The human heme biosynthetic pathway. The pathway consists of 8 enzymatic steps: 4 localized in mitochondria and 4 in the cytosol. Only the type III isomers of uroporphyrinogen and coproporphyrinogen are metabolized to heme. Heme is exported from mitochondria for incorporation into cellular hemoproteins and, particularly in liver, exerts feedback regulation on 5-aminolevulinic acid synthase (ALAS1).