Which compound chelates iron and is the immediate precursor of heme formation?

Which compound chelates iron and is the immediate precursor of heme formation?

• A. Porphobilinogen
• B. Protoporphyrinogen IX
• C. Uroporphyrinogen III
• D. Protoporphyrin IX

Answer: (D)

Iron is inserted into a porphyrin ring in the final step of heme synthesis, and the molecule that actually binds that iron just before heme forms is protoporphyrin IX. After protoporphyrinogen IX is oxidized to protoporphyrin IX, ferrochelatase adds ferrous iron to create heme. The other intermediates—porphobilinogen and uroporphyrinogen III—are earlier building blocks in the ring system and do not carry iron in the step that yields heme. Protoporphyrinogen IX is one step earlier and does not yet chelate iron, so it is not the immediate iron-chelating precursor.

Iron is inserted into a porphyrin ring in the final step of heme synthesis, and the molecule that actually binds that iron just before heme forms is protoporphyrin IX. After protoporphyrinogen IX is oxidized to protoporphyrin IX, ferrochelatase adds ferrous iron to create heme. The other intermediates—porphobilinogen and uroporphyrinogen III—are earlier building blocks in the ring system and do not carry iron in the step that yields heme. Protoporphyrinogen IX is one step earlier and does not yet chelate iron, so it is not the immediate iron-chelating precursor.