Fig. 1. Bacterial heme biosynthesis.

The heme synthesis pathway of most bacteria begins with charged glutamyl-tRNA^Glu to form the universal precursor ALA, and coproporphyrinogen III isformed through a series of conserved enzymatic steps. The classical pathway (blue) forms heme through the protoporphyrinogen IX intermediate; most organisms including Gram-negative bacteria and eukaryotes use this pathway. The noncanonical pathway (green), performed by most Gram-positive bacteria, produces heme through the coproporphyrin III intermediate. Shown for each step is the enzyme name followed by the common protein annotation in bold.