Phycoerythrin

Phycoerythrin is a water-soluble phycobiliprotein pigment found in cyanobacteria and red algae. It is a component of the phycobilisome light-harvesting complexes that capture light energy and transfer it to chlorophyll a during photosynthesis. The pigment broadens the range of light absorbed by photosynthetic apparatus, especially in green to yellow-green regions, enabling efficient energy capture in certain aquatic environments.

Phycoerythrin consists of protein subunits with covalently attached chromophores, primarily phycoerythrobilin and phycourobilin. These chromophores are linked to specific cysteine residues on the alpha and beta subunits, forming a pigment-protein complex. The protein commonly assembles as oligomeric structures, such as trimers or hexamers of alpha and beta subunits, arranged within the larger phycobilisome assembly.

Natural phycoerythrin is classified into several forms, with the major commercially relevant types being R-phycoerythrin (R-PE) and B-phycoerythrin (B-PE). R-PE contains both phycoerythrobilin and phycourobilin chromophores and is noted for high fluorescence brightness, while B-PE has a different chromophore composition and spectral profile. In practice, R-PE is widely used as a fluorescent label in biomedical research.

Spectral properties of phycoerythrin include strong absorption in the green to yellow-green region and emission in the red to orange-red region, with emission commonly around 575–580 nm when excited near 488–532 nm. This makes it a popular fluorophore for flow cytometry and fluorescence microscopy, frequently conjugated to antibodies or used as a reporter protein.

Sources include red algae and certain cyanobacteria; commercially, phycoerythrin is harvested, purified, and formulated for laboratory use. Safety considerations follow standard protein handling guidelines, with typical sensitivity to light and pH affecting stability.