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. Author manuscript; available in PMC: 2020 Dec 1.
Published in final edited form as: Curr Opin Struct Biol. 2019 Feb 22;59:19–28. doi: 10.1016/j.sbi.2019.01.006

Figure 1. Multiple biosynthesis pathways leading to heme b.

Figure 1.

Eukaryotes and gram-negative bacteria share the canonical pathway (blue). Gram-positive bacteria (purple) and denitrifying/sulfate reducing bacteria and archaea (orange) have distinct pathways that branch off following the coproporphyrinogen III (copro’gen) intermediate. The oxidation of porphyrinogens can use O2 as a 2e-/2H+ acceptor, generating H2O2 as a byproduct (i.e., steps catalyzed by CgdC, PgoX); alternatively, NAD(P)+ can serve as a 2e-/H+ acceptor in the presence of an active site acid (PgdH1, PgdH2). CgdH is a radical SAM-dependent enzyme. The electron acceptors for CgdH and CgoX have not been experimentally verified.