoglobin performs
two important biological functions concerned with respiration-

1. Delivery of 0₂  from the lungs to the tissues.
2. Transport of CO₂ and protons from tissues to lungs for excretion.
3.  Hemoglobin gives the red color to blood.
4. Hemoglobin maintains the shape of the red blood cells (RBC).
5. Hemoglobin acts as a buffer.
6.  Hemoglobin interacts with other ligands.
7. Hemoglobin degradation accumulates physiologically active catabolites.

Structure of Hemoglobin:

Hemoglobin (mol. wt. 64,450) is a conjugated protein, containing globin-the apoprotein part-and the heme-the non-protein part (prosthetic group). Hemoglobin is a tetrameric allosteric protein (Fig.10.1).

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 a-chains and two b-chains (a₂b₂). Some authors consider hemoglobin consisting of two identical dimers- (ab)₁. and (ab)₂..Each a -chain contains 141 amino acids while b-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 lX, with iron at its center. Protoporphyrin lX consists of four pyrrole rings to which four methyl, two propionyl and two vinyl groups are attached (Fig.10.2)

Besides the adult hemoglobin (HbA₁) described above, other minor hemoglobins are also found in humans (Table.10.1 ). ln adults a small fraction (< 5%) of hemoglobin, known as HbA₂ is present. HbA₂ is composed of two a and two d (delta) chains. Fetal hemoglobin (HbF) is synthesized during the fetal development and a little of it may be present even in adults. Glycosylated hemoglobin (HbA1C), formed by covalent binding of glucose is also found in low concentration. It is increased in diabetes mellitus which is successfully utilized for the prognosis of these patients.