Fig. 1.

Comparison of the ‘alternative’ and ‘classic’ pathways for the biosynthesis of haem.A. Direct comparison of the two pathways showing the transformation of uroporphyrinogen III into haem via the ‘classic’ and ‘alternative’ routes with the relevant intermediates. The numbering of the tetrapyrrole macrocycle is highlighted in uroporphyrinogen and the labelling of the 4 pyrrole-derived rings (A-D) is shown in haem.B. Comparison of the sequences of D. desulfuricans AhbA and AhbB, which constitute the sirohaem decarboxylase. Conserved residues are highlighted.