Significations et usages de Phycoerythrin

Phycoerythrin is a red protein from the light-harvesting phycobiliprotein family, present in cyanobacteria, red algae and cryptomonads.^[1]

Like all phycobiliproteins, phycoerythrin is composed of a protein part, organised in a hexameric structure of alpha and beta chains, covalently binding chromophores called phycobilins. In the phycoerythrin family, the phycobilins are: phycoerythrobilin, the typical phycoerythrin acceptor chromophore, and sometimes phycourobilin (marine organisms). Phycoerythrins are the phycobiliproteins that bind the highest number of phycobilins (up to six per alpha-beta subunit dimer).

Absorption peaks in the visible light spectrum are at 495 and 545/566 nm, depending on the chromophores bound and the considered organism. A strong emission peak exists at 575 ± 10 nm. (i.e., phycoerythrin absorbs slightly blue-green/yellowish light and emits slightly orange-yellow light.)

Phycoerythrin is an accessory pigment to the main chlorophyll pigments responsible for photosynthesis. The light energy is captured by phycoerythrin and is then passed on to the reaction centre chlorophyll pair, most of the time via the phycobiliproteins phycocyanin and allophycocyanin.

R-Phycoerythrin, or PE, is useful in the laboratory as a fluorescence-based indicator for the presence of cyanobacteria and for labeling antibodies in a technique called immunofluorescence, among other applications. There are also other types of phycoerythrins, such as B-Phycoerythrin, which has slightly different spectral properties. B-Phycoerythrin absorbs strongly at about 545 nm (slightly yellowish green) and emits strongly at 572 nm (yellow) instead and could be better suited for some instruments. B-Phycoerythrin may also be less "sticky" than R-Phycoerythrin and contributes less to background signal due to non-specific binding in certain applications.

R-Phycoerythrin and B-Phycoerythrin are among the brightest fluorescent dyes ever identified.

  Structural Characteristics

  Phycoerythrobilin is the typical chromophore in phycoerythrin

R-phycoerythrin is predominantly produced by red algae. The protein is made up of at least three different subunits and varies according to the species of algae that produces it. The subunit structure of the most common R-PE is (αβ)₆γ. The α subunit has two phycoerythrobilins (PEB), the β subunit has 2 or 3 PEBs and one phycourobilin (PUB), while the different gamma subunits are reported to have 3 PEB and 2 PUB (γ₁) or 1 or 2 PEB and 1 PUB (γ₂). The molecular weight of R-PE is 250,000 Daltons.

  Spectral Characteristics

  Fluorescent spectra of phycoerythrin

  R-Phycoerythrin Variations

  Excitation and emission profiles for r-phycoerythrin from two different algae. Common laser excitation wavelengths are also noted.

As mentioned above, phycoerythrin can be found in a variety of algal species. As such, there can be variations in the efficiency of absorbance and emission of light required for facilitation of photosynthesis. This could be a result of where in the water column a specific alga resides and a consequent need for greater or less efficiency of the accessory pigments.

With advances in imaging and detection technology which can avoid rapid photobleaching, protein fluorophores have become a viable and powerful tool for researchers in fields such as microscopy, microarray analysis and Western blotting. In light of this, it may be beneficial for researchers to screen these variable R-phycoerythrins to determine which one is most appropriate for their particular application. (RPE tech data) Even a small increase in fluorescent efficiency could reduce background noise and lower the rate of false-negative results.

  References

  External links

• Phycoerythrin at the US National Library of Medicine Medical Subject Headings (MeSH)

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