Structure of haemoglobin
Hb = 4 polypeptide subunits + 4 haem groups + 4 Fe²⁺ → can bind 4 O₂.
Haemoglobin (Hb) is the red, oxygen-carrying protein of red blood cells. It is a quaternary protein: four polypeptide subunits are held together by hydrogen bonds, ionic interactions and hydrophobic forces.
In adult haemoglobin (HbA):
- Two α-globin chains (each ~141 amino acids).
- Two β-globin chains (each ~146 amino acids).
Each subunit folds into a globular tertiary structure (the 'globin fold', containing several α-helices) and cradles a prosthetic group called haem.
A haem group is a porphyrin ring (an organic molecule of four pyrrole rings) containing a central iron(II) ion (Fe²⁺). The Fe²⁺ can reversibly bind one O₂ molecule:
So each Hb molecule has 4 haem groups, 4 Fe²⁺ ions and can carry up to 4 O₂. A single RBC contains about 250 million Hb molecules, so each RBC can carry roughly 10⁹ O₂.
When O₂ leaves Hb at the tissues, the protein returns to its 'deoxy' conformation. The Fe²⁺ remains in the +2 oxidation state throughout — if it were oxidised to Fe³⁺ (forming methaemoglobin), it could no longer carry O₂.
- Quaternary protein: 4 polypeptide chains.
- HbA = 2α + 2β; 4 haem groups; 4 Fe²⁺; up to 4 O₂.
- Oxygen binding is REVERSIBLE.
- Carbon monoxide binds the same Fe²⁺ — irreversibly — and is therefore toxic.