phycobilin

American Heritage Dictionary:

phy·co·bi·lin

(fī'kō-bī'lĭn) pronunciation

n.
Any of a group of water-soluble proteinaceous pigments that occur in red algae and cyanobacteria.

[PHYCO- + Latin bīlis, bile + -IN.]

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Phycobilin

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Any member of a class of intensely colored pigments found in some algae that absorb light for photosynthesis. Phycobilins are structurally related to mammalian bile pigments, and they are unique among photosynthetic pigments in being covalently bound to proteins (phycobiliproteins). In at least two groups of algae, phycobiliproteins are aggregated in a highly ordered protein complex called a phycobilisome.

Phycobilins occur only in three groups of algae: cyanobacteria (blue-green algae), Rhodophyta (red algae), and Cryptophyceae (cryptophytes), and are largely responsible for their distinctive colors, including blue-green, yellow, and red. Five different phycobilins have been identified to date, but the two most common are phycocyanobilin, a blue pigment, and phycoerythrobilin, a red pigment. In the cell, these pigments absorb light maximally in the orange (620-nanometers) and green (550-nm) portion of the visible light spectrum, respectively. A blue-green light (495-nm) absorbing pigment, phycourobilin, is found in some cyanobacteria and red algae. A yellow light (575-nm) absorbing pigment, phycobiliviolin (also called cryptoviolin) is apparently found in all cryptophytes but in only a few cyanobacteria. A fifth phycobilin, which absorbs deep-red light (697 nm), has been identified spectrally in some cryptophytes, but its chemical properties are unknown. See also Cryptophyceae; Cyanophyceae; Rhodophyceae.

Phycobilins are associated with the photosynthetic light-harvesting system in chloroplasts of red algae and cryptophytes and with the photosynthetic membranes of cyanobacteria, which lack chloroplasts. Phycobilins are covalently bound to a water-soluble protein that aggregates on the surface of the photosynthetic membrane. All other photosynthetic pigments (for example, chlorophylls and carotenoids) are bound to photosynthetic membrane proteins by hydrophobic attraction. Phycobiliprotein can constitute a major fraction of an alga. In some cyanobacteria, phycobiliproteins can account for more than 50% of the soluble protein and one-quarter of the dry weight of the cell. See also Cell plastids.

Phycobilins are photosynthetic accessory pigments that absorb light efficiently in the yellow, green, orange, or red portion of the light spectrum, where chlorophyll a only weakly absorbs. Light energy absorbed by phycobilins is transferred with greater than 90% efficiency to chlorophyll a, where it is used for photosynthesis. See also Chlorophyll; Photosynthesis.

Oxford Dictionary of Biochemistry:

phycobilin

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any bilin found in phycobiliproteins. The two most common are phycocyanobilin and phycoerythrobilin. They are linear tetrapyrroles or open tetrapyrroles, i.e. chains of four pyrroles, the terminal pyrroles not being linked to each other to form the cyclic tetrapyrrole structure. The structure of phycocyanobilin is shown; phycoerythrobilin is similar, except that the ethyl group on ring A is −CH=CH₂, and the methyne group between rings A and B is a methylene group.

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Wikipedia on Answers.com:

Phycobilin

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Phycobilins (from Greek: φύκος (phykos) meaning "alga", and from Latin: bilis meaning "bile") are chromophores (light-capturing molecules) found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and higher plants). They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for photosynthesis.

The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a. Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, while those at greater depth often contain more of the phycobilins that can capture green light, which is relatively more abundant there.

The phycobilins fluoresce at a particular wavelength, and are, therefore, often used in research as chemical tags, e.g., by binding phycobiliproteins to antibodies in a technique known as immunofluorescence.

Types

There are four types of phycobilins:

Phycoerythrobilin, which is red
Phycourobilin, which is orange
Phycoviolobilin (also known as phycobiliviolin) found in phycoerythrocyanin
Phycocyanobilin (also known as phycobiliverdin), which is blue.

They can be found in different combinations attached to phycobiliproteins to confer specific spectroscopic properties.

Structural relation to other molecules

In chemical terms, phycobilins consist of an open chain of four pyrrole rings (tetrapyrrole) and are structurally similar to the bile pigment bilirubin, which explains the name. (Bilirubin's conformation is also affected by light, a fact used for the phototherapy of jaundiced newborns.) Phycobilins are also closely related to the chromophores of the light-detecting plant pigment phytochrome, which also consist of an open chain of four pyrroles. Chlorophylls are composed of four pyrroles as well, but there the pyrroles are arranged in a ring and contain a metal atom in the center.

References

O'Carra P, Murphy RF, Killilea SD (May 1980). "The native forms of the phycobilin chromophores of algal biliproteins. A clarification". Biochem. J. 187 (2): 303–9. PMC 1161794. PMID 7396851. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1161794.

Types of tetrapyrroles

Bilanes
(Linear)

Bilirubin · Biliverdin · Stercobilinogen · Stercobilin · Urobilinogen · Urobilin

Phytobilins

Phytochromobilin

Phycobilins	Phycoerythrobilin · Phycocyanobilin · Phycourobilin · Phycoviolobilin

Macrocycle

Corrinoids

Methylcobalamin · Adenosylcobalamin · Cyanocobalamin

Porphyrins

Protoporphyrins	Protoporphyrin IX · Heme (b, c, a, o) · Zinc protoporphyrin · Magnesium protoporphyrin

Phytoporphyrins	Chlorophyll c1 · Chlorophyll c2 · Protochlorophyllide

Reduced
porphyrins

Porphyrinogens	Uroporphyrinogen (I, III) · Coproporphyrinogen (I, III) · Protoporphyrinogen IX

Chlorins	Chlorophyllide (a, b) · Chlorophyll (a, b) · Phaeophytin (a, b) · Bacteriochlorophyll c

Bacteriochlorins	Bacteriochlorophyll a

Isobacteriochlorins	Siroheme · Sirohydrochlorin

Corphins	Cofactor F430

biochemical families: prot · nucl · carb (glpr, alco, glys) · lipd (fata/i, phld, strd, gllp, eico) · amac/i · ncbs/i · ttpy/i

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		American Heritage Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more
		McGraw-Hill Science & Technology Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more
		Oxford Dictionary of Biochemistry. Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved. Read more
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