Figure 1.

Schematic of the human heme biosynthetic pathway and the porphyrias resulting from the indicated enzymatic defect. Heme is synthesized from succinyl CoA and glycine through eight enzymatic steps. The heme biosynthetic enzymes are italicized, and the resulting porphyrias are shown in boxes. Note that there are two isozymes for the first enzyme, 5-aminolevulinic acid synthase (ALAS): a housekeeping enzyme, ALAS1, encoded by a gene that is under negative feedback regulation of the end product heme, and an erythroid-specific enzyme, ALAS2, that is regulated by iron response proteins and erythroid transcription factors.