Editing Xanthophyll (section)

==Xanthophyll cycle==
[[File:Violaxanthin cycle.png|thumb|400px|The 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]].<ref>Falkowski, P. G. & J. A. Raven, 1997, Aquatic photosynthesis. Blackwell Science, 375 pp</ref> 
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, i.e. reduced, 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 ([[Enzyme Commission number|EC]] [https://www.brenda-enzymes.org/enzyme.php?ecno=1.23.5.1 1.23.5.1]), while the reverse reaction, i.e. oxidation, is performed by zeaxanthin epoxidase ([[Enzyme Commission number|EC]] [https://www.brenda-enzymes.org/enzyme.php?ecno=1.14.15.21 1.14.15.21]).<ref>Taiz, Lincoln and Eduardo Zeiger. 2006. ''Plant Physiology''. Sunderland, MA: Sinauer Associates, Inc. Publishers, Fourth edition, 764 pp</ref>

In [[diatoms]] and [[dinoflagellates]], the xanthophyll cycle consists of the pigment [[diadinoxanthin]], which is transformed into [[diatoxanthin]] (diatoms) or [[dinoxanthin]] (dinoflagellates) under high-light conditions.<ref>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.
</ref>

Wright et al. (Feb 2011) found that, "The increase in zeaxanthin appears to surpass the decrease in violaxanthin in spinach" and commented that the discrepancy could be explained by "a synthesis of zeaxanthin from beta-carotene", however they noted further study is required to explore this hypothesis.<ref>{{cite journal|last=Wright|title=The interrelationship between the lower oxygen limit, chlorophyll fluorescence and the xanthophyll cycle in plants|journal=Photosynthesis Research|volume=107|issue=3|pages=223–235|display-authors=etal|doi=10.1007/s11120-011-9621-9|pmid=21290261|year=2011|s2cid=8454497}}</ref>