Biosynthesis of Haem

All tetrapyrroles are biosynthesised from the same precursor 5-aminolaevulinic acid (ALA). In plants and most bacteria ALA is made from glutamate; this pathway is known as the C5 pathway. In animals, yeasts and purple photosynthetic bacteria ALA is made from glycine and succinyl CoA. This reaction is known as the Shemin pathway. The rest of the biosynthetic pathway to haem is common to all organisms.

Pathway from Gly or Glu to HMB

Enzymes: A, Glutamyl tRNA reductase; B, Glutamate semialdehyde aminomutase; C, 5-Aminolaevulinic acid synthase; D, Porphobilinogen synthase (ALA dehydratase); E, Hydroxymethylbilane synthase (PBG deaminase).

Pathway from HMB to haem

Enzymes: F, Uroporphyrinogen III synthase (cosynthetase); G, Uroporphyrinogen decarboxylase; H, Coproporphyrinogen oxidase; I, Protoporphyrinogen oxidase; J, Ferrochelatase.

The pathways to Vitamin B12, Sirohaem, Factor F430, and Haem d1 branch from the Haem biosynthetic pathway at Uro'gen III

Alternative view of Haem Biosynthesis