phycobilin

Phycobilins are a class of water-soluble pigment molecules found in cyanobacteria and some algae. They function as chromophores that are covalently bound to protein subunits to form phycobiliproteins, which assemble into large light-harvesting complexes called phycobilisomes that associate with photosynthetic membranes.

The main phycobiliproteins are phycocyanin, phycoerythrin, and allophycocyanin. Each contains specific bilin chromophores: phycocyanin carries phycocyanobilin, phycoerythrin contains phycoerythrobilin and/or phycourobilin, and allophycocyanin contains allophycocyanobilin. These pigments confer distinctive colors—blue for phycocyanin, red to orange for phycoerythrin, and reddish hues for allophycocyanin—and extend the usable light spectrum for photosynthesis.

Phycobilins broaden the range of light absorption by capturing wavelengths that chlorophylls absorb less efficiently, particularly in the green to orange region. The absorbed energy is efficiently transferred through the phycobiliprotein network to chlorophyll a, fueling photosynthetic reactions.

Biosynthesis involves bilin chromophores derived from biliverdin, produced via heme degradation, followed by enzymatic modification to form PCB, PEB, and PUB. These chromophores are covalently attached to specific cysteine residues on phycobiliprotein apoproteins by phycobilin lyases.

Ecologically, phycobiliproteins are characteristic of cyanobacteria and red algae, where they support light harvesting in chloroplast-containing organisms. Green algae and land plants lack phycobilisomes and rely more on chlorophylls and carotenoids.

In addition to their biological role, phycobiliproteins have notable applications as natural colorants and fluorescent labels, with phycoerythrin and phycocyanin used in biotechnology, diagnostics, and food industries.