Phycobilin

Phycobilins (from Greek: φύκος (phykos) meaning "alga", and from Latin: bilis meaning "bile") are light-capturing bilins found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and plants).^[1] Most of their molecules consist of a chromophore which makes them coloured.^[1] 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.^[1]

The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a.^[2] Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light,^[3] 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.^[4]

Types

There are four types of phycobilins:^[1]

1. Phycoerythrobilin, which is red
2. Phycourobilin, which is orange
3. Phycoviolobilin (also known as phycobiliviolin) found in phycoerythrocyanin
4. 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)^[5] and are structurally similar to the bile pigment bilirubin,^[6] which explains the name. (Bilirubin's conformation is also affected by light, a fact used for the phototherapy of jaundiced newborns.)^[7] Phycobilins are also closely related to the chromophores of the light-detecting plant pigment phytochrome,^[8] 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

1. Frank, H. A.; Cogdell, R. J. (2012-01-01), Egelman, Edward H. (ed.), "8.6 Light Capture in Photosynthesis", Comprehensive Biophysics, Amsterdam: Elsevier, pp. 94–114, doi:10.1016/b978-0-12-374920-8.00808-0, ISBN 978-0-08-095718-0, retrieved 2024-01-04
2. González, A.; Sevilla, E.; Bes, M. T.; Peleato, M. L.; Fillat, M. F. (2016-01-01), "Chapter Five - Pivotal Role of Iron in the Regulation of Cyanobacterial Electron Transport", in Poole, Robert K. (ed.), Advances in Bacterial Electron Transport Systems and Their Regulation, Advances in Microbial Physiology, vol. 68, Academic Press, pp. 169–217, doi:10.1016/bs.ampbs.2016.02.005, PMID 27134024, retrieved 2024-01-04
3. Crichton, Robert R. (2012-01-01), Crichton, Robert R. (ed.), "Chapter 10 - Magnesium–Phosphate Metabolism and Photoreceptors", Biological Inorganic Chemistry (Second ed.), Oxford: Elsevier, pp. 197–214, doi:10.1016/b978-0-444-53782-9.00010-3, ISBN 978-0-444-53782-9, retrieved 2024-01-04
4. Mysliwa-Kurdziel, Beata; Solymosi, Katalin (2017). "Phycobilins and Phycobiliproteins Used in Food Industry and Medicine" (PDF). Mini-Reviews in Medicinal Chemistry. 17 (13): 1173–1193. doi:10.2174/1389557516666160912180155. PMID 27633748. S2CID 6563485.
5. Stirbet, Alexandrina; Lazár, Dušan; Papageorgiou, George C.; Govindjee (2019-01-01), Mishra, A. K.; Tiwari, D. N.; Rai, A. N. (eds.), "Chapter 5 - Chlorophyll a Fluorescence in Cyanobacteria: Relation to Photosynthesis☆", Cyanobacteria, Academic Press, pp. 79–130, doi:10.1016/b978-0-12-814667-5.00005-2, ISBN 978-0-12-814667-5, S2CID 104302759, retrieved 2024-01-04
6. Sibiya, Thabani; Ghazi, Terisha; Chuturgoon, Anil (2022). "The Potential of Spirulina platensis to Ameliorate the Adverse Effects of Highly Active Antiretroviral Therapy (HAART)". Nutrients. 14 (15): 3076. doi:10.3390/nu14153076. ISSN 2072-6643. PMC 9332774. PMID 35893930.
7. Ennever, John F. (1988), Douglas, Ron H.; Moan, Johan; Dall’Acqua, F. (eds.), "Clinical and in Vitro Photochemistry of Bilirubin", Light in Biology and Medicine: Volume 1, Boston, MA: Springer US, pp. 143–151, doi:10.1007/978-1-4613-0709-9_19, ISBN 978-1-4613-0709-9, retrieved 2024-01-04
8. Cornejo, Juan; et al. (1992). "Phytochrome Assembly: THE STRUCTURE AND BIOLOGICAL ACTIVITY OF 2(R),3(E)-PHYTOCHROMOBILINDERIVED FROM PHYCOBILIPROTEINS*". The Journal of Biological Chemistry. 267 (21): 14790–14798. doi:10.1016/S0021-9258(18)42109-6. PMID 1634523.

• 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. doi:10.1042/bj1870303. PMC 1161794. PMID 7396851.