Phycobilins (from Greek: '' (phykos)'' meaning "alga", and from

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

: ''bilis'' meaning "bile") are light-capturing bilins found in

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

and in the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...

s of red algae, glaucophytes and some cryptomonads (though not in green algae and

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

s). Most of their molecules consist of a chromophore which makes them coloured. They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

. 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 An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of ...

in a technique known as

immunofluorescence Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on microbiological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to spe ...

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 of it.

References

* {{Plant_pigments Photosynthetic pigments Tetrapyrroles Fluorescent dyes