All arederivatives oftetraterpenes, meaning that they are produced from 8isoprene 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 inautumn leaf coloration of about 15-30% of tree species,[3] but many plant colors, especially reds and purples, are due topolyphenols.
Carotenoids are produced by all photosynthetic organisms and are primarily used asaccessory pigments tochlorophyll in the light-harvesting part of photosynthesis.
Most carotenoids aretetraterpenoids, regularisoprenoids. Several modifications to these structures exist: includingcyclization, varying degrees ofsaturation or unsaturation, and otherfunctional groups.[6] Carotenes typically contain only carbon and hydrogen, i.e., they arehydrocarbons. Prominent members includeα-carotene,β-carotene, andlycopene, are known ascarotenes. Carotenoids containing oxygen includelutein andzeaxanthin. They are known asxanthophylls.[3] Their color, ranging from pale yellow through bright orange to deep red, is directly related to their structure, especially the length of the conjugation.[3] Xanthophylls are often yellow, giving their class name.
Carotenoids also participate in different types of cell signaling.[7] They are able to signal the production ofabscisic acid, which regulates plant growth,seed dormancy, embryo maturation andgermination,cell division and elongation, floral growth, and stress responses.[8]
The length of the multipleconjugated double bonds determines their color and photophysics.[9][10] After absorbing a photon, the carotenoid transfers its excited electron tochlorophyll for use in photosynthesis.[9] Upon absorption of light, carotenoids transfer excitation energy to and fromchlorophyll. The singlet-singlet energy transfer is a lower energy state transfer and is used during photosynthesis.[7] The triplet-triplet transfer is a higher energy state and is essential in photoprotection.[7] Light produces damaging species during photosynthesis, with the most damaging beingreactive oxygen species (ROS).[11] 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.[9]
Carotenoids defend plants againstsinglet oxygen, by both energy transfer and by chemical reactions. They also protect plants by quenching triplet chlorophyll.[12] By protecting lipids from free-radical damage, which generate chargedlipid peroxides and other oxidised derivatives, carotenoids support crystalline architecture and hydrophobicity of lipoproteins and cellular lipid structures, hence oxygen solubility and its diffusion therein.[13]
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.[15]
Hormonal Regulation: Phytohormones such asauxins andabscisic acid modulate carotenoid biosynthesis. Notably, abscisic acid enhances carotenoid accumulation under stress conditions.[16]
Environmental Factors: Stressors likedrought orpathogen attack can trigger carotenoid accumulation as a protective response, thereby enhancing plant resilience.[17]
Beta-carotene, found inpumpkins,sweet potato,carrots andwinter squash, is responsible for their orange-yellow colors.[3] Dried carrots have the highest amount of carotene of any food per 100-gram serving, measured in retinol activity equivalents (provitamin A equivalents).[3][20] Vietnamesegac fruit contains the highest known concentration of the carotenoidlycopene.[21] Although green,kale,spinach,collard greens, andturnip greens contain substantial amounts of beta-carotene.[3] The diet offlamingos is rich in carotenoids, imparting the orange-colored feathers of these birds.[22]
Carotenoids, especially provitamin A carotenoids such asβ-carotene, are essential for human health. Their benefits include:
Supportingvision, particularly in low-light conditions.[23]
Reviews of preliminary research in 2015 indicated that foods high in carotenoids may reduce the risk ofhead and neck cancers[27] andprostate cancer.[28] There is no correlation between consumption of foods high in carotenoids and vitamin A and the risk ofParkinson's disease.[29]
Humans and otheranimals 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 ofsalmon, and the red coloring of cookedlobsters and scales of the yellow morph ofcommon wall lizards are due to carotenoids.[30][citation needed] It has been proposed that carotenoids are used in ornamental traits (for extreme examples seepuffin 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.[31]
Carotenoids from the diet are stored in the fatty tissues of animals,[3] and exclusivelycarnivorous animals obtain the compounds from animal fat. In the human diet,absorption of carotenoids is improved when consumed with fat in a meal.[32] Cooking carotenoid-containing vegetables in oil and shredding the vegetable both increase carotenoidbioavailability.[3][32][33]
Yellow and orange leaf colors in autumn are due to carotenoids, which are visible after chlorophyll degrades for the season.Apricots, rich in carotenoids
