Carotenoid

The orange ring surrounding Grand Prismatic Spring is due to carotenoid molecules, produced by huge mats of algae and bacteria.

Carotenoids are organic pigments that are naturally occurring in chromoplasts of plants and some other photosynthetic organisms like algae, some types of fungus and some bacteria. There are over 600 known carotenoids; they are split into two classes, xanthophylls and carotenes. They absorb blue light. Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage.[1] In humans, carotenoids such as beta-carotene are a precursor to vitamin A, a pigment essential for good vision, and carotenoids can also act as antioxidants.

People consuming diets rich in carotenoids from natural foods, such as fruits and vegetables, are healthier and have lower mortality from a number of chronic illnesses. However, a recent meta-analysis of 68 reliable antioxidant supplementation experiments involving a total of 232,606 individuals concluded that consuming additional beta-carotene from supplements is unlikely to be beneficial and may actually be harmful,[2] although this conclusion may be due to the inclusion of studies involving smokers.[3] With the notable exception of Vietnam Gac and crude palm oil, most carotenoid-rich fruits and vegetables are low in lipids. Since dietary lipids have been hypothesized to be an important factor for carotenoid bioavailability, a 2005 study investigated whether addition of avocado fruit or oil, as lipid sources, would enhance carotenoid absorption in humans. The study found that the addition of both avocado fruit and oil significantly enhanced the subjects' absorption of all carotenoids tested (alpha-carotene, beta-carotene, lycopene, and lutein).[4]

Contents

Properties

Main articles: carotenes and xanthophylls

Carotenoids belong to the category of tetraterpenoids (i.e. they contain 40 carbon atoms). Structurally they are in the form of a polyene chain which is sometimes terminated by rings.

Probably the most well-known carotenoid is the one that gives this second group its name, carotene, found in carrots (also apricots) and responsible for their bright orange colour. Crude palm oil, however, is the richest source of carotenoids in nature in terms of retinol (provitamin A) equivalent[5]. Vietnam Gac fruit contains the highest known concentration of the carotenoid lycopene.

Their colour, ranging from pale yellow through bright orange to deep red, is directly linked to their structure. Xanthophylls are often yellow, hence their class name. The double carbon-carbon bonds interact with each other in a process called conjugation, which allows electrons in the molecule to move freely across these areas of the molecule. As the number of double bonds increases, electrons associated with conjugated systems have more room to move, and require less energy to change states. This causes the range of energies of light absorbed by the molecule to decrease. As more frequencies of light are absorbed from the short end of the visible spectrum, the compounds acquire an increasingly red appearance.

Physiological effects

In photosynthetic organisms, specifically flora, carotenoids play a vital role in the photosynthetic reaction centre. They either participate in the energy-transfer process, or protect the reaction center from auto-oxidation. In non-photosynthesizing organisms, specifically humans, carotenoids have been linked to oxidation-preventing mechanisms.

Carotenoids disposition in proteins. Left: in cyanobacterium photosystem I carotenoids are outside (orange) PDB 1jb0. Right: in rhodopsin retinal is deep inside (pink) PDB 1f88.

Carotenoids have many physiological functions. Given their structure (above), carotenoids are efficient free-radical scavengers, and they enhance the vertebrate immune system. There are several dozen carotenoids in foods people consume, and most carotenoids have antioxidant activity. [6] Epidemiological studies have shown that people with high beta-carotene intake and high plasma levels of beta-carotene have a significantly reduced risk of lung cancer. However, studies of supplementation with large doses of beta-carotene in smokers have shown an increase in cancer risk (possibly because excessive beta-carotene results in breakdown products that reduce plasma vitamin A and worsen the lung cell proliferation induced by smoke[7]). Similar results have been found in other animals. Not all carotenoids are helpful, e.g. etretinate is a teratogen.

Humans and Animals are incapable of synthesizing carotenoids, and must obtain them through their diet, yet they are common and often in ornamental features. For example, the pink colour of flamingos and salmon, and the red colouring of lobsters are due to carotenoids. It has been proposed that carotenoids are used in ornamental traits because, given their physiological and chemical properties, they can be used as honest indicators of individual health, and hence they can be used by animals when selecting potential mates.

Simplified carotenoid synthesis pathway.

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. Lutein and the other carotenoid pigments found in mature leaves are often not obvious because of the presence of chlorophyll. However, when chlorophyll is not present, as in young folliage and also dying deciduous folliage (such as autumn leaves), the yellows, reds, and oranges of the carotenoids are predominant. For the same reason, carotenoid colors often predominate in ripe fruit (e.g., oranges, tomatoes, bananas), after being unmasked by the disappearance of chlorophyll.

