View source for Carotenoid

{{Short description|Class of chemical compounds; yellow, orange or red plant pigments}}
[[File:Beta-Carotin.svg|thumb|Chemical structure of β-[[carotene]], a common natural pigment.|420px]]

'''Carotenoids''' ({{IPAc-en|k|ə|ˈ|r|ɒ|t|ɪ|n|ɔɪ|d}}) are yellow, orange, and red [[organic compound|organic]] [[pigment]]s that are produced by [[plant]]s and [[algae]], as well as several bacteria, archaea, and [[Fungus|fungi]].<ref>{{Lehninger4th}}</ref> Carotenoids give the characteristic color to [[pumpkin]]s, [[carrot]]s, [[parsnip]]s, [[maize|corn]], [[tomato]]es, [[Domestic Canary|canaries]], [[flamingo]]s, [[salmon]], [[lobster]], [[shrimp]], and [[daffodil]]s. Over 1,100 identified carotenoids can be further categorized into two classes {{ndash}} [[xanthophyll]]s (which contain oxygen) and [[carotene]]s (which are purely [[hydrocarbon]]s and contain no oxygen).<ref>{{Cite journal|last=Yabuzaki|first=Junko|date=2017-01-01|title=Carotenoids Database: structures, chemical fingerprints and distribution among organisms|journal=Database|language=en|volume=2017|issue=1|doi=10.1093/database/bax004|pmid=28365725|pmc=5574413}}</ref>

All are [[derivative (chemistry)|derivatives]] of [[tetraterpene]]s, meaning that they are produced from 8 [[isoprene]] units and contain 40 carbon atoms. In general, carotenoids absorb wavelengths ranging from 400 to 550 nanometers (violet to green light). This causes the compounds to be deeply colored yellow, orange, or red. Carotenoids are the dominant pigment in [[autumn leaf color]]ation of about 15-30% of tree species,<ref name=lpi/> but many plant colors, especially reds and purples, are due to [[polyphenol]]s.
[[File:Updated graphic 24.2.16.png|thumb|Macular pigments of the human eye]]

Carotenoids serve two key roles in plants and algae: they absorb light energy for use in [[photosynthesis]], and they provide [[photoprotection]] via [[non-photochemical quenching]].<ref>{{cite journal |vauthors=Armstrong GA, Hearst JE |title=Carotenoids 2: Genetics and molecular biology of carotenoid pigment biosynthesis |journal=FASEB J. |volume=10 |issue=2 |pages=228–37 |year=1996 |pmid=8641556 |doi= 10.1096/fasebj.10.2.8641556|doi-access=free |s2cid=22385652 |url=http://www.fasebj.org/cgi/pmidlookup?view=long&pmid=8641556}}</ref> Carotenoids that contain unsubstituted beta-ionone rings  (including [[β-carotene]], [[α-carotene]], [[β-cryptoxanthin]], and [[γ-carotene]]) have [[vitamin A]] activity (meaning that they can be converted to [[retinol]]). In the eye, [[lutein]], [[Meso-Zeaxanthin|''meso''-zeaxanthin]], and [[zeaxanthin]] are present as [[macula|macular pigment]]s whose importance in visual function, as of 2016, remains under [[clinical research]].<ref name=lpi/><ref name="Li">{{cite journal|pmc=4698241|year=2015|last1=Bernstein|first1=P. S.|title=Lutein, Zeaxanthin, and meso-Zeaxanthin: The Basic and Clinical Science Underlying Carotenoid-based Nutritional Interventions against Ocular Disease|journal=Progress in Retinal and Eye Research|volume=50|pages=34–66|last2=Li|first2=B|last3=Vachali|first3=P. P.|last4=Gorusupudi|first4=A|last5=Shyam|first5=R|last6=Henriksen|first6=B. S.|last7=Nolan|first7=J. M.|doi=10.1016/j.preteyeres.2015.10.003|pmid=26541886}}</ref>

== Structure and function ==
{{main|carotenes|xanthophylls}}
[[File:72 - gac.jpg|thumb|[[Gac]] fruit, rich in lycopene]]
[[File:Akita Omoriyama Zoo - panoramio.jpg|thumb|Ingesting carotenoid-rich foods affects the [[plumage]] of [[flamingo]]s.]]
[[File:Luteine - Lutein.svg|thumb|385px|[[Lutein]], a [[Xanthophyll]].]]
Carotenoids are produced by all photosynthetic organisms and are primarily used as [[accessory pigment]]s to [[chlorophyll]] in the light-harvesting part of photosynthesis.

They are highly [[Unsaturated hydrocarbon|unsaturated]] with [[Conjugated system|conjugated double bonds]], which enables carotenoids to absorb light of various [[wavelength]]s. At the same time, the terminal groups regulate the [[Chemical polarity|polarity]] and properties within [[Lipid bilayer|lipid membranes]].

Most carotenoids are [[tetraterpenoids]], regular <chem>C40</chem> [[isoprenoids]]. Several modifications to these structures exist: including [[Cyclic compound|cyclization]], varying degrees of [[Saturated fat|saturation]] or unsaturation, and other [[functional group]]s.<ref name="Maresca-2008">{{Cite journal |last1=Maresca |first1=Julia A. |last2=Romberger |first2=Steven P. |last3=Bryant |first3=Donald A. |date=2008-05-28 |title=Isorenieratene Biosynthesis in Green Sulfur Bacteria Requires the Cooperative Actions of Two Carotenoid Cyclases |journal=Journal of Bacteriology |language=en |volume=190 |issue=19 |pages=6384–6391 |doi=10.1128/JB.00758-08 |issn=0021-9193 |pmc=2565998 |pmid=18676669}}</ref>  Carotenes typically contain only carbon and hydrogen, i.e., they are [[hydrocarbon]]s. Prominent members include [[α-carotene]], [[β-carotene]], and [[lycopene]], are known as ''[[carotene]]s''. Carotenoids containing oxygen include [[lutein]] and [[zeaxanthin]]. They are known as ''[[xanthophyll]]s''.<ref name=lpi/>  Their color, ranging from pale yellow through bright orange to deep red, is directly related to their structure, especially the length of the conjugation.<ref name=lpi/> Xanthophylls are often yellow, giving their class name.

