2.3.7—Haemoglobin function

Haemoglobin transports oxygen by binding it reversibly to the Fe²⁺ ion in each prosthetic haem group. The haem group provides the oxygen-binding site; the globin protein provides the soluble, organised structure that presents these sites.

• Binding site: Each haem group contains an Fe²⁺ ion that can reversibly combine with one oxygen molecule, forming oxyhaemoglobin.
• Loading and release: Reversible binding allows oxygen to be picked up where it is available and released where it is needed, rather than making oxygen permanently part of the pigment.
• Capacity: A haemoglobin molecule has four haem groups, so it can bind four oxygen molecules when all sites are occupied.
• Why the protein matters: Haemoglobin is soluble in blood, and its globin subunits position the haem groups so oxygen can be carried efficiently.

The haemoglobin structure therefore links composition to function: Fe²⁺ supplies the reversible oxygen-binding site, while the globin assembly carries those sites in a soluble protein. Oxygen binding can also alter the protein’s quaternary arrangement, helping subsequent oxygen molecules bind more readily; this is a structural basis for cooperative loading, not a claim that oxygen is covalently fixed.

The oxygen-binding site is the Fe²⁺-containing haem group, not an arbitrary amino acid in the globin chain. “Reversible” means oxygen can bind and be released; it does not mean the haem group is absent or that haemoglobin permanently stores oxygen.