Haemoglobin (Hb) is the protein in red blood cells that carries oxygen. It is a globular, conjugated protein with a quaternary structure.Each haemoglobin molecule is made of four polypeptide chains: two α-globin chains and two β-globin chains. Each chain is folded into a compact shape and is attached to a haem group, which is a non-protein prosthetic group. Each haem group contains an iron ion (Fe2+).
Each Fe2+ ion can combine reversibly with one oxygen molecule (O2), so one haemoglobin molecule can carry four oxygen molecules (eight oxygen atoms). Haemoglobin combined with oxygen is called oxyhaemoglobin:Hb + 4O2 ⇌ Hb(O2)4 The reaction is reversible: oxygen binds in the lungs and is released in respiring tissues.Its structure makes haemoglobin suited to its role:• as a globular protein, it is soluble in the cytoplasm of red blood cells, with hydrophilic R groups on the outside and hydrophobic R groups inside
• the four subunits interact, so the binding of one oxygen molecule changes the shape of the molecule and affects how easily the others bind
• each red blood cell contains around 270 million haemoglobin molecules, so blood carries far more oxygen than could dissolve in plasma.Worked example: a red blood cell contains 2.7 × 108 haemoglobin molecules. When fully saturated it carries 4 × 2.7 × 108 = 1.08 × 109 oxygen molecules.
Each Fe2+ ion can combine reversibly with one oxygen molecule (O2), so one haemoglobin molecule can carry four oxygen molecules (eight oxygen atoms). Haemoglobin combined with oxygen is called oxyhaemoglobin:• the four subunits interact, so the binding of one oxygen molecule changes the shape of the molecule and affects how easily the others bind
• each red blood cell contains around 270 million haemoglobin molecules, so blood carries far more oxygen than could dissolve in plasma.Worked example: a red blood cell contains 2.7 × 108 haemoglobin molecules. When fully saturated it carries 4 × 2.7 × 108 = 1.08 × 109 oxygen molecules.