A quaternary protein
Haemoglobin belongs to a family of closely related molecules found in many different organisms. A haemoglobin molecule is built from several polypeptide subunits and therefore has a quaternary structure.
There are four polypeptide chains in total: two α-globin chains and two β-globin chains.
Haem groups
Each globin subunit contains a haem group. Within each haem group is an iron(II) ion, Fe2+, at the site where an oxygen molecule can bind reversibly.
One haem group can bind one O2 molecule. A complete haemoglobin molecule therefore has four oxygen-binding sites and can carry a maximum of four O2 molecules.
- Four polypeptide subunits make up one haemoglobin molecule.
- Two subunits are α-globins and two are β-globins.
- Each subunit contains one haem group.
- Each haem group contains Fe2+.
- Each haem group can bind one molecule of oxygen.
Hydrophobic R groups are directed towards the interior of the folded protein, helping maintain its compact globular form. Hydrophilic R groups face the surrounding aqueous environment, helping haemoglobin remain soluble.
Exam Tip: When describing haemoglobin structure, connect the four polypeptide subunits with the four haem groups, then state that each haem group can bind one O2 molecule. This links quaternary structure directly to oxygen-carrying capacity.