thumb|class=skin-invert-image|Chemical structure of β-[[carotene, a common natural pigment|420px]]

Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, canaries, flamingos, salmon, lobster, shrimp, and daffodils. Over 1,100 identified carotenoids can be further categorized into two classes xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons and contain no oxygen).

All are derivatives of tetraterpenes, 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 coloration of about 15-30% of tree species, 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, and zeaxanthin are present as macular pigments whose importance in visual function, as of 2016, remains under clinical research.

Structure and function

thumb|[[Gac fruit, rich in lycopene]]

thumb|Ingesting carotenoid-rich foods affects the [[plumage of flamingos.]]

thumb|class=skin-invert-image|385px|[[Lutein, a Xanthophyll]]

Carotenoids are produced by all photosynthetic organisms and are primarily used as accessory pigments to chlorophyll in the light-harvesting part of photosynthesis.

They are highly unsaturated with conjugated double bonds, which enables carotenoids to absorb light of various wavelengths. At the same time, the terminal groups regulate the polarity and properties within lipid membranes.

Most carotenoids are tetraterpenoids, regular <chem>C40</chem> isoprenoids. Several modifications to these structures exist: including cyclization, varying degrees of saturation or unsaturation, and other functional groups. Carotenes typically contain only carbon and hydrogen, i.e., they are hydrocarbons. Prominent members include α-carotene, β-carotene, and lycopene, are known as carotenes. Carotenoids containing oxygen include lutein and zeaxanthin. They are known as xanthophylls. 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.

Photophysics

Carotenoids inherit majority of their photophysical properties from polyenes. After absorption of a photon, they are promoted to the second excited electronic state (S<sub>2</sub>). This state undergoes ultrafast relaxation (hundreds of femtoseconds or faster) to the first excited state (S<sub>1</sub>) that may be mediated by certain intermediate electronic states. β-carotene dissolved in benzene has 8 ps S<sub>1</sub> state relaxation time. The length of the multiple conjugated double bonds determines their color and photophysics, the UV-vis absorption band shifts to red for longer carotenoids. There is a rule of thumb saying that the S<sub>1</sub> state lifetime in carotenoids shortens approximately by a factor of two when the central conjugated bond system is extended by additional C=C bond. Presence of additional groups on terminal rings does not affect the S<sub>1</sub> state unless carotenoid is surrounded by polar environment. 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. By protecting lipids from free-radical damage, which generate charged 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.

Structure-property relationships

Like some fatty acids, carotenoids are lipophilic due to the presence of long unsaturated aliphatic chains.

Regulation

The regulation of carotenoid biosynthesis is influenced by various factors, including:

  • Gene Expression: Many carotenoid biosynthetic genes are upregulated by light, enhancing the expression of PSY and subsequently increasing carotenoid production.
  • Hormonal Regulation: Phytohormones such as auxins and abscisic acid modulate carotenoid biosynthesis. Notably, abscisic acid enhances carotenoid accumulation under stress conditions.
  • Environmental Factors: Stressors like drought or pathogen attack can trigger carotenoid accumulation as a protective response, thereby enhancing plant resilience.

Morphology

Carotenoids are located primarily outside the cell nucleus in different cytoplasm organelles, lipid droplets, cytosomes and granules. They have been visualised and quantified by raman spectroscopy in an algal cell.

With the development of monoclonal antibodies to trans-lycopene it was possible to localise this carotenoid in different animal and human cells.

thumb|The orange ring surrounding [[Grand Prismatic Spring is due to carotenoids produced by cyanobacteria and other bacteria.]]

Foods

Beta-carotene, found in pumpkins, sweet potato, carrots and winter squash, is responsible for their orange-yellow colors. Vietnamese gac fruit contains the highest known concentration of the carotenoid lycopene. Although green, kale, spinach, collard greens, and turnip greens contain substantial amounts of beta-carotene.

Carotenoids, especially provitamin A carotenoids such as β-carotene, are essential for human health. Their benefits include:

  • Supporting vision, particularly in low-light conditions.
  • Enhancing immune function.
  • Contributing to skin health.
  • Providing antioxidant properties that may reduce the risk of chronic diseases, including cardiovascular diseases and certain cancers.