Carotenoids also participate in different types of cell signaling.<ref name="Cogdell-1978">{{Cite journal|last=Cogdell|first=R. J.|date=1978-11-30|title=Carotenoids in photosynthesis|journal=Phil. Trans. R. Soc. Lond. B|language=en|volume=284|issue=1002|pages=569–579|doi=10.1098/rstb.1978.0090|bibcode=1978RSPTB.284..569C|issn=0080-4622}}</ref> They are able to signal the production of [[abscisic acid]], which regulates plant growth, [[seed dormancy]], embryo maturation and [[germination]], [[cell division]] and elongation, floral growth, and stress responses.<ref>{{Cite journal|last=Finkelstein|first=Ruth|date=2013-11-01|title=Abscisic Acid Synthesis and Response|journal=The Arabidopsis Book|volume=11|doi=10.1199/tab.0166|issn=1543-8120|pmc=3833200|pmid=24273463|page=e0166}}</ref>

===Photophysics===
The length of the multiple [[Conjugated system|conjugated double bonds]] determines their color and photophysics.<ref name=":2">{{Cite journal|last=Vershinin|first=Alexander|date=1999-01-01|title=Biological functions of carotenoids - diversity and evolution|journal=BioFactors|language=en|volume=10|issue=2–3|pages=99–104|doi=10.1002/biof.5520100203|pmid=10609869|s2cid=24408277|issn=1872-8081}}</ref><ref>{{cite journal |doi=10.1021/cr020674n |title=Ultrafast Dynamics of Carotenoid Excited States−From Solution to Natural and Artificial Systems |year=2004 |last1=Polívka |first1=Tomáš |last2=Sundström |first2=Villy |journal=Chemical Reviews |volume=104 |issue=4 |pages=2021–2072 |pmid=15080720 }}</ref> After absorbing a photon, the carotenoid transfers its excited electron to [[chlorophyll]] for use in photosynthesis.<ref name=":2" /> Upon absorption of light, carotenoids transfer excitation energy to and from [[chlorophyll]]. The singlet-singlet energy transfer is a lower energy state transfer and is used during photosynthesis.<ref name="Cogdell-1978"/> The triplet-triplet transfer is a higher energy state and is essential in photoprotection.<ref name="Cogdell-1978"/> Light produces damaging species during photosynthesis, with the most damaging being [[reactive oxygen species]] (ROS).<ref>{{Cite journal |last1=Aizpuru |first1=Aitor |last2=González-Sánchez |first2=Armando |date=2024-07-20 |title=Traditional and new trend strategies to enhance pigment contents in microalgae |journal=World Journal of Microbiology and Biotechnology |language=en |volume=40 |issue=9 |pages=272 |doi=10.1007/s11274-024-04070-3 |issn=1573-0972 |pmc=11271434 |pmid=39030303}}</ref> As these high energy ROS are produced in the chlorophyll the energy is transferred to the carotenoid’s polyene tail and undergoes a series of reactions in which electrons are moved between the carotenoid bonds in order to find the most balanced (lowest energy) state for the carotenoid.<ref name=":2" />

Carotenoids defend plants against [[singlet oxygen]], by both energy transfer and by chemical reactions. They also protect plants by quenching triplet chlorophyll.<ref>{{cite journal |doi=10.1104/pp.111.182394 |title=Chemical Quenching of Singlet Oxygen by Carotenoids in Plants |year=2012 |last1=Ramel |first1=Fanny |last2=Birtic |first2=Simona |last3=Cuiné |first3=Stéphan |last4=Triantaphylidès |first4=Christian |last5=Ravanat |first5=Jean-Luc |last6=Havaux |first6=Michel |journal=Plant Physiology |volume=158 |issue=3 |pages=1267–1278 |pmid=22234998 |pmc=3291260 }}</ref> By protecting lipids from free-radical damage, which generate charged [[Lipid peroxidation|lipid peroxides]] and other oxidised derivatives, carotenoids support crystalline architecture and hydrophobicity of lipoproteins and cellular lipid structures, hence oxygen solubility and its diffusion therein.<ref>{{Cite book|url=https://www.worldcat.org/oclc/148650411|title=Carotenoids: physical, chemical, and biological functions and properties|date=2010|publisher=CRC Press|author=John Thomas Landrum|isbn=978-1-4200-5230-5|location=Boca Raton|oclc=148650411}}</ref>

===Structure-property relationships===
Like some [[fatty acid]]s, carotenoids are [[lipophilic]] due to the presence of long [[Saturated and unsaturated compounds|unsaturated]] [[aliphatic]] chains.<ref name=lpi/> As a consequence, carotenoids are typically present in plasma [[lipoprotein]]s and cellular lipid structures.<ref>{{Citation|last=Gruszecki|first=Wieslaw I.|title=Carotenoids in Membranes|date=2004|url=http://link.springer.com/10.1007/0-306-48209-6_20|work=The Photochemistry of Carotenoids|series=Advances in Photosynthesis and Respiration|volume=8|pages=363–379|editor-last=Frank|editor-first=Harry A.|place=Dordrecht|publisher=Kluwer Academic Publishers|language=en|doi=10.1007/0-306-48209-6_20|isbn=978-0-7923-5942-5|access-date=2021-03-28|editor2-last=Young|editor2-first=Andrew J.|editor3-last=Britton|editor3-first=George|editor4-last=Cogdell|editor4-first=Richard J.}}</ref>

== Morphology ==
Carotenoids are located primarily outside the [[cell nucleus]] in different cytoplasm organelles, [[lipid droplet]]s, [[Microbody|cytosomes]] and granules. They have been visualised and quantified by [[raman spectroscopy]] in an [[algal]] cell.<ref>{{Citation|last1=Timlin|first1=Jerilyn A.|title=Localizing and Quantifying Carotenoids in Intact Cells and Tissues|date=2017-06-14|work=Carotenoids|publisher=InTech|isbn=978-953-51-3211-0|last2=Collins|first2=Aaron M.|last3=Beechem|first3=Thomas A.|last4=Shumskaya|first4=Maria|last5=Wurtzel|first5=Eleanore T.|doi=10.5772/68101|s2cid=54807067 |doi-access=free}}</ref>

With the development of [[Monoclonal antibody|monoclonal antibodies]] to ''trans-''[[lycopene]] it was possible to localise this carotenoid in different animal and human cells.<ref name="Petyaev etal-2018">{{Cite journal|last1=Petyaev|first1=Ivan M.|last2=Zigangirova|first2=Naylia A.|last3=Pristensky|first3=Dmitry|last4=Chernyshova|first4=Marina|last5=Tsibezov|first5=Valeriy V.|last6=Chalyk|first6=Natalya E.|display-authors=3|last7=Morgunova|first7=Elena Y.|last8=Kyle|first8=Nigel H.|last9=Bashmakov|first9=Yuriy K.|date=2018|title=Non-Invasive Immunofluorescence Assessment of Lycopene Supplementation Status in Skin Smears|url=http://www.liebertpub.com/doi/10.1089/mab.2018.0012|journal=Monoclonal Antibodies in Immunodiagnosis and Immunotherapy|language=en|volume=37|issue=3|pages=139–146|doi=10.1089/mab.2018.0012|pmid=29901405|s2cid=49190846 |issn=2167-9436}}</ref>
[[File:Aerial image of Grand Prismatic Spring (view from the south).jpg|thumb|The orange ring surrounding [[Grand Prismatic Spring]] is due to carotenoids produced by [[cyanobacteria]] and other [[bacteria]].]]

