Heme synthesis occurs partly in the mitochondria and partly in the cytoplasm. The process begins in the mitochondria because one of the precursors is found only there. Since this reaction is regulated in part by the concentration of heme, the final step (which produces the heme) is also mitochondrial. Many of the intermediate steps are cytoplasmic. Notice in the diagram of the pathway that there is a branch with no apparent useful end product.
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d-Aminolevulinic acid synthase (ALA synthase).
d-Aminolevulinic acid dehydratase (ALA dehydratase)
Uroporphyrinogen I synthase and uroporphyrinogen III cosynthase
Uroporphyrinogen decarboxylase
Coproporphyrinogen III oxidase
Protoporphyrinogen IX oxidase
Ferrochelatase
Summary of regulation of heme synthesis