phycourobilin

Phycourobilin (PUB) is a linear, open-chain tetrapyrrole pigment that functions as a bilin chromophore in phycoerythrin, a light-harvesting complex found in cyanobacteria and red algae (rhodophytes). Along with phycoerythrobilin (PEB) and phycocyanobilin (PCB), PUB helps tune the absorption properties of phycoerythrin.

PUB absorbs blue-green light and transfers excitation energy to chlorophyll within phycobiliproteins, contributing to photosynthesis under light environments where blue light is prevalent. The relative abundance of PUB in phycoerythrin can vary with light quality, an adjustment known as chromatic acclimation that optimizes light capture.

Biosynthesis begins with biliverdin IXα, which is reduced by bilin reductases as part of the phycobilin pathway to produce PUB. The chromophore is covalently bound to phycobiliprotein apoproteins by lyases, forming PUB-containing phycoerythrin that participates in energy transfer to photosystems.

Distribution includes many cyanobacteria and some red algae. PUB-containing phycoerythrin broadens the spectral range of light harvesting, illustrating the diversity of strategies cyanobacteria and rhodophytes use to adapt to varying light environments.