Antheraxanthin (from ''ánthos'', Greek for "flower" and ''xanthos'', Greek for "yellow") is a bright yellow

accessory pigment Accessory pigments are light-absorbing compounds, found in photosynthetic organisms, that work in conjunction with chlorophyll ''a''. They include other forms of this pigment, such as chlorophyll ''b'' in green algal and higher plant antennae, w ...

found in many organisms that perform

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. It is a

xanthophyll cycle Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek (, "yellow") and (, "l ...

pigment, an oil-soluble alcohol within the

xanthophyll Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek (, "yellow") and (, "lea ...

subgroup of

carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, co ...

. Antheraxanthin is both a component in and product of the cellular

photoprotection Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative st ...

mechanisms in photosynthetic green algae, red algae, euglenoids, and

plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

In the xanthophyll cycle

Antheraxanthin is an intermediate molecule of the

in most photosynthetic eukaryotes, namely plants, and some bacteria. In the

, specific carotenoid pigments are transformed via enzymatic reactions into either more or less photoprotective

biological pigments Biological pigments, also known simply as pigments or biochromes, are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments. Many biol ...

. A plant can increase its capacity for

non-photochemical quenching Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls (Chl) via enhanced internal con ...

(NPQ) and excess heat dissipation by converting the orange pigment

violaxanthin Violaxanthin is a xanthophyll pigment with an orange color found in a variety of plants. Violaxanthin is the product of the epoxidation of zeaxanthin where the oxygen atoms are from reactive oxygen species (ROS). Such ROS's arise when a plant ...

to antheraxanthin and then to light-yellow pigment

zeaxanthin Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji ( wolfberries ...

. The xanthophyll pool, or total xanthophyll cycle pigment levels are sometimes abbreviated as “VAZ” in scientific literature. “VAZ” is an abbreviation of the cycle’s main pigments in order of lowest to highest

each pigment provides. Antheraxanthin is denoted by the “A” in the middle, between violaxanthin (V) and zeaxanthin (Z).Kováč D, et al 2013. Response of green reflectance continuum removal index to the xanthophyll de-epoxidation cycle in Norway spruce needles. ''Journal of Experimental Botany'' 64, 1817-1827

Location in thylakoid membrane

The xanthophyll cycle takes place, along with other photopigment reactions, in the membranes of

thylakoids Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast th ...

within

chloroplasts A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

. Antheraxanthin provides more heat and light stability (via NPQ) to the thylakoid membranes than violaxanthin, but less than zeaxanthin. Xanthophyll cycle reactions, where antheraxanthin is an intermediate stage, are usually responses to changes in light or radiation exposure, which cause changes in the internal pH of thylakoids. Lessening or heightening photoprotection through the xanthophyll cycle allows plants to regulate their own light uptake for

. The majority of chloroplasts are located in a plant's

mesophyll A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, s ...

tissue cells, just below the epidermis (surface) layer of leaves and stems. Since thylakoids are contained within chloroplasts, antheraxanthin and other photopigments are in their highest concentrations in the leaves of plants under high solar radiation or light/heat exposure. A high-altitude study with

dandelions ''Taraxacum'' () is a large genus of flowering plants in the family Asteraceae, which consists of species commonly known as dandelions. The scientific and hobby study of the genus is known as taraxacology. The genus is native to Eurasia and Nor ...

(''Taraxacum officianale'') in the Andes Mountains found overall high rates of antheraxanthin accumulation in highly exposed leaves on a NW-facing slope at 1600 meter and even higher rates on a summit at 3600 meter.Molina-Montenegro MA, Penuelas J, Munne-Bosch S, & Sardans J, 2012
Higher plasticity in ecophysiological traits enhances the performance and invasion success of ''Taraxacum officinale'' (dandelion) in alpine environments
''Biology of Invasions'' 14, 21-33

Enzymes and reactions

Antheraxanthin is a partially de-epoxidized (see de-epoxidation) form of violaxanthin. One of violaxanthin’s two

epoxide In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale ...

groups are removed from its chemical structure to generate antheraxanthin.Eskling M, Arvidsson P, & Åkerlund H, 1997
The xanthophyll cycle, its regulation and components
''Physiologia Plantarum'' 100, 806-816 For this reason, the xanthophyll cycle is sometimes called the violaxanthin cycle. Violaxanthin de-epoxidase is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

that reduces one

group from violaxanthin into a double bond to create antheraxanthin. It also functions to create zeaxanthin, where it catalyses the reduction of two epoxide groups from violaxanthin. Zeaxanthin epoxidase catalyses the attachment of one epoxide group to zeaxanthin to generate antheraxanthin, and two epoxide groups to generate violaxanthin.

References

{{Carotenoids Carotenoids Epoxides Cyclohexenes