Structure and Function of Hemoglobin
Hemoglobin (Hb) is the red blood pigment, exclusively found in erythrocytes. 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.
Structure of Hemoglobin
Max Perutz described the molecular structure of haemoglobin in 1959. Hemoglobin has a quaternary structure as it is made up of four polypeptide chains; two α-chains (141 amino acids each) and two β-chains (146 amino acids each). The formula is represented as α2β2.
The Heme Group: The Chemical Heart
Each of the four polypeptide chains is associated with a Heme prosthetic group. From a chemical perspective, Heme is an iron-porphyrin complex. It consists of:
- Protoporphyrin IX: A tetrapyrrole ring system.
- Iron (Fe2+): Located at the center of the porphyrin ring.
Function of Hemoglobin
1. Oxygen Transport (Oxygenation)
Hemoglobin binds with oxygen to form Oxyhemoglobin. This is not an oxidation reaction (the iron remains in the +2 state), but an oxygenation process.
The binding is "cooperative"—once the first oxygen molecule binds, the structure of the entire protein shifts (from a Tense (T) state to a Relaxed (R) state), making it much easier for the remaining three oxygens to bind.
2. The Bohr Effect (Transport of CO2 and H+)
In the tissues, where CO2 concentration is high and pH is low, Hemoglobin's affinity for oxygen decreases. This causes Hb to release O2 to the cells. Simultaneously, Hb binds to H+ ions and CO2 (forming carbaminohemoglobin) to carry them back to the lungs.
3. Buffering Action
Due to the presence of many histidine residues, hemoglobin acts as a powerful chemical buffer in the blood, helping to maintain a stable pH despite the constant influx of acidic CO2 from metabolic activities.