==Foods==
[[Beta-carotene]], found in [[pumpkin]]s, [[sweet potato]], [[carrot]]s and [[winter squash]], is responsible for their orange-yellow colors.<ref name=lpi/> Dried carrots have the highest amount of carotene of any food per 100-gram serving, measured in retinol activity equivalents (provitamin A equivalents).<ref name="lpi">{{cite web|publisher=Micronutrient Information Center, Linus Pauling Institute, Oregon State University|url=http://lpi.oregonstate.edu/infocenter/phytochemicals/carotenoids/|title=Carotenoids| date=1 August 2016|access-date=17 April 2019}}</ref><ref>{{Cite web|title = Foods Highest in Retinol Activity Equivalent|url = http://nutritiondata.self.com/foods-000100000000000000000-w.html|website = nutritiondata.self.com|access-date = 2015-12-04}}</ref> Vietnamese [[gac]] fruit contains the highest known concentration of the carotenoid [[lycopene]].<ref>{{cite journal|pmc=4779482|year=2015|last1=Tran|first1=X. T.|title=Effects of maturity on physicochemical properties of Gac fruit (Momordica cochinchinensis Spreng.)|journal=Food Science & Nutrition|volume=4|issue=2|pages=305–314|last2=Parks|first2=S. E.|last3=Roach|first3=P. D.|last4=Golding|first4=J. B.|last5=Nguyen|first5=M. H.|doi=10.1002/fsn3.291|pmid=27004120}}</ref> Although green, [[kale]], [[spinach]], [[collard greens]], and [[turnip greens]] contain substantial amounts of beta-carotene.<ref name=lpi/> The diet of [[flamingo]]s is rich in carotenoids, imparting the orange-colored feathers of these birds.<ref>{{cite journal|pmc=4639753|year=2015|last1=Yim|first1=K. J.|title=Occurrence of viable, red-pigmented haloarchaea in the plumage of captive flamingoes|journal=Scientific Reports|volume=5|pages=16425|last2=Kwon|first2=J|last3=Cha|first3=I. T.|last4=Oh|first4=K. S.|last5=Song|first5=H. S.|last6=Lee|first6=H. W.|last7=Rhee|first7=J. K.|last8=Song|first8=E. J.|last9=Rho|first9=J. R.|last10=Seo|first10=M. L.|last11=Choi|first11=J. S.|last12=Choi|first12=H. J.|last13=Lee|first13=S. J.|last14=Nam|first14=Y. D.|last15=Roh|first15=S. W.|doi=10.1038/srep16425|pmid=26553382|bibcode=2015NatSR...516425Y}}</ref>

Reviews of preliminary research in 2015 indicated that foods high in carotenoids may reduce the risk of [[head and neck cancers]]<ref>{{cite journal | last1 = Leoncini | last2 = Sources | first2 = Natural | last3 = Head | last4 = Cancer | first4 = Neck  |display-authors=et al | date = July 2015 | title = A Systematic Review and Meta-analysis of Epidemiological Studies | journal = Cancer Epidemiol Biomarkers Prev | volume = 24 | issue = 7| pages = 1003–11 | pmid = 25873578 | doi=10.1158/1055-9965.EPI-15-0053| s2cid = 21131127 }}</ref> and [[prostate cancer]].<ref>{{cite journal | last1 = Soares Nda | first1 = C  |display-authors=et al | date = October 2015 | title = Anticancer properties of carotenoids in prostate cancer. A review | journal = Histol Histopathol | volume = 30 | issue = 10| pages = 1143–54 | pmid = 26058846 | doi=10.14670/HH-11-635|url=https://www.hh.um.es/pdf/Vol_30/30_10/Soares-30-1143-1154-2015.pdf}}</ref> There is no correlation between consumption of foods high in carotenoids and vitamin A and the risk of [[Parkinson's disease]].<ref>{{cite journal | last1 = Takeda | first1 = A  |display-authors=et al | year = 2014 | title = Vitamin A and carotenoids and the risk of Parkinson's disease: a systematic review and meta-analysis | journal = Neuroepidemiology | volume = 42 | issue = 1| pages = 25–38 | pmid = 24356061 | doi=10.1159/000355849| s2cid = 12396064 | doi-access = free }}</ref>

Humans and other [[animal]]s are mostly incapable of synthesizing carotenoids, and must obtain them through their diet. Carotenoids are a common and often ornamental feature in animals. For example, the pink color of [[salmon]], and the red coloring of cooked [[lobster]]s and scales of the yellow morph of [[common wall lizard]]s are due to carotenoids.<ref>{{Cite journal|title = Colour variation in the polymorphic common wall lizard (Podarcis muralis): An analysis using the RGB colour system|last = Sacchi|first = Roberto|date = 4 June 2013|journal = Zoologischer Anzeiger|volume = 252|issue = 4|pages = 431–439|doi = 10.1016/j.jcz.2013.03.001| bibcode=2013ZooAn.252..431S }}</ref>{{citation needed|date=December 2015}} It has been proposed that carotenoids are used in ornamental traits (for extreme examples see [[puffin]] birds) because, given their physiological and chemical properties, they can be used as visible indicators of individual health, and hence are used by animals when selecting potential mates.<ref>{{cite journal|journal=Evol Psychol|year=2012|volume=10|issue=5|pages=842–54|title=Attractive skin coloration: harnessing sexual selection to improve diet and health|vauthors=Whitehead RD, Ozakinci G, Perrett DI|pmid=23253790|doi=10.1177/147470491201000507|s2cid=8655801|doi-access=free|pmc=10429994}}</ref>

Carotenoids from the diet are stored in the fatty tissues of animals,<ref name=lpi/> and exclusively [[Carnivore|carnivorous]] animals obtain the compounds from animal fat. In the human diet, [[Small intestine#Absorption|absorption]] of carotenoids is improved when consumed with fat in a meal.<ref name="jfst">{{cite journal | last1=Mashurabad | first1=Purna Chandra | last2=Palika | first2=Ravindranadh | last3=Jyrwa | first3=Yvette Wilda | last4=Bhaskarachary | first4=K. | last5=Pullakhandam | first5=Raghu | title=Dietary fat composition, food matrix and relative polarity modulate the micellarization and intestinal uptake of carotenoids from vegetables and fruits | journal=Journal of Food Science and Technology | volume=54 | issue=2 | date=3 January 2017 | issn=0022-1155 | doi=10.1007/s13197-016-2466-7 | pages=333–341|pmid=28242932|pmc=5306026}}</ref> Cooking carotenoid-containing vegetables in oil and shredding the vegetable both increase carotenoid [[bioavailability]].<ref name=lpi/><ref name=jfst/><ref>{{cite journal |last1=Rodrigo |first1=María Jesús |last2=Cilla |first2=Antonio |last3=Barberá |first3=Reyes |last4=Zacarías |first4=Lorenzo |title=Carotenoid bioaccessibility in pulp and fresh juice from carotenoid-rich sweet oranges and mandarins |journal=Food & Function |date=2015 |volume=6 |issue=6 |pages=1950–1959 |doi=10.1039/c5fo00258c |pmid=25996796}}</ref>

