phycobilines

Phycobilins are a class of linear tetrapyrrole pigments that serve as chromophores in phycobiliproteins, the light-harvesting proteins of cyanobacteria, red algae, and some other groups. They are water-soluble pigments derived from bilins and include phycoerythrobilin, phycocyanobilin, and allophycocyanobilin. Phycobilins are covalently bound to apoproteins as part of phycobiliproteins through thioether linkages formed at cysteine residues by specialized lyases.

Organized into phycobilisomes attached to the photosynthetic membranes, the phycobiliproteins assemble in rods and a core to form antenna complexes. The pigments absorb light in spectral regions where chlorophyll a is less effective, notably in the green to orange wavelengths, and relay excitation energy via resonance energy transfer to chlorophyll a in the photosystems.

Biosynthesis begins with heme; heme oxygenase releases biliverdin, which is then enzymatically reduced to phycoerythrobilin, phycocyanobilin, and allophycocyanobilin that are subsequently attached to specific cysteine residues on apoproteins by bilin lyases.

Spectral characteristics differ among phycobilins: phycoerythrobilin absorbs in the green region; phycocyanobilin absorbs in the orange-red region; allophycocyanobilin absorbs in the red region.

Collectively they enhance light-harvesting efficiency in aquatic environments and contribute to the overall photosynthetic performance of the organisms that contain them. Phycobiliproteins and their phycobilin chromophores are studied for their unique spectral properties and their applications in biotechnology, including fluorescent labeling.