Biology – Proteins | e-Consult

Haemoglobin is a protein molecule found in red blood cells responsible for transporting oxygen throughout the body. It consists of four polypeptide chains, each linked to a haem group. These chains are arranged in a tetrameric structure, meaning they assemble as a dimer of two alpha (α) globin chains and a dimer of two beta (β) globin chains (α₂β₂). This specific arrangement is crucial for cooperative binding of oxygen.

Each haem group contains a central iron (Fe²⁺) ion, which is essential for oxygen binding. The iron ion is coordinated to four nitrogen atoms within a porphyrin ring structure. This arrangement creates a favourable electronic environment for oxygen to bind reversibly. The iron ion can exist in two oxidation states: Fe²⁺ (ferrous) and Fe³⁺ (ferric). Only the Fe²⁺ state is capable of binding oxygen. The binding of one oxygen molecule to the iron ion causes a conformational change in the haemoglobin molecule, which is transmitted to the other subunits. This is the basis of cooperative binding – the binding of oxygen to one subunit increases the affinity of the other subunits for oxygen, making oxygen uptake more efficient in the lungs and oxygen release more efficient in the tissues.

The quaternary structure of haemoglobin, specifically the interactions between the subunits, is also important. These interactions ensure that the subunits remain associated and that the conformational changes induced by oxygen binding are effectively propagated throughout the entire molecule.