== Plant colors ==
[[File:Acer japonicum Vitifolium JPG1fu.jpg|thumb|Yellow and orange leaf colors in autumn are due to carotenoids, which are visible after chlorophyll degrades for the season.]]
[[File:Prunus armeniaca Nubra Valley.jpg|thumb|[[Apricot]]s, rich in carotenoids]]

The most common carotenoids include lycopene and the vitamin A precursor β-carotene. In plants, the xanthophyll [[lutein]] is the most abundant carotenoid and its role in preventing age-related eye disease is currently under investigation.<ref name=Li/> Lutein and the other carotenoid pigments found in mature leaves are often not obvious because of the masking presence of [[chlorophyll]]. When chlorophyll is not present, as in autumn foliage, the yellows and oranges of the carotenoids are predominant. For the same reason, carotenoid colors often predominate in ripe fruit after being unmasked by the disappearance of chlorophyll.

Carotenoids are responsible for the brilliant yellows and oranges that tint [[deciduous]] foliage (such as dying [[Autumn leaf color|autumn leaves]]) of certain hardwood species as [[hickory|hickories]], [[ash (tree)|ash]], [[maple]], [[yellow poplar]], [[aspen]], [[birch]], [[black cherry]], [[sycamore]], [[Populus sect. Aegiros|cottonwood]], [[sassafras]], and [[alder]]. Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species.<ref name=tree1044>{{cite journal |last1=Archetti |first1=Marco |last2=Döring |first2=Thomas F. |last3=Hagen |first3=Snorre B. |last4=Hughes |first4=Nicole M. |last5=Leather |first5=Simon R. |last6=Lee |first6=David W. |last7=Lev-Yadun |first7=Simcha |last8=Manetas |first8=Yiannis |last9=Ougham |first9=Helen J. |title=Unravelling the evolution of autumn colours: an interdisciplinary approach |journal=Trends in Ecology & Evolution |volume=24 |pages=166–73 |year=2011 |doi=10.1016/j.tree.2008.10.006 |pmid=19178979 |issue=3 }}</ref> However, the reds, the purples, and their blended combinations that decorate autumn foliage usually come from another group of pigments in the cells called [[anthocyanin]]s. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.<ref>{{Cite book |year=2004 |editor1-last=Davies |editor1-first=Kevin M. |title=Plant pigments and their manipulation |url=https://books.google.com/books?id=QF4_vIXK9HEC |series=Annual Plant Reviews |volume=14 |location=Oxford |publisher=Blackwell Publishing |page=6 |isbn=978-1-4051-1737-1}}</ref>

== Bird colors and sexual selection ==
Dietary carotenoids and their metabolic derivatives are responsible for bright yellow to red coloration in birds.<ref>{{Cite journal|last1=Delhey|first1=Kaspar|last2=Peters|first2=Anne|date=2016-11-16|title=The effect of colour-producing mechanisms on plumage sexual dichromatism in passerines and parrots|journal=Functional Ecology|volume=31|issue=4|pages=903–914|doi=10.1111/1365-2435.12796|issn=0269-8463|doi-access=free}}</ref> Studies estimate that around 2956 modern bird species display carotenoid coloration and that the ability to utilize these pigments for external coloration has evolved independently many times throughout avian evolutionary history.<ref>{{Cite journal|last1=Thomas|first1=Daniel B.|last2=McGraw|first2=Kevin J.|last3=Butler|first3=Michael W.|last4=Carrano|first4=Matthew T.|last5=Madden|first5=Odile|last6=James|first6=Helen F.|date=2014-08-07|title=Ancient origins and multiple appearances of carotenoid-pigmented feathers in birds|url=http://dx.doi.org/10.1098/rspb.2014.0806|journal=Proceedings of the Royal Society B: Biological Sciences|volume=281|issue=1788|pages=20140806|doi=10.1098/rspb.2014.0806|pmid=24966316|pmc=4083795|issn=0962-8452}}</ref> Carotenoid coloration exhibits high levels of [[sexual dimorphism]], with adult male birds generally displaying more vibrant coloration than females of the same species.<ref name="Cooney-2019">{{Cite journal|last1=Cooney|first1=Christopher R.|last2=Varley|first2=Zoë K.|last3=Nouri|first3=Lara O.|last4=Moody|first4=Christopher J. A.|last5=Jardine|first5=Michael D.|last6=Thomas|first6=Gavin H.|date=2019-04-16|title=Sexual selection predicts the rate and direction of colour divergence in a large avian radiation|url=http://dx.doi.org/10.1038/s41467-019-09859-7|journal=Nature Communications|volume=10|issue=1|page=1773|doi=10.1038/s41467-019-09859-7 |pmid=30992444| pmc=6467902|bibcode=2019NatCo..10.1773C |issn=2041-1723}}</ref>

These differences arise due to the selection of yellow and red coloration in males by [[Mate choice|female preference]].<ref>{{Cite journal|last=Hill|first=Geoffrey E.|date=September 1990|title=Female house finches prefer colourful males: sexual selection for a condition-dependent trait|url=http://dx.doi.org/10.1016/s0003-3472(05)80537-8|journal=Animal Behaviour|volume=40|issue=3|pages=563–572|doi=10.1016/s0003-3472(05)80537-8|s2cid=53176725|issn=0003-3472}}</ref><ref name="Cooney-2019" /> In many species of birds, females invest greater time and resources into raising offspring than their male partners. Therefore, it is imperative that female birds carefully select high quality mates. Current literature supports the theory that vibrant carotenoid coloration is correlated with male quality—either though direct effects on immune function and oxidative stress,<ref>{{Cite journal|last1=Weaver|first1=Ryan J.|last2=Santos|first2=Eduardo S. A.|last3=Tucker|first3=Anna M.|last4=Wilson|first4=Alan E.|last5=Hill|first5=Geoffrey E.|date=2018-01-08|title=Carotenoid metabolism strengthens the link between feather coloration and individual quality|url=http://dx.doi.org/10.1038/s41467-017-02649-z|journal=Nature Communications|volume=9|issue=1|page=73|doi=10.1038/s41467-017-02649-z|pmid=29311592|pmc=5758789|bibcode=2018NatCo...9...73W |issn=2041-1723}}</ref><ref>{{Cite journal|last1=Simons|first1=Mirre J. P.|last2=Cohen|first2=Alan A.|last3=Verhulst|first3=Simon|date=2012-08-14|title=What Does Carotenoid-Dependent Coloration Tell? Plasma Carotenoid Level Signals Immunocompetence and Oxidative Stress State in Birds–A Meta-Analysis|journal=PLOS ONE|volume=7|issue=8|pages=e43088| pmid=22905205|doi=10.1371/journal.pone.0043088|pmc=3419220|bibcode=2012PLoSO...743088S |issn=1932-6203|doi-access=free}}</ref><ref>{{Cite journal|last1=Koch|first1=Rebecca E.|last2=Hill|first2=Geoffrey E.|date=2018-05-14|title=Do carotenoid-based ornaments entail resource trade-offs? An evaluation of theory and data|journal=Functional Ecology|volume=32|issue=8|pages=1908–1920|doi=10.1111/1365-2435.13122|issn=0269-8463|doi-access=free|bibcode=2018FuEco..32.1908K }}</ref> or through a connection between carotenoid metabolizing pathways and pathways for cellular respiration.<ref>{{Cite journal|last1=Hill|first1=Geoffrey E.|last2=Johnson|first2=James D.|date=November 2012|title=The Vitamin A–Redox Hypothesis: A Biochemical Basis for Honest Signaling via Carotenoid Pigmentation|url=http://dx.doi.org/10.1086/667861|journal=The American Naturalist|volume=180|issue=5|pages=E127–E150|doi=10.1086/667861|pmid=23070328|s2cid=2013258|issn=0003-0147}}</ref><ref>{{Cite journal|last1=Powers|first1=Matthew J|last2=Hill|first2=Geoffrey E|date=2021-05-03|title=A Review and Assessment of the Shared-Pathway Hypothesis for the Maintenance of Signal Honesty in Red Ketocarotenoid-Based Coloration|journal=Integrative and Comparative Biology|volume=61|issue=5|pages=1811–1826|doi=10.1093/icb/icab056|pmid=33940618|issn=1540-7063|doi-access=free}}</ref>