The most common carotenoids include lycopene and the vitamin A precursor β-carotene. In plants, the xanthophylllutein is the most abundant carotenoid and its role in preventing age-related eye disease is currently under investigation.[5] Lutein and the other carotenoid pigments found in mature leaves are often not obvious because of the masking presence ofchlorophyll. 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 tintdeciduous foliage (such as dyingautumn leaves) of certain hardwood species ashickories,ash,maple,yellow poplar,aspen,birch,black cherry,sycamore,cottonwood,sassafras, andalder. Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species.[34] However, the reds, the purples, and their blended combinations that decorate autumn foliage usually come from another group of pigments in the cells calledanthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.[35]
Dietary carotenoids and their metabolic derivatives are responsible for bright yellow to red coloration in birds.[36] 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.[37] Carotenoid coloration exhibits high levels ofsexual dimorphism, with adult male birds generally displaying more vibrant coloration than females of the same species.[38]
These differences arise due to the selection of yellow and red coloration in males byfemale preference.[39][38] 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,[40][41][42] or through a connection between carotenoid metabolizing pathways and pathways for cellular respiration.[43][44]
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 speciesParus major, the more colorfully ornamented males produce sperm that is better protected againstoxidative stress due to increased presence of carotenoidantioxidants.[45] However, there is also evidence that attractive male coloration may be a faulty signal of male quality. Amongstickleback fish, males that are more attractive to females due to carotenoid colorants appear to under-allocate carotenoids to their germline cells.[46] Since carotinoids are beneficial antioxidants, their under-allocation togermline cells can lead to increased oxidativeDNA damage to these cells.[46] Therefore, female sticklebacks may riskfertility and the viability of their offspring by choosing redder, but more deteriorated partners with reducedsperm quality.
Products of carotenoid degradation such asionones,damascones anddamascenones are also important fragrance chemicals that are used extensively in theperfumes and fragrance industry. Both β-damascenone and β-ionone although low in concentration inrose distillates are the key odor-contributing compounds in flowers. In fact, the sweet floral smells present inblack tea, agedtobacco,grape, and manyfruits are due to the aromatic compounds resulting from carotenoid breakdown.
Some carotenoids are produced by bacteria to protect themselves from oxidative immune attack. Theaureus (golden) pigment that gives some strains ofStaphylococcus aureus their name is a carotenoid calledstaphyloxanthin. This carotenoid is a virulence factor with anantioxidant action that helps the microbe evade death byreactive oxygen species used by the host immune system.[47]
The basic building blocks of carotenoids areisopentenyl diphosphate (IPP) anddimethylallyl diphosphate (DMAPP).[48] These two isoprene isomers are used to create various compounds depending on the biological pathway used to synthesize the isomers.[49] Plants are known to use two different pathways for IPP production: the cytosolicmevalonic acid pathway (MVA) and the plastidicmethylerythritol 4-phosphate (MEP).[48] In animals, the production ofcholesterol starts by creating IPP and DMAPP using the MVA.[49] For carotenoid production plants use MEP to generate IPP and DMAPP.[48] The MEP pathway results in a 5:1 mixture of IPP:DMAPP.[49] 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.[48]
Glyceraldehyde 3-phosphate andpyruvate, intermediates ofphotosynthesis, are converted to deoxy-D-xylulose 5-phosphate (DXP) catalyzed byDXP synthase (DXS).DXP reductoisomerase catalyzes the reduction byNADPH and subsequent rearrangement.[48][49] 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 ofATP, to 2-phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol (CDP-ME2P). The conversion to CDP-ME2P is catalyzed byCDP-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) viaHMBDP synthase in the presence offlavodoxin and NADPH. HMBDP is reduced to IPP in the presence offerredoxin and NADPH by the enzymeHMBDP reductase. The last two steps involving HMBPD synthase and reductase can only occur in completelyanaerobic environments. IPP is then able toisomerize to DMAPP via IPP isomerase.[49]
The conversion of phytoene to lycopene in plants and cyanobacteria (left) differs compared to bacteria and fungi (right).
