Figure 1.

Late steps of tetrapyrrole biosynthesis in cyanobacteria (Synechocystis 6803). Conversion of Coprogen to Protogen is catalyzed by CPO. In Synechocystis 6803, HemF works as the sole CPO under oxic conditions, and HemN works under anoxic conditions. The CPO product Protogen is oxidized to Proto by PPO. In most cyanobacteria, this reaction is performed by HemJ PPO isoform, in others by HemG or HemY. Proto is than channeled to ferrochelatase (FeCH) leading to Protoheme or to magnesium-chelatase (MgCH), first enzyme of the dedicated chlorophyll pathway. Coprogen and Protogen can be spontaneously oxidized to Copro and Proto, respectively.