FIG. 1.

Heme synthesis pathway. Heme biosynthesis is catalyzed by 5-aminolevulonate synthase (ALAS). The first step of condensation of glycine and succinyl CoA to 5-aminolevulinic acid (ALA) occurs in mitochondria. Coproporphyrinogen III (COPRO III) is formed as a first porphyrin and transported back to mitochondria from the cytosol, where is gets converted to protoporphyrinogen IX. The final conversion of protoporphyrinogen IX to protoporphyrin IX (PPIX) and insertion of the iron atom into the ring system generates heme b.