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Light Protection in Plants: Characterisation of Violaxanthin de-epoxidase

Hallin, Erik LU (2016)
Abstract (Swedish)
Växter behöver ljus. Ljuset omvandlas genom fotosyntesen till kemisk energi som kan lagras i växten. Mer ljus ger således mer energi men bara upp till en viss gräns. När växten inte längre kan hantera mängden ljus som den utsätts för kommer flaskhalsar i processen orsaka att högenergetiska tillstånd blir mer långlivade. Dessa reaktiva ämnen kan då orsaka skada på närliggande komponenter, vilket kan kräva en kostsam reparation eller till och med död. Växter måste därför kunna reglera mängden ljus som når det fotosyntetiska maskineriet. Detta måste också ske i sådan hastighet att ljusvariationer orsakade på tidsskalan sekunder till minuter, från till exempelvis moln kan hanteras. Då växter inte har förmågan att förflytta sig när de utsätts... (More)
Växter behöver ljus. Ljuset omvandlas genom fotosyntesen till kemisk energi som kan lagras i växten. Mer ljus ger således mer energi men bara upp till en viss gräns. När växten inte längre kan hantera mängden ljus som den utsätts för kommer flaskhalsar i processen orsaka att högenergetiska tillstånd blir mer långlivade. Dessa reaktiva ämnen kan då orsaka skada på närliggande komponenter, vilket kan kräva en kostsam reparation eller till och med död. Växter måste därför kunna reglera mängden ljus som når det fotosyntetiska maskineriet. Detta måste också ske i sådan hastighet att ljusvariationer orsakade på tidsskalan sekunder till minuter, från till exempelvis moln kan hanteras. Då växter inte har förmågan att förflytta sig när de utsätts för starkt ljus har evolutionen tagit fram ett antal alternativ. Riktningen av blad mot eller från ljuskällan och förmågan av de fotosyntetiska komponenterna att skugga varandra är två metoder för att reglera mängden inkommande ljus. Därefter kan upptaget ljus delvis omvandlas till värme, som är en mindre reaktiv form av energi. Exakt hur denna omvandlingsprocess går till är inte känd ännu men vi vet att en av de inblandade komponenterna är molekylen zeaxantin, som bildas tillfälligt då växten utsätts för ljusstress. Zeaxantin omvandlas från violaxantin av enzymet violaxantin de-epoxidas (VDE), som aktiveras då fotosyntesen börjar få svårt att hinna med att hantera det inkommande ljuset.

Detta arbete går ut på att karaktärisera VDE och därigenom ge en djupare förståelse om hur växter och alger kan försvara sig mot ljus. Ett enzyms funktion är väl sammankopplat till dess struktur, vilket gör karaktärisering av strukturen till en viktig del av analysen. Med hjälp av ett antal metoder har information erhållits om strukturen hos VDE och hur denna förändras vid aktivering. Exempelvis kunde vi genom detta visa att VDE molekyler kopplas samman vid aktiveringen, vilket kan vara en viktig del av förmågan att omvandla violaxantin till zeaxantin. Ett enzym består vanligtvis av en kedja av aminosyror. I vissa fall finns interna länkar bildade av aminosyran cystein. Hur dessa cysteiner är länkade inom VDE samt deras innebörd för aktiviteten har studerats. För att analysera vilka delar av VDE som är viktiga för dess aktivitet har dessa delar byts ut och även tagits bort i olika kombinationer. Detta har också gett antydningar om var i VDE som violaxantin binder. Genom att också analysera hur mycket av den bildade slutprodukten samt reaktionens intermediat som förekommer vid olika tidspunkter kunde påvisa att intermediatet lämnar VDE och måste bindas igen för att slutprodukten zeaxantin ska bildas.
