Haem

Haem, or heme, is an iron-containing prosthetic group found in a wide range of proteins. It consists of a porphyrin ring, specifically protoporphyrin IX, with a single central iron atom. The iron can undergo redox changes, enabling roles in oxygen binding, electron transfer, and catalysis across different biological contexts.

In most haem-containing proteins, the iron is coordinated within the plane of the porphyrin and linked to an axial ligand, typically a histidine residue from the protein. The opposite axial site can bind small ligands such as oxygen, carbon monoxide, or water, enabling functions in gas transport, sensing, and redox chemistry.

Biosynthesis of haem proceeds via the porphyrin pathway, beginning with glycine and succinyl-CoA to form δ-aminolevulinic acid (ALA). ALA synthase is the rate-limiting enzyme. The process spans mitochondria and the cytosol, with iron insertion by ferrochelatase in mitochondria completing haem formation.

Haem is a prosthetic group in hemoglobin and myoglobin for oxygen transport and storage, in cytochromes of the mitochondrial electron transport chain for electron transfer, and in catalases, peroxidases, and other enzymes involved in detoxification and chemical transformation.

Haem is degraded by haem oxygenase to biliverdin, iron, and carbon monoxide. Biliverdin is reduced to bilirubin and excreted via the liver. Recycled iron is stored by ferritin and reused for new haem synthesis; free haem can be toxic and is tightly regulated.

Clinically, disorders of haem synthesis include porphyrias, characterized by the accumulation of porphyrin intermediates. Dietary sources provide heme iron, mainly from animal products, while non-heme iron is found in plants. The spelling haem is the British variant of heme.