2.3.6—Haemoglobin quaternary structure

Haemoglobin is a globular protein with quaternary structure because four polypeptide subunits assemble into one protein: two α-globin and two β-globin chains.

• Subunit organisation: The four globin chains form a compact, roughly spherical assembly. Interactions between subunits, including disulfide bonds, help hold the quaternary structure together.
• Within each subunit: Each globin chain contains one prosthetic haem group. Therefore one haemoglobin molecule has four haem groups.
• Iron position: Each haem group contains an Fe²⁺ ion. The iron is located in the haem prosthetic group rather than being part of the polypeptide backbone.
• Shape and solubility: Hydrophobic R groups tend to face inwards and hydrophilic R groups outwards, helping maintain the folded, water-compatible protein.

The four-subunit arrangement positions four haem groups within the globin assembly, while the Fe²⁺ ion in each haem provides the site associated with reversible oxygen binding. Thus the quaternary structure and prosthetic groups create the structural basis for haemoglobin’s oxygen-carrying role; detailed oxygen-transport behaviour is treated separately.

Do not describe haemoglobin as one single polypeptide or as having only one haem group. The structure card establishes four globin subunits and four haem groups; it does not replace the separate card on haemoglobin function.