Reviews of preliminary research in 2015 indicated that foods high in carotenoids may reduce the risk of head and neck cancers and prostate cancer. There is no correlation between consumption of foods high in carotenoids and vitamin A and the risk of Parkinson's disease.

Humans and other animals 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 lobsters and scales of the yellow morph of common wall lizards are due to carotenoids. 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.

Carotenoids from the diet are stored in the fatty tissues of animals, Cooking carotenoid-containing vegetables in oil and shredding the vegetable both increase carotenoid bioavailability.

Plant colors

thumb|Yellow and orange leaf colors in autumn are due to carotenoids, which are visible after chlorophyll degrades for the season.

thumb|[[Apricots, 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. However, the reds, the purples, and their blended combinations that decorate autumn foliage usually come from another group of pigments in the cells called anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.

Bird colors and sexual selection

Dietary carotenoids and their metabolic derivatives are responsible for bright yellow to red coloration in birds. 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. Carotenoid coloration exhibits high levels of sexual dimorphism, with adult male birds generally displaying more vibrant coloration than females of the same species.

These differences arise due to the selection of yellow and red coloration in males by female preference. or through a connection between carotenoid metabolizing pathways and pathways for cellular respiration.

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 Parus major, the more colorfully ornamented males produce sperm that is better protected against oxidative stress due to increased presence of carotenoid antioxidants. 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. Since carotinoids are beneficial antioxidants, their under-allocation to germline cells can lead to increased oxidative DNA damage to these cells.

Biosynthesis

thumb|class=skin-invert-image|Pathway of carotenoid synthesis

The basic building blocks of carotenoids are isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These two isoprene isomers are used to create various compounds depending on the biological pathway used to synthesize the isomers. Plants are known to use two different pathways for IPP production: the cytosolic mevalonic acid pathway (MVA) and the plastidic methylerythritol 4-phosphate (MEP).

MEP pathway

Glyceraldehyde 3-phosphate and pyruvate, intermediates of photosynthesis, are converted to deoxy-D-xylulose 5-phosphate (DXP) catalyzed by DXP synthase (DXS). DXP reductoisomerase catalyzes the reduction by NADPH and subsequent rearrangement.

  1. Desaturation to lycopene: Phytoene undergoes a series of desaturation reactions facilitated by enzymes such as phytoene desaturase (PDS) and ζ-carotene isomerase (Z-ISO), resulting in the formation of lycopene, a red carotenoid.
  2. Cyclization to carotenoids: Lycopene is cyclized into various carotenoids, including α-carotene and β-carotene, through the action of lycopene cyclase (LCY), which catalyzes cyclization at the ends of the lycopene molecule.
  3. Further modifications: Subsequent modifications, such as hydroxylation and oxidation, lead to the formation of xanthophylls (e.g., lutein and zeaxanthin) and other derivatives.

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 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 bacterial phytoene desaturase (CRTI) for the catalysis. Plants and cyanobacteria however utilize four enzymes for this process. The first of these enzymes is a 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 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 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-lycopene into an all-trans lycopene in the presence of reduced FAD.

This all-trans lycopene is cyclized; cyclization gives rise to carotenoid diversity, which can be distinguished based on the end groups. There can be either a 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 photosystems but they can still be further converted into xanthophylls by using beta-hydrolase and epsilon-hydrolase, leading to a variety of xanthophylls.

  1. Phytoene desaturase (PDS): Introduces double bonds into phytoene, facilitating its conversion into lycopene.
  2. Lycopene cyclase (LCY): Responsible for the cyclization of lycopene into α-carotene or β-carotene.
  3. Carotenoid hydroxylases: Enzymes such as lutein epoxide cyclase (LUT) introduce hydroxyl groups into carotenoids, leading to the formation of xanthophylls.

Regulation

It is believed that both DXS and DXR are rate-determining enzymes, allowing them to regulate carotenoid levels. Regulation may also be caused by external toxins that affect enzymes and proteins required for synthesis. Ketoclomazone is derived from herbicides applied to soil and binds to DXP synthase. 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
  • Apocarotenoids
  • 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

  • Carotenoid complex
  • List of phytochemicals in food
  • CRT (genetics), gene cluster responsible for the biosynthesis of carotenoids
  • E number#E100–E199 (colours)
  • Phytochemistry

References

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