Figure 6. The biosynthetic pathway of phycobilins. Bilin synthesis starts from the cleavage of heme b (bilin b) by hemeoxygenase (HO) to yield biliverdin IXa, the central molecule of bilin biosysnthesis pathways. Biliverdin IXα is further reduced by either NAD(P)H- or ferredoxin-dependent bilin reductases (FDBR). FDBR reductase like PebA synthase and PebB synthase transfer two electrons to their substrate to give phycoerythrobilin in two step catalysis. PcyA and PebS catalyze the transfer of four electrons from ferredoxin to two distinct double bonds of biliverdin to yield phycocyanobilin and phycoerythrobilin, respectively in single step. Phycocyanobilin and phycoerythrobilin are further reduced to phycoviobilin and phycourobilin, respectively by PecE/F of Rpc G reductase.