It is generally considered that sexually selected traits, such as carotenoid-based coloration, evolve because they are honest signals of phenotypic and genetic quality.  For instance, among males of the bird species ''[[great tit|Parus major]]'', the more colorfully ornamented males produce sperm that is better protected against [[oxidative stress]] due to increased presence of carotenoid [[antioxidant]]s.<ref>{{Cite journal |last1=Helfenstein |first1=Fabrice |last2=Losdat |first2=Sylvain |last3=Møller |first3=Anders Pape |last4=Blount |first4=Jonathan D. |last5=Richner |first5=Heinz |date=February 2010 |title=Sperm of colourful males are better protected against oxidative stress |journal=Ecology Letters |volume=13 |issue=2 |pages=213–222 |doi=10.1111/j.1461-0248.2009.01419.x |issn=1461-0248 |pmid=20059524|doi-access=free |bibcode=2010EcolL..13..213H }}</ref>  However, there is also evidence that attractive male coloration may be a faulty signal of male quality.  Among [[stickleback]] fish, males that are more attractive to females due to carotenoid colorants appear to under-allocate carotenoids to their germline cells.<ref name="Kim2020">{{Cite journal |last1=Kim |first1=Sin-Yeon |last2=Velando |first2=Alberto |date=January 2020 |title=Attractive male sticklebacks carry more oxidative DNA damage in the soma and germline |url=https://pubmed.ncbi.nlm.nih.gov/31610052 |journal=Journal of Evolutionary Biology |volume=33 |issue=1 |pages=121–126 |doi=10.1111/jeb.13552 |issn=1420-9101 |pmid=31610052|s2cid=204702365 }}</ref>   Since carotinoids are beneficial antioxidants, their under-allocation to [[germline]] cells can lead to increased oxidative [[DNA damage (naturally occurring)|DNA damage]] to these cells.<ref name = Kim2020/>  Therefore, female sticklebacks may risk [[fertility]] and the viability of their offspring by choosing redder, but more deteriorated partners with reduced [[sperm]] quality.

==Aroma chemicals==
Products of carotenoid degradation such as [[ionone]]s, [[damascone]]s and [[damascenone]]s are also important fragrance chemicals that are used extensively in the [[perfume]]s and fragrance industry. Both β-damascenone and β-ionone although low in concentration in [[rose]] distillates are the key odor-contributing compounds in flowers. In fact, the sweet floral smells present in [[black tea]], aged [[tobacco]], [[grape]], and many [[fruit]]s are due to the aromatic compounds resulting from carotenoid breakdown.

==Disease==
Some carotenoids are produced by bacteria to protect themselves from oxidative immune attack. The ''aureus'' (golden) pigment that gives some strains of ''[[Staphylococcus aureus]]'' their name is a carotenoid called [[staphyloxanthin]]. This carotenoid is a virulence factor with an [[antioxidant]] action that helps the microbe evade death by [[reactive oxygen species]] used by the host immune system.<ref>{{cite journal |vauthors=Liu GY, Essex A, Buchanan JT |title=Staphylococcus aureus golden pigment impairs neutrophil killing and promotes virulence through its antioxidant activity |journal=J. Exp. Med. |volume=202 |issue=2 |pages=209–15 |year=2005  |pmid=16009720 |pmc=2213009 |doi=10.1084/jem.20050846 |display-authors=etal}}</ref>

== Biosynthesis ==
[[File:Carotenoid synthetic pathway.svg|thumb|Pathway of carotenoid synthesis]]
{{Merge from|Carotenoid biosynthesis|date=October 2024|section=TRUE}}
The basic building blocks of carotenoids are [[Isopentenyl pyrophosphate|isopentenyl diphosphate]] (IPP) and [[Dimethylallyl pyrophosphate|dimethylallyl diphosphate]] (DMAPP).<ref name="Nisar-2015">{{Cite journal|last1=Nisar|first1=Nazia|last2=Li|first2=Li|last3=Lu|first3=Shan|last4=Khin|first4=Nay Chi|last5=Pogson|first5=Barry J.|date=2015-01-05|title=Carotenoid Metabolism in Plants|journal=Molecular Plant|series=Plant Metabolism and Synthetic Biology|volume=8|issue=1|pages=68–82|doi=10.1016/j.molp.2014.12.007|pmid=25578273|s2cid=26818009 |doi-access=free}}</ref> These two isoprene isomers are used to create various compounds depending on the biological pathway used to synthesize the isomers.<ref name="Kuzuyama-2012">{{Cite journal|last1=KUZUYAMA|first1=Tomohisa|last2=SETO|first2=Haruo|date=2012-03-09|title=Two distinct pathways for essential metabolic precursors for isoprenoid biosynthesis|journal=Proceedings of the Japan Academy. Series B, Physical and Biological Sciences|volume=88|issue=3|pages=41–52|doi=10.2183/pjab.88.41|issn=0386-2208|pmc=3365244|pmid=22450534|bibcode=2012PJAB...88...41K}}</ref> Plants are known to use two different pathways for IPP production: the cytosolic [[mevalonic acid]] pathway (MVA) and the plastidic [[Non-mevalonate pathway|methylerythritol 4-phosphate]] (MEP).<ref name="Nisar-2015" /> In animals, the production of [[cholesterol]] starts by creating IPP and DMAPP using the MVA.<ref name="Kuzuyama-2012" /> For carotenoid production plants use MEP to generate IPP and DMAPP.<ref name="Nisar-2015" /> The MEP pathway results in a 5:1 mixture of IPP:DMAPP.<ref name="Kuzuyama-2012" /> IPP and DMAPP undergo several reactions, resulting in the major carotenoid precursor, [[geranylgeranyl diphosphate]] (GGPP). GGPP can be converted into carotenes or xanthophylls by undergoing a number of different steps within the carotenoid biosynthetic pathway.<ref name="Nisar-2015" />

