Figure 1.

Heme biosynthesis. A: The first cyclic tetrapyrrole uroporphyrinogen III is formed from the precursor 5-aminolevulinic acid in three enzymatic steps via the intermediates porphobilinogen and pre-uroporphyrinogen. Depending on the organism, ALA is either synthesized by condensation of glycine with succinyl-CoA or from tRNA-bound glutamate via glutamate-1-semialdehyde. B: Uroporphyrinogen III is converted into heme in four consecutive enzymatic steps via the intermediates coproporphyrinogen III, protoporphyrinogen IX, and protoporphyrin IX. Structures of all heme biosynthesis enzymes have been determined with the exception of oxygen-independent PPO (n.d., structure not determined).