Phycoerythrobilin synthase (, PebS) is an enzyme with systematic name (3Z)-phycoerythrobilin:ferredoxin oxidoreductase (from biliverdin IX alpha). It catalyses the following chemical reaction
This enzyme, from a cyanophage infecting oceanic cyanobacteria of the Prochlorococcus genus carries out in one step the reduction of biliverdin to (3Z)-phycoerythrobilin. The same overall reaction results from the action of and in oxygenic organisms.