=== MEP pathway ===
[[Glyceraldehyde 3-phosphate]] and [[Pyruvic acid|pyruvate]], intermediates of [[photosynthesis]], are converted to deoxy-D-xylulose 5-phosphate (DXP) catalyzed by [[DXP synthase]] (DXS). [[DXP reductoisomerase]] catalyzes the reduction by [[Nicotinamide adenine dinucleotide phosphate|NADPH]] and subsequent rearrangement.<ref name="Nisar-2015" /><ref name="Kuzuyama-2012" /> The resulting MEP is converted to 4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol (CDP-ME) in the presence of CTP using the enzyme MEP cytidylyltransferase. CDP-ME is then converted, in the presence of [[Adenosine triphosphate|ATP]], to 2-phospho-4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol (CDP-ME2P). The conversion to CDP-ME2P is catalyzed by [[4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol kinase|CDP-ME kinase]]. Next, CDP-ME2P is converted to 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MECDP). This reaction occurs when MECDP synthase catalyzes the reaction and CMP is eliminated from the CDP-ME2P molecule. MECDP is then converted to (e)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate (HMBDP) via [[4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase|HMBDP synthase]] in the presence of [[flavodoxin]] and NADPH. HMBDP is reduced to IPP in the presence of [[ferredoxin]] and NADPH by the enzyme [[4-Hydroxy-3-methylbut-2-enyl diphosphate reductase|HMBDP reductase]]. The last two steps involving HMBPD synthase and reductase can only occur in completely [[Anaerobic respiration|anaerobic]] environments. IPP is then able to [[Isomerization|isomerize]] to DMAPP via IPP isomerase.<ref name="Kuzuyama-2012" />

=== Carotenoid biosynthetic pathway ===
[[Image:Phytoene desaturation PLOS ONE.png|thumb|right| The conversion of phytoene to lycopene in plants and cyanobacteria (left) differs compared to bacteria and fungi (right).]]
Two GGPP molecules condense via [[phytoene synthase]] (PSY), forming the 15-cis [[isomer]] of [[phytoene]]. PSY belongs to the [[squalene/phytoene synthase family]] and is homologous to [[Farnesyl-diphosphate farnesyltransferase|squalene synthase]] that takes part in [[steroid]] biosynthesis. The subsequent conversion of phytoene into all-trans-[[lycopene]] depends on the organism. Bacteria and fungi employ a single enzyme, the [[Phytoene desaturase (lycopene-forming)|bacterial phytoene desaturase]] (CRTI) for the catalysis. Plants and cyanobacteria however utilize four enzymes for this process.<ref>{{cite journal|last1=Moise|first1=Alexander R.|last2=Al-Babili|first2=Salim |last3=Wurtzel|first3=Eleanore T.|title=Mechanistic aspects of carotenoid biosynthesis.|journal=Chemical Reviews|date=31 October 2013|volume=114|issue=1|pages=164–93|doi=10.1021/cr400106y|pmc=3898671|pmid=24175570}}</ref> The first of these enzymes is a [[15-Cis-phytoene desaturase|plant-type phytoene desaturase]] which introduces two additional double bonds into 15-cis-phytoene by  [[dehydrogenation]] and isomerizes two of its existing double bonds from [[Cis–trans isomerism|trans to cis]] producing 9,15,9’-tri-cis-ζ-carotene. The central double bond of this tri-cis-ζ-carotene is isomerized by the [[zeta-carotene isomerase]] Z-ISO and the resulting 9,9'-di-cis-ζ-carotene is dehydrogenated again via a [[9,9'-Dicis-zeta-carotene desaturase|ζ-carotene desaturase (ZDS)]]. This again introduces two double bonds, resulting in 7,9,7’,9’-tetra-cis-lycopene. [[CRTISO]], a carotenoid isomerase, is needed to convert the [[Cis–trans isomerism|cis]]-lycopene into an [[Cis–trans isomerism|all-trans]] lycopene in the presence of reduced [[Flavin adenine dinucleotide|FAD]].

This all-trans lycopene is cyclized; [[Cyclic compound|cyclization]] gives rise to carotenoid diversity, which can be distinguished based on the end groups. There can be either a [[Beta-ring|beta ring]] or an epsilon ring, each generated by a different enzyme ([[lycopene beta-cyclase]]  [beta-LCY] or [[lycopene epsilon-cyclase]] [epsilon-LCY]). [[α-Carotene]] is produced when the all-trans lycopene first undergoes reaction with epsilon-LCY then a second reaction with beta-LCY; whereas [[β-carotene]] is produced by two reactions with beta-LCY. α- and β-Carotene are the most common carotenoids in the plant [[photosystem]]s but they can still be further converted into xanthophylls by using beta-hydrolase and epsilon-hydrolase, leading to a variety of xanthophylls.<ref name="Nisar-2015" />

=== Regulation ===
It is believed that both DXS and DXR are rate-determining enzymes, allowing them to regulate carotenoid levels.<ref name="Nisar-2015" /> This was discovered in an experiment where DXS and DXR were genetically overexpressed, leading to increased carotenoid expression in the resulting seedlings.<ref name="Nisar-2015" /> Also, J-protein (J20) and heat shock protein 70 (Hsp70) chaperones are thought to be involved in post-transcriptional regulation of DXS activity, such that mutants with defective J20 activity exhibit reduced DXS enzyme activity while accumulating inactive DXS protein.<ref>{{cite journal|last1=Nisar|first1=Nazia|last2=Li|first2=Li|last3=Lu|first3=Shan|last4=ChiKhin|first4=Nay|last5=Pogson|first5=Barry J.|title=Carotenoid Metabolism in Plants|journal=Molecular Plant|date=5 January 2015|volume=8|issue=1|pages=68–82|doi=10.1016/j.molp.2014.12.007|pmid=25578273|s2cid=26818009 |doi-access=free}}</ref> Regulation may also be caused by external [[toxin]]s that affect enzymes and proteins required for synthesis. Ketoclomazone is derived from [[herbicide]]s applied to soil and binds to DXP synthase.<ref name="Kuzuyama-2012" /> This inhibits DXP synthase, preventing synthesis of DXP and halting the MEP pathway.<ref name="Kuzuyama-2012" /> The use of this toxin leads to lower levels of carotenoids in plants grown in the contaminated soil.<ref name="Kuzuyama-2012" /> [[Fosmidomycin]], an [[Antibiotics|antibiotic]], is a [[Competitive inhibition|competitive inhibitor]] of DXP reductoisomerase due to its similar structure to the enzyme.<ref name="Kuzuyama-2012" /> Application of said antibiotic prevents reduction of DXP, again halting the MEP pathway.&nbsp;<ref name="Kuzuyama-2012" />

