[Unusual pathways and environmentally regulated genes of bacterial heme biosynthesis]

The majority of bacteria, all investigated archaea and plants form the general precursor molecule of all tetrapyrroles 5-aminolevulinic acid by a unique transformation of transfer RNA bound glutamate. Only the alpha-group of the proteobacteria, mammals and yeast synthesize 5-aminolevulinic acid via the well known condensation of succinyl-CoA and glycine. The late steps in tetrapyrrole biosynthesis also contain alternative biosynthetic pathways for the formation and oxidative decarboxylation of coproporphyrinogen III. Unusual enzymatic reactions including the utilization of two substrate molecules as cofactor by the porphobilinogen deaminase and the formation of a spiro intermediate are involved in the formation of uroporphyrinogen III. The biosynthesis of hemes in bacteria is strictly regulated at the formation of 5-aminolevulinic acid and the oxidative decarboxylation of coproporphyrinogen III. The involved heme biosynthetic genes are regulated by the environmental concentrations of oxygen, iron, nitrate, growth phase and intracellular levels of heme. The current knowledge on the various enzymatic reactions and gene regulatory mechanisms is reviewed.