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Abstract
Plants and algae need light to drive the photosynthetic machinery. An excess of light will, however, result in damage to the photosynthetic machinery and to the rest of the organism. The surplus of energy, when exposed to light stress is converted into less harmful heat energy through a process called non-photochemical quenching. This quenching is partly dependent on the xanthophyll pool located inside the thylakoid membrane. During light stress the xanthophyll violaxanthin is converted to zeaxanthin by violaxanthin de-epoxidase (VDE), triggering the zeaxanthin dependent light quenching. VDE is located on the lumen side of the thylakoid and is activated by the reduction of pH caused by photosynthesis. The sequence of VDE has been divided... (More)
Plants and algae need light to drive the photosynthetic machinery. An excess of light will, however, result in damage to the photosynthetic machinery and to the rest of the organism. The surplus of energy, when exposed to light stress is converted into less harmful heat energy through a process called non-photochemical quenching. This quenching is partly dependent on the xanthophyll pool located inside the thylakoid membrane. During light stress the xanthophyll violaxanthin is converted to zeaxanthin by violaxanthin de-epoxidase (VDE), triggering the zeaxanthin dependent light quenching. VDE is located on the lumen side of the thylakoid and is activated by the reduction of pH caused by photosynthesis. The sequence of VDE has been divided into three domains, a conserved N-terminal domain rich in cysteines, a lipocalin-like domain expected to bind the substrates and a negatively charged C-terminal domain rich in glutamic acid. In this work we have constructed cysteine mutants that revealed the importance of 12 of the 13 cysteines of VDE to the activity. These 12 cysteines were found to form disulphides in a pattern giving two hairpin structures and also increased the thermal stability of VDE.

The active site of VDE does not appear to be exclusively located in the cysteine rich N-terminal domain. The expression of the N-terminal domain without the rest of VDE did not show catalytic activity. The rest of VDE without the N-terminal domain was also not able to catalyse the reaction, but after mixing of these two constructs the activity returned. This shows that the N-terminal domain and the rest of VDE can fold independently and also indicates that the active site is localised in the interface between the N-terminal domain and the lipocalin-like domain. Crosslinking of monomeric VDE could localise the N-terminal domain near the opening of the lipocalin barrel, where violaxanthin is predicted to bind.

The glutamic rich C-terminal domain could be truncated from VDE while the rest of VDE remained active. This showed that the C-terminal domain was not required for the catalytic activity of VDE. The truncation did, however, cause a great loss of activity and a shift in how the VDE activity depends on pH. The C-terminal domain could be linked with the ability of VDE to oligomerise at the pH required for activity. The pH dependent oligomerisation of VDE was lost after truncation of the C-terminal domain. A reduction in pH towards the pH required for optimal activity also causes a strong formation of α-helical structures involved in coiled coils. This formation of secondary structure was also lost after truncation of the C-terminal domain, which is predicted to contain coiled coils. A likely scenario is that the pH activation of VDE involves an oligomerisation event caused by coiled coils at the C-terminal domain. The oligomerisation of VDE could also be seen using chemical crosslinking at different pH, showing a monomeric state at neutral pH while oligomeric interactions occur at lower pH. Small angle x-ray scattering gave indications of a dimeric symmetry of the oligomeric state, while also revealing an elongated shape of monomeric VDE with the lipocalin-like domain localised in the centre. We also show that the previously proposed symmetric docking of violaxanthin into the dimeric state of the lipocalin-like domain appears less likely compared to a non-symmetric binding, based on the observation that only one side of violaxanthin is converted per substrate binding of VDE.