==Naturally occurring carotenoids==
* Hydrocarbons
** [[Lycopersene]] 7,8,11,12,15,7',8',11',12',15'-Decahydro-γ,γ-carotene
** [[Phytofluene]]
** [[Lycopene]]
** [[Hexahydrolycopene]] 15-''cis''-7,8,11,12,7',8'-Hexahydro-γ,γ-carotene
** [[Torulene]] 3',4'-Didehydro-β,γ-carotene
** [[α-Zeacarotene]] 7',8'-Dihydro-ε,γ-carotene
** [[α-Carotene]]
** [[β-Carotene]]
** [[γ-Carotene]]
** [[δ-Carotene]]
** [[ε-Carotene]]
** [[ζ-Carotene]]
* Alcohols
** [[Alloxanthin]]
** [[Bacterioruberin]] 2,2'-Bis(3-hydroxy-3-methylbutyl)-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-γ,γ-carotene-1,1'-diol
** [[Cynthiaxanthin]]
** [[Pectenoxanthin]]
** [[Cryptomonaxanthin]] (3R,3'R)-7,8,7',8'-Tetradehydro-β,β-carotene-3,3'-diol
** [[Crustaxanthin]] β,-Carotene-3,4,3',4'-tetrol
** [[Gazaniaxanthin]] (3R)-5'-cis-β,γ-Caroten-3-ol
** [[OH-Chlorobactene]] 1',2'-Dihydro-f,γ-caroten-1'-ol
** [[Loroxanthin]] β,ε-Carotene-3,19,3'-triol
** [[Lutein]] (3R,3′R,6′R)-β,ε-carotene-3,3′-diol
** [[Lycoxanthin]] γ,γ-Caroten-16-ol
** [[Rhodopin]] 1,2-Dihydro-γ,γ-caroten-l-ol
** [[Rhodopinol]] a.k.a. [[Warmingol]]  13-''cis''-1,2-Dihydro-γ,γ-carotene-1,20-diol
** [[Saproxanthin]] 3',4'-Didehydro-1',2'-dihydro-β,γ-carotene-3,1'-diol
** [[Zeaxanthin]]
* Glycosides
** [[Oscillaxanthin]] 2,2'-Bis(β-L-rhamnopyranosyloxy)-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-γ,γ-carotene-1,1'-diol
** [[Phleixanthophyll]] 1'-(β-D-Glucopyranosyloxy)-3',4'-didehydro-1',2'-dihydro-β,γ-caroten-2'-ol
* Ethers
** [[Rhodovibrin]] 1'-Methoxy-3',4'-didehydro-1,2,1',2'-tetrahydro-γ,γ-caroten-1-ol
** [[Spheroidene]] 1-Methoxy-3,4-didehydro-1,2,7',8'-tetrahydro-γ,γ-carotene
* Epoxides
** [[Diadinoxanthin]] 5,6-Epoxy-7',8'-didehydro-5,6-dihydro—carotene-3,3-diol
** [[Luteoxanthin]] 5,6: 5',8'-Diepoxy-5,6,5',8'-tetrahydro-β,β-carotene-3,3'-diol
** [[Mutatoxanthin]]
** [[Citroxanthin]]
** [[Zeaxanthin furanoxide]] 5,8-Epoxy-5,8-dihydro-β,β-carotene-3,3'-diol
** [[Neochrome (Carotenoid)|Neochrome]]         5',8'-Epoxy-6,7-didehydro-5,6,5',8'-tetrahydro-β,β-carotene-3,5,3'-triol
** [[Foliachrome]]
** [[Trollichrome]]
** [[Vaucheriaxanthin]] 5',6'-Epoxy-6,7-didehydro-5,6,5',6'-tetrahydro-β,β-carotene-3,5,19,3'-tetrol
* Aldehydes
** [[Rhodopinal]]
** [[Warmingone]] 13-cis-1-Hydroxy-1,2-dihydro-γ,γ-caroten-20-al
** [[Torularhodinaldehyde]] 3',4'-Didehydro-β,γ-caroten-16'-al
* Acids and acid esters
** [[Torularhodin]] 3',4'-Didehydro-β,γ-caroten-16'-oic acid
** [[Torularhodin methyl ester]] Methyl 3',4'-didehydro-β,γ-caroten-16'-oate
* Ketones{{anchor|Ketocarotenoids}}
** [[Astacene]]
** [[Astaxanthin]]
** [[Canthaxanthin]]<ref>{{cite journal | last1 = Choi | first1 = Seyoung | last2 = Koo | first2 = Sangho | year = 2005 | title = Efficient Syntheses of the Keto-carotenoids Canthaxanthin, Astaxanthin, and Astacene | journal = J. Org. Chem. | volume = 70 | issue = 8| pages = 3328–3331 | doi = 10.1021/jo050101l | pmid = 15823009 }}</ref> a.k.a. Aphanicin, Chlorellaxanthin β,β-Carotene-4,4'-dione
** [[Capsanthin]] (3R,3'S,5'R)-3,3'-Dihydroxy-β,κ-caroten-6'-one
** [[Capsorubin]] (3S,5R,3'S,5'R)-3,3'-Dihydroxy-κ,κ-carotene-6,6'-dione
** [[Cryptocapsin]] (3'R,5'R)-3'-Hydroxy-β,κ-caroten-6'-one
** [[2,2'-Diketospirilloxanthin]] 1,1'-Dimethoxy-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-γ,γ-carotene-2,2'-dione
** [[Echinenone]] β,β-Caroten-4-one
** [[3'-Hydroxyechinenone]]
** [[Flexixanthin]] 3,1'-Dihydroxy-3',4'-didehydro-1',2'-dihydro-β,γ-caroten-4-one
** 3-OH-Canthaxanthin a.k.a. Adonirubin a.k.a. [[Phoenicoxanthin]] 3-Hydroxy-β,β-carotene-4,4'-dione
** [[Hydroxyspheriodenone]] 1'-Hydroxy-1-methoxy-3,4-didehydro-1,2,1',2',7',8'-hexahydro-γ,γ-caroten-2-one
** [[Okenone]] 1'-Methoxy-1',2'-dihydro-c,γ-caroten-4'-one
** [[Pectenolone]] 3,3'-Dihydroxy-7',8'-didehydro-β,β-caroten-4-one
** [[Phoeniconone]] a.k.a. [[Dehydroadonirubin]] 3-Hydroxy-2,3-didehydro-β,β-carotene-4,4'-dione
** [[Phoenicopterone]] β,ε-caroten-4-one
** [[Rubixanthone]] 3-Hydroxy-β,γ-caroten-4'-one
** [[Siphonaxanthin]] 3,19,3'-Trihydroxy-7,8-dihydro-β,ε-caroten-8-one
* Esters of alcohols
