Figure 1.

Summary of the heme synthetic pathway, highlighting the enzymatic defects associated with the porphyrias. The heme synthetic pathway involves eight enzymes, four of which are active in the mitochondria and four of which are active in the cytoplasm. The pathway is initiated and completed in the mitochondria. Intermediate steps in the cytoplasm begin with the activity of ALA dehydratase, also known as PBG synthase. Open arrows indicate progression through the pathway. Deficiency (indicated by blocked red arrows) in any of the eight enzymes involved in the pathway may contribute to the development of acute or chronic hepatic porphyrias or erythropoietic porphyrias, as shown in red. Abbreviations: Ac, acetate; CoA, coenzyme A; Copro’gen, coproporphyrinogen; Pr, propionate; proto’gen, protoporphyrinogen; Uro’gen, uroporphyrinogen; and Vi, vinyl. (From 43, used with permission of the authors and publisher.)