Carotenoid biosynthesis occurs primarily in theplastids of plant cells, particularly withinchloroplasts andchromoplasts. The biosynthetic pathway initiates with the condensation of two molecules ofgeranylgeranyl pyrophosphate (GGPP), a 20-carbon isoprenoid precursor. The key steps in this pathway are as follows:
Formation of phytoene: The enzymephytoene synthase (PSY) catalyzes the condensation of two GGPP molecules to producephytoene, a colorless carotenoid.[51]
Desaturation to lycopene: Phytoene undergoes a series of desaturation reactions facilitated by enzymes such asphytoene desaturase (PDS) and ζ-carotene isomerase (Z-ISO), resulting in the formation oflycopene, a red carotenoid.
Cyclization to carotenoids: Lycopene is cyclized into various carotenoids, includingα-carotene andβ-carotene, through the action of lycopene cyclase (LCY), which catalyzes cyclization at the ends of the lycopene molecule.[52]
Further modifications: Subsequent modifications, such as hydroxylation and oxidation, lead to the formation ofxanthophylls (e.g.,lutein andzeaxanthin) and other derivatives.
Two GGPP molecules condense viaphytoene synthase (PSY), forming the 15-cisisomer ofphytoene. PSY belongs to thesqualene/phytoene synthase family and is homologous tosqualene synthase that takes part insteroid biosynthesis. The subsequent conversion of phytoene into all-trans-lycopene depends on the organism. Bacteria and fungi employ a single enzyme, thebacterial phytoene desaturase (CRTI) for the catalysis. Plants and cyanobacteria however utilize four enzymes for this process.[53] The first of these enzymes is aplant-type phytoene desaturase which introduces two additional double bonds into 15-cis-phytoene bydehydrogenation and isomerizes two of its existing double bonds fromtrans to cis producing 9,15,9'-tri-cis-ζ-carotene. The central double bond of this tri-cis-ζ-carotene is isomerized by thezeta-carotene isomerase Z-ISO and the resulting 9,9'-di-cis-ζ-carotene is dehydrogenated again via aζ-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 thecis-lycopene into anall-trans lycopene in the presence of reducedFAD.
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 abeta ring or an epsilon ring, each generated by a different enzyme (lycopene beta-cyclase [beta-LCY] orlycopene 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; whereasβ-carotene is produced by two reactions with beta-LCY. α- and β-Carotene are the most common carotenoids in the plantphotosystems but they can still be further converted into xanthophylls by using beta-hydrolase and epsilon-hydrolase, leading to a variety of xanthophylls.[48]
Several enzymes play critical roles in the carotenoid biosynthetic pathway:
Phytoene synthase (PSY): Catalyzes the first committed step in carotenoid biosynthesis, converting GGPP into phytoene.[54]
Phytoene desaturase (PDS): Introduces double bonds into phytoene, facilitating its conversion into lycopene.[55]
Lycopene cyclase (LCY): Responsible for the cyclization of lycopene into α-carotene or β-carotene.[56]
Carotenoid hydroxylases: Enzymes such as lutein epoxide cyclase (LUT) introduce hydroxyl groups into carotenoids, leading to the formation of xanthophylls.[57]
It is believed that both DXS and DXR are rate-determining enzymes, allowing them to regulate carotenoid levels.[48] This was discovered in an experiment where DXS and DXR were genetically overexpressed, leading to increased carotenoid expression in the resulting seedlings.[48] 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.[58] Regulation may also be caused by externaltoxins that affect enzymes and proteins required for synthesis. Ketoclomazone is derived fromherbicides applied to soil and binds to DXP synthase.[49] This inhibits DXP synthase, preventing synthesis of DXP and halting the MEP pathway.[49] The use of this toxin leads to lower levels of carotenoids in plants grown in the contaminated soil.[49]Fosmidomycin, anantibiotic, is acompetitive inhibitor of DXP reductoisomerase due to its similar structure to the enzyme.[49] Application of said antibiotic prevents reduction of DXP, again halting the MEP pathway.[49]
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
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
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