** [[Astacein]] 3,3'-Bispalmitoyloxy-2,3,2',3'-tetradehydro-β,β-carotene-4,4'-dione or 3,3'-dihydroxy-2,3,2',3'-tetradehydro-β,β-carotene-4,4'-dione dipalmitate
** [[Fucoxanthin]] 3'-Acetoxy-5,6-epoxy-3,5'-dihydroxy-6',7'-didehydro-5,6,7,8,5',6'-hexahydro-β,β-caroten-8-one
** [[Isofucoxanthin]] 3'-Acetoxy-3,5,5'-trihydroxy-6',7'-didehydro-5,8,5',6'-tetrahydro-β,β-caroten-8-one
** [[Physalien]]
** [[Siphonein]] 3,3'-Dihydroxy-19-lauroyloxy-7,8-dihydro-β,ε-caroten-8-one or 3,19,3'-trihydroxy-7,8-dihydro-β,ε-caroten-8-one 19-laurate
* [[Apocarotenoid]]s
** [[β-Apo-2'-carotenal]] 3',4'-Didehydro-2'-apo-b-caroten-2'-al
** [[Apo-2-lycopenal]]
** [[Apo-6'-lycopenal]] 6'-Apo-y-caroten-6'-al
** [[Azafrinaldehyde]] 5,6-Dihydroxy-5,6-dihydro-10'-apo-β-caroten-10'-al
** [[Bixin]] 6'-Methyl hydrogen 9'-cis-6,6'-diapocarotene-6,6'-dioate
** [[Citranaxanthin]] 5',6'-Dihydro-5'-apo-β-caroten-6'-one or 5',6'-dihydro-5'-apo-18'-nor-β-caroten-6'-one or 6'-methyl-6'-apo-β-caroten-6'-one
** [[Crocetin]] 8,8'-Diapo-8,8'-carotenedioic acid
** [[Crocetinsemialdehyde]] 8'-Oxo-8,8'-diapo-8-carotenoic acid
** [[Crocin]] Digentiobiosyl 8,8'-diapo-8,8'-carotenedioate
** [[Hopkinsiaxanthin]] 3-Hydroxy-7,8-didehydro-7',8'-dihydro-7'-apo-b-carotene-4,8'-dione or 3-hydroxy-8'-methyl-7,8-didehydro-8'-apo-b-carotene-4,8'-dione
** [[Methyl apo-6'-lycopenoate]] Methyl 6'-apo-y-caroten-6'-oate
** [[Paracentrone]] 3,5-Dihydroxy-6,7-didehydro-5,6,7',8'-tetrahydro-7'-apo-b-caroten-8'-one or 3,5-dihydroxy-8'-methyl-6,7-didehydro-5,6-dihydro-8'-apo-b-caroten-8'-one
** [[Sintaxanthin]] 7',8'-Dihydro-7'-apo-b-caroten-8'-one or 8'-methyl-8'-apo-b-caroten-8'-one
* Nor- and seco-carotenoids
** [[Actinioerythrin]] 3,3'-Bisacyloxy-2,2'-dinor-b,b-carotene-4,4'-dione
** [[β-Carotenone]] 5,6:5',6'-Diseco-b,b-carotene-5,6,5',6'-tetrone
** [[Peridinin]] 3'-Acetoxy-5,6-epoxy-3,5'-dihydroxy-6',7'-didehydro-5,6,5',6'-tetrahydro-12',13',20'-trinor-b,b-caroten-19,11-olide
** [[Pyrrhoxanthininol]] 5,6-epoxy-3,3'-dihydroxy-7',8'-didehydro-5,6-dihydro-12',13',20'-trinor-b,b-caroten-19,11-olide
** [[Semi-α-carotenone]] 5,6-Seco-b,e-carotene-5,6-dione
** [[Semi-β-carotenone]] 5,6-seco-b,b-carotene-5,6-dione or 5',6'-seco-b,b-carotene-5',6'-dione
** [[Triphasiaxanthin]] 3-Hydroxysemi-b-carotenone 3'-Hydroxy-5,6-seco-b,b-carotene-5,6-dione or 3-hydroxy-5',6'-seco-b,b-carotene-5',6'-dione
* Retro-carotenoids and retro-apo-carotenoids
** [[Eschscholtzxanthin]] 4',5'-Didehydro-4,5'-retro-b,b-carotene-3,3'-diol
** [[Eschscholtzxanthone]] 3'-Hydroxy-4',5'-didehydro-4,5'-retro-b,b-caroten-3-one
** [[Rhodoxanthin]] 4',5'-Didehydro-4,5'-retro-b,b-carotene-3,3'-dione
** [[Tangeraxanthin]] 3-Hydroxy-5'-methyl-4,5'-retro-5'-apo-b-caroten-5'-one or 3-hydroxy-4,5'-retro-5'-apo-b-caroten-5'-one
* Higher carotenoids
** [[Nonaprenoxanthin]] 2-(4-Hydroxy-3-methyl-2-butenyl)-7',8',11',12'-tetrahydro-e,y-carotene
** [[Decaprenoxanthin]] 2,2'-Bis(4-hydroxy-3-methyl-2-butenyl)-e,e-carotene
** [[C.p. 450]] 2-[4-Hydroxy-3-(hydroxymethyl)-2-butenyl]-2'-(3-methyl-2-butenyl)-b,b-carotene
** [[C.p. 473]] 2'-(4-Hydroxy-3-methyl-2-butenyl)-2-(3-methyl-2-butenyl)-3',4'-didehydro-l',2'-dihydro-β,γ-caroten-1'-ol
** [[Bacterioruberin]] 2,2'-Bis(3-hydroxy-3-methylbutyl)-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-γ,γ-carotene-1,1'-diol

==See also==
* [[List of phytochemicals in food]]
* [[CRT (genetics)]], [[gene cluster]] responsible for the [[biosynthesis]] of carotenoids
* [[E number#E100–E199 (colours)]]
* [[Phytochemistry]]

==References==
<!--This article uses the Cite.php citation mechanism. If you would like more information on how to add references to this article, please see http://meta.wikimedia.org/wiki/Cite/Cite.php -->
{{Reflist}}

==External links==
{{commons category|Carotenoids}}
* {{MeshName|Carotenoids}}

{{Plant Pigments}}
{{Carotenoids}}
{{Terpenoids}}
{{Cholesterol metabolism intermediates}}
{{Aryl hydrocarbon receptor modulators}}
{{Authority control}}

[[Category:Bioindicators]]
[[Category:Carotenoids| ]]
[[Category:Photosynthesis]]