Editing Bilirubin (section)

== Structure ==
Bilirubin consists of an open-chain [[tetrapyrrole]]. It is formed by oxidative cleavage of a [[porphyrin]] in heme, which affords biliverdin. Biliverdin is reduced to bilirubin. After conjugation with [[glucuronic acid]], bilirubin is water-soluble and can be excreted.<ref name=":3" />

Bilirubin is structurally similar to the [[pigment]] [[phycobilin]] used by certain algae to capture light energy, and to the pigment [[phytochrome]] used by plants to sense light. All of these contain an open chain of four pyrrolic rings.<ref>{{cite web
|url=https://hmdb.ca/metabolites/HMDB0000054
|title=Showing metabocard for Bilirubin (HMDB0000054)
|date=2022-07-12
|website=hmdb.ca
|publisher=The Human Metabolome Database (HMDB)
|access-date=2024-08-22
}}</ref>

Like these other pigments, some of the double-bonds in bilirubin [[Cis-trans isomerism|isomerize]] when exposed to light. This isomerization is relevant to the [[phototherapy]] of jaundiced newborns: the E,Z-isomers of bilirubin formed upon light exposure are more soluble than the unilluminated Z,Z-isomer, as the possibility of intramolecular hydrogen bonding is removed.<ref>{{cite journal | vauthors = McDonagh AF, Palma LA, Lightner DA | title = Blue light and bilirubin excretion | journal = Science | volume = 208 | issue = 4440 | pages = 145–51 | date = April 1980 | pmid = 7361112 | doi = 10.1126/science.7361112 | bibcode = 1980Sci...208..145M }}</ref> Increased solubility allows the excretion of unconjugated bilirubin in bile.{{cn|date=July 2024}}

Some textbooks and research articles show the incorrect geometric isomer of bilirubin.<ref>{{cite web |url=http://www.nsta.org/main/news/stories/college_science.php?news_story_ID=48991 |title=Bilirubin's Chemical Formula |access-date=2007-08-14 |archive-date=4 May 2011 |archive-url=https://web.archive.org/web/20110504235852/http://nsta.org/main/news/stories/college_science.php?news_story_ID=48991 |url-status=dead }}</ref> The naturally occurring isomer is the Z,Z-isomer.