FIG 1.

Tetrapyrrole biosynthetic pathways in prokaryotes. An outline of prokaryotic tetrapyrrole biosynthetic pathways is shown with the conserved three-enzymatic-step core path from ALA (5-aminolevulinate) to uroporphyrinogen III boxed in blue. Among prokaryotes, one finds one of two generally nonoverlapping paths to ALA. The synthesis of ALA from glutamyl-tRNA (5-carbon pathway) is most common, with a limited number of bacteria possessing the enzyme AlaS to form ALA from glycine and succinyl-CoA (4-carbon pathway). The 4-carbon pathway to ALA is found only in bacteria that possess the protoporphyrin-dependent pathway branch. Uroporphyrinogen III is the precursor to Ni-containing F₄₃₀ in methanogens, Co-containing cobalamin, and Fe-containing siroheme and heme d₁ as well as coproporphyrinogen, which is the common precursor of Fe-containing protoheme and Mg-containing chlorophylls. Abbreviations: PBG, porphobilinogen; HMB, hydroxymethylbilane; uro'gen III, uroporphyrinogen III.