Xanthophyll#Xanthophyll cycle
The xanthophyll cycle, violaxanthin cycle also, is a protective mechanism of the photosynthetic apparatus of higher plants and some green algae. It helps dissipate excess excitation energy as heat, before the formation of reactive oxygen species. Thereby damage to the antenna complex of photosystem II (PS II) is minimized. The xanthophyll cycle is an important step for the non- photochemical quenching of Excitonenenergie.
This can happen at Überenergetisierung the electron transport chain, for example by excessive light ( high light stress).
- 3.1 Diadinoxanthin - Deepoxidase (DDE)
- 3.2 Diatoxanthin epoxidase (DEP )
- 3.3 Importance of
Non - photochemical quenching of light energy
During photosynthesis the energy of light is used. Photosynthesis rate depends on the light intensity, but it is limited to a certain degree. At high light intensities are therefore many chlorophyll molecules in the antenna complexes excited before, but due to the exhausted their photosynthetic capacity light energy not forward in the reaction center of the photosynthetic system II (PS II). This has the consequence that the excited molecules of chlorophyll generate reactive oxygen species (ROS ), such as singlet oxygen 1O2. ROS damage pigments, proteins and lipids of the thylakoid membrane, thereby inhibiting photosynthesis or destroy photosynthetic systems.
To cope with excess energy, plants use the xanthophyll cycle as a protective mechanism.
Biochemistry
Violaxanthin - Deepoxidase (VDE )
During intense light as a result of efficient photosynthesis many protons are translocated into the thylakoid lumen. These are usually used for the operation of a membrane-bound ATP synthase. At high light levels, however, the proton gradient is broken down quickly enough, since the ATP requirement of the plant is covered. As a result, the lumen is acidified strongly ( pH reduction ). This activates the enzyme - violaxanthin Deepoxidase ( EC 1.10.99.3 ), which reduces the diepoxide violaxanthin on the monoepoxide antheraxanthin to zeaxanthin. VDE is a nuclear -encoded enzyme which is located in the lumen of the chloroplast and has a pH optimum at 5.0. In the dark, the enzyme is inactive and is not membrane associated before.
For the reduction of ascorbate ( Asch ) is needed, which also activates the enzyme. This is oxidized to dehydroascorbate (DHA ), with also water is released. DHA can not be regenerated to ascorbate in the lumen. In addition, ( al- ) can diffuse through the thylakoid membrane as an anion not easy. Maybe there is a transporter system which transports ascorbate into and DHA from the lumen. In the stroma of the chloroplast DHA can be reduced with consumption of NADPH and glutathione to ascorbate.
Zeaxanthin epoxidase (ZE )
The back reaction of zeaxanthin via antheraxanthin to violaxanthin is catalyzed by another membrane-associated enzyme, the zeaxanthin epoxidase ( EC 1.14.13.90 ). It is located in the stroma, and performs, using oxygen and a NADPH one epoxide group. The optimum pH is 7.5 this monooxygenase. The reaction takes place on plants in the dark or low light, but was also observed for bright light. For the reaction further cofactors such as FAD and ferredoxin are required.
VDE and ZE are among the vegetable Lipocalinproteinen and have common structural similarities.
Importance
For the protection of photosystem plants have many options, but include carotenoids as the main protection systems. This obtains the xanthophyll cycle as non- constitutive defense mechanism against an oversupply of light energy are the most important. The light harvesting complexes zeaxanthin binds to a subunit of the LHCII, which is protonated at the low pH. This allows effective recording of the energy of the triplet state of chlorophyll ( 3Chl a *), which is then radiated as heat. This is because that excited zeaxanthin has only a short life time of 10 ps. Contrast, violaxanthin directs excitation energy on to chlorophyll and therefore functions as accessory pigment. A high excitation of PS II leads to a high pH gradient, only thereby zeaxanthin can be formed. It is estimated that 50 to 70 % of the absorbed photons are converted by the heat cycle.
Diadinoxanthinzyklus
In diatoms, a similar cycle is for the non- photochemical quenching used in which zeaxanthin Diatoxanthin (DTX) is used instead. This is implemented by Dtx - Epoixdase (DEP ) to Diadionoxanthin ( Ddx ) under consumption of NADPH and O2. Diadionoxanthin therefore is functionally equivalent to violaxanthin. Starting from Dtx Ddx is recycled through the Diadinoxanthin - Deepoxidase (DDE).
This cycle does not occur with a diepoxide compound, the conversion of DTX Ddx is also very fast.
Diadinoxanthin - Deepoxidase (DDE)
The enzyme has a pH optimum at DDE 5.5, but also shows in a neutral medium activity. It requires as a cofactor ascorbate ( Asch ), to reduce the epoxy. In this case, water is released as the violaxanthin - Deepoxidase (VDE ). In contrast to VDE DDE shows some differences: DDE requires less ascorbate and its activity is stimulated by lower concentration of Monogalactosyldiacylglycerol.
Diatoxanthin epoxidase (DEP )
For the oxidation of Diatoxanthin to Diadionoxanthin DEP requires the same cofactors such as zeaxanthin epoxidase (PA): NAD (P) H, FAD, ferredoxin and oxygen. This catalyzed reaction takes place at a pH - optimum of 7.5. In the dark and with a strong (light -powered ) the pH gradient across the membrane DEP is completely inhibited. Probably there is not enough NADPH available during hours of darkness.
Importance
For diatoms the Diadinoxanthinzyklus is the most important safeguard in case of excessive excitation of the photosynthetic apparatus. In contrast to the xanthophyll cycle, the reactions proceed much faster.
Lutein Epoxidzyklus
In some plant species of lutein Epoxidzyklus serves as a protective mechanism.