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Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr. Kieselbach, Thomas, Department of Chemistry, Umeå University, Sweden
organization
publishing date
type
Thesis
publication status
published
subject
keywords
violaxanthin de-epoxidase, cysteine, disulphide bond, function, pH activation, oligomerisation, circular dichroism, crosslinking, small angle x-ray scattering
pages
114 pages
publisher
Lund University, Faculty of Science, Department of Biochemistry and Structural Biology
defense location
Center for chemistry and chemical engineering, lecture hall C, Naturvetarvägen 14, Lund
defense date
2016-05-13 13:15
ISBN
978-91-7422-439-9
language
English
LU publication?
yes
id
25d79f5d-292c-42ec-900b-60510e78e129
date added to LUP
2016-04-30 12:23:21
date last changed
2016-09-19 08:45:20
@phdthesis{25d79f5d-292c-42ec-900b-60510e78e129,
  abstract     = {Plants and algae need light to drive the photosynthetic machinery. An excess of light will, however, result in damage to the photosynthetic machinery and to the rest of the organism. The surplus of energy, when exposed to light stress is converted into less harmful heat energy through a process called non-photochemical quenching. This quenching is partly dependent on the xanthophyll pool located inside the thylakoid membrane. During light stress the xanthophyll violaxanthin is converted to zeaxanthin by violaxanthin de-epoxidase (VDE), triggering the zeaxanthin dependent light quenching. VDE is located on the lumen side of the thylakoid and is activated by the reduction of pH caused by photosynthesis. The sequence of VDE has been divided into three domains, a conserved N-terminal domain rich in cysteines, a lipocalin-like domain expected to bind the substrates and a negatively charged C-terminal domain rich in glutamic acid. In this work we have constructed cysteine mutants that revealed the importance of 12 of the 13 cysteines of VDE to the activity. These 12 cysteines were found to form disulphides in a pattern giving two hairpin structures and also increased the thermal stability of VDE.<br/> <br/>The active site of VDE does not appear to be exclusively located in the cysteine rich N-terminal domain. The expression of the N-terminal domain without the rest of VDE did not show catalytic activity. The rest of VDE without the N-terminal domain was also not able to catalyse the reaction, but after mixing of these two constructs the activity returned. This shows that the N-terminal domain and the rest of VDE can fold independently and also indicates that the active site is localised in the interface between the N-terminal domain and the lipocalin-like domain. Crosslinking of monomeric VDE could localise the N-terminal domain near the opening of the lipocalin barrel, where violaxanthin is predicted to bind.<br/> <br/>The glutamic rich C-terminal domain could be truncated from VDE while the rest of VDE remained active. This showed that the C-terminal domain was not required for the catalytic activity of VDE. The truncation did, however, cause a great loss of activity and a shift in how the VDE activity depends on pH. The C-terminal domain could be linked with the ability of VDE to oligomerise at the pH required for activity. The pH dependent oligomerisation of VDE was lost after truncation of the C-terminal domain. A reduction in pH towards the pH required for optimal activity also causes a strong formation of α-helical structures involved in coiled coils. This formation of secondary structure was also lost after truncation of the C-terminal domain, which is predicted to contain coiled coils. A likely scenario is that the pH activation of VDE involves an oligomerisation event caused by coiled coils at the C-terminal domain. The oligomerisation of VDE could also be seen using chemical crosslinking at different pH, showing a monomeric state at neutral pH while oligomeric interactions occur at lower pH. Small angle x-ray scattering gave indications of a dimeric symmetry of the oligomeric state, while also revealing an elongated shape of monomeric VDE with the lipocalin-like domain localised in the centre. We also show that the previously proposed symmetric docking of violaxanthin into the dimeric state of the lipocalin-like domain appears less likely compared to a non-symmetric binding, based on the observation that only one side of violaxanthin is converted per substrate binding of VDE.<br/>},
  author       = {Hallin, Erik},
  isbn         = {978-91-7422-439-9},
  keyword      = {violaxanthin de-epoxidase,cysteine,disulphide bond,function,pH activation,oligomerisation,circular dichroism,crosslinking,small angle x-ray scattering},
  language     = {eng},
  pages        = {114},
  publisher    = {Lund University, Faculty of Science, Department of Biochemistry and Structural Biology},
  school       = {Lund University},
  title        = {Light Protection in Plants: Characterisation of Violaxanthin de-epoxidase},
  year         = {2016},
}