xanthophyll

The chemical structure of cryptoxanthin

The color of an egg yolk is from the xanthophyll carotenoids lutein and zeaxanthin

Xanthophylls (originally phylloxanthins) are yellow pigments from the carotenoid group. The name is from Greek xanthos (ξανθος, "yellow") + phyllon (φύλλον, "leaf"), due to their contribution to the yellow band in early chromatography of leaf pigments. Their molecular structure is based on carotenes, with additional oxidation. Thus, they are carotenoids but no longer carotenes.

Xanthophylls either contain hydroxyl groups and/or pairs of hydrogen atoms that are substituted by oxygen atoms. For this reason they are more polar than the purely hydrocarbon carotenes. Due to this difference, the carotenes travel further than xanthophylls in paper chromatography, since the support paper is always hydrophillic.

Xanthophylls are found in the leaves of most plants, where they act to modulate light energy. The xanthopylls found in the bodies of animals come from their food, and are ultimately derived from plant sources, even in dietary animal products which contain them. For example, the yellow color of chicken egg yolks, fat, and skin comes from ingested xanthophylls (primarily lutein, which is often added to feed for this purpose). The yellow color of the human macula lutea (literally, yellow spot) in the retina of the eye, comes from the lutein and zeaxanthin it contains, both xanthophylls again derived from the diet.

The group of xanthophylls includes lutein, zeaxanthin, neoxanthin, violaxanthin, and α- and β-cryptoxanthin. The latter compound is the only known xanthophyll to contain a beta-ionone ring, and thus β-cryptoxanthin is the only xanthophyll which is known to possess pro-vitamin A activity. Even then, it is a vitamin only for plant-eating species which possess the enzyme to make retinal from those carotenoids which contain beta-ionone.

Xanthophyll cycle

The xanthophyll cycle involves the enzymatic removal of epoxy groups from xanthophylls (e.g. violaxanthin, antheraxanthin, diadinoxanthin) to create so-called de-epoxidised xanthophylls (e.g. diatoxanthin, zeaxanthin). These enzymatic cycles were found to play a key role in stimulating energy dissipation within light harvesting antenna proteins by non-photochemical quenching- a mechanism to reduce the amount of energy that reaches the photosynthetic reaction centers. Non-photochemical quenching is one of the main ways of protecting against photoinhibition.^[1] In higher plants there are three carotenoid pigments that are active in the xanthophyll cycle: violaxanthin, antheraxanthin and zeaxanthin. During light stress violaxanthin is converted to zeaxanthin via the intermediate antheraxanthin, which plays a direct photoprotective role acting as a lipid-protective anti-oxidant and by stimulating non-photochemical quenching within light harvesting proteins. This conversion of violaxanthin to zeaxanthin is done by the enzyme violaxanthin de-epoxidase, while the reverse reaction is performed by zeaxanthin epoxidase^[2]

In diatoms and dinoflagellates the xanthophyll cycle consists of the pigment diadinoxanthin, which is transformed into diatoxanthin (diatoms) or dinoxanthin (dinoflagellates), at high light. ^[3]

References

1. ^ Falkowski, P. G. & J. A. Raven, 1997, Aquatic photosynthesis. Blackwell Science, 375 pp
2. ^ Taiz, Lincoln and Eduardo Zeiger. 2006. Plant Physiology. Sunderland, MA: Sinauer Associates, Inc. Publishers, Fourth edition, 764 pp
3. ^ Jeffrey, S. W. & M. Vesk, 1997. Introduction to marine phytoplankton and their pigment signatures. In Jeffrey, S. W., R. F. C. Mantoura & S. W. Wright (eds.), Phytoplankton pigments in oceanography, pp 37-84. – UNESCO Publishing, Paris.

• Demmig-Adams, B & W. W. Adams, 2006. Photoprotection in an ecological context: the remarkable complexity of thermal energy dissipation, New Phytologist, 172: 11–21.

External links

• MeSH Xanthophylls