Phycobiliproteins

Phycobiliproteins are a group of water-soluble pigment–protein complexes that act as light-harvesting antennae in photosynthesis. They are found mainly in cyanobacteria and in red algae (Rhodophyta) and function to absorb light in spectral regions where chlorophylls are less effective, transferring the excitation energy to chlorophyll a in the photosynthetic reaction centers. The chromophores, called phycobilins, are covalently bound to apoproteins and extend the range of light absorption beyond that of chlorophyll.

The major phycobiliproteins are phycoerythrin, phycocyanin, and allophycocyanin. They assemble as alpha and beta subunit–containing complexes and carry covalently attached phycobilin chromophores such as phycoerythrobilin, phycocyanobilin, and related derivatives. In cyanobacteria and many red algae, these proteins form phycobilisomes, large light-harvesting complexes that dock to the thylakoid membranes with a core of allophycocyanin and peripheral rods rich in phycocyanin and phycoerythrin. Spectrally, phycoerythrin emits around 575 nm, phycocyanin around 620–650 nm, and allophycocyanin around 660 nm, enabling efficient energy funneling to chlorophyll a through resonance transfer.

Applications of phycobiliproteins include their use as fluorescent dyes in biotechnology and research. Phycoerythrin, allophycocyanin, and phycocyanin provide bright, narrow emission spectra suitable for fluorescence tagging in flow cytometry, microscopy, and diagnostics. They are valued for high brightness and便利 complementary spectral properties, though they can be sensitive to pH and temperature changes.