Aroma chemicals

Products of carotenoid degradation such as ionones, damascones, and damascenones are also important fragrance chemicals that are used extensively in the perfumes and fragrance industry. Both beta-damascenone and beta-ionone although low in concentration in rose distillates are the key odour-contributing compounds in flowers. In fact, the sweet floral smells present in black tea, aged tobacco, grape, and many fruits are due to the aromatic compounds resulting from carotenoid breakdown.

Disease

Despite being important in nutrition, some carotenoids are produced by bacteria to protect themselves from immune attack, such as MRSA. The golden pigment of S. aureus allows it to survive competitive attack by Lactobaccillus as well as the human immune system.[8]

List of Naturally occurring carotenoids

2,2'-Diketospirilloxanthin 1,1'-Dimethoxy-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-y,y-carotene-2,2'-dione

(3R,3'R)-b,b-carotene-3,3'-diol dipalmitate

3,19,3'-trihydroxy-7,8-dihydro-b,e-caroten-8-one 19-laurate

5',6'-dihydro-5'-apo-18'-nor-b-caroten-6'-one or 6'-methyl-6'-apo-b-caroten-6'-one

3-hydroxy-8'-methyl-7,8-didehydro-8'-apo-b-carotene-4,8'-dione

See also

References

  1. ↑ Armstrong GA, Hearst JE (1996). "Carotenoids 2: Genetics and molecular biology of carotenoid pigment biosynthesis". FASEB J. 10 (2): 228–37. PMID 8641556. http://www.fasebj.org/cgi/pmidlookup?view=long&pmid=8641556. 
  2. ↑ Bjelakovic G, et al (2007). "Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis". JAMA 297 (8): 842–57. doi:10.1001/jama.297.8.842. PMID 17327526. 
  3. ↑ It is known that taking beta-carotene supplements is harmful for smokers, and the meta-analysis of Bjelakovic et al. was influenced by inclusion of these studies. See the letter to JAMA by Philip Taylor and Sanford Dawsey and the reply by the authors of the original paper.
  4. ↑ Unlu N, et al (2005). "Carotenoid Absorption from Salad and Salsa by Humans Is Enhanced by the Addition of Avocado or Avocado Oil". Human Nutrition and Metabolism 135 (3): 431–6. PMID 15735074. http://jn.nutrition.org/cgi/pmidlookup?view=long&pmid=15735074. 
  5. ↑ Choo Yuen May Palm oil carotenoids
  6. ↑ Beta-carotene and other carotenoids as antioxidants. From U.S. National Library of Medicine. November, 2008.
  7. ↑ Alija AJ, Bresgen N, Sommerburg O, Siems W, Eckl PM (2004). "Cytotoxic and genotoxic effects of {beta}-carotene breakdown products on primary rat hepatocytes". Carcinogenesis 25 (5): 827–31. doi:10.1093/carcin/bgh056. PMID 14688018. http://carcin.oxfordjournals.org/cgi/content/full/25/5/827. 
  8. ↑ Liu GY, Essex A, Buchanan JT, et al (2005). "Staphylococcus aureus golden pigment impairs neutrophil killing and promotes virulence through its antioxidant activity". J. Exp. Med. 202 (2): 209–15. doi:10.1084/jem.20050846. PMID 16009720. PMC: 2213009. http://www.jem.org/cgi/content/full/202/2/209. 

Classifications

Carotenoids can have many classifications. Some are alcohols, hydrocarbons, ethers, epoxides, ketones, acids, etc. They can be classified also into Apo Carotenoids, Nor and Seco Carotenoids, retro Carotenoids, retro Apo Carotenoids and Higher Carotenoids.

External links

Types of Plant Pigments
Flavonoids
Anthocyanins • Anthocyanidins • Anthoxanthins • Proanthocyanidins • Tannins
Betalains
Betacyanins • Betaxanthins
Carotenoids
Xanthophylls • Carotenes • Retinoids
Other
Chlorophyll • Allophycocyanin • Phycocyanin • Phycoerythrin • Phycoerythrocyanin • Quinones • Xanthones
Carotenoids
Carotenes:
Carotene (Alpha-carotene · Beta-carotene)  · Lycopene · Phytoene • Phytofluene · Neurosporene
Retinoids:
Acitretin · Alitretinoin · Apocarotenal · Bexarotene · Etretinate · Fenretinide · Isotretinoin · Retinaldehyde · Tazarotene · Vitamin A (Retinol, Tretinoin)
Xanthophylls:
Antheraxanthin · Astaxanthin · Canthaxanthin · Citranaxanthin · Cryptoxanthin · Diadinoxanthin · Diatoxanthin · Dinoxanthin  · Flavoxanthin · Fucoxanthin · Lutein · Neoxanthin · Rhodoxanthin · Rubixanthin · Violaxanthin · Zeaxanthin
Other:
Crocin · Crocetin · Peridinin · Food orange 7