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Identifier 000383143
Title Ο ρόλος των πολυαμινών στη διαχείριση των φωτονίων και των πρωτονίων από τη φωτοσυλλεκτική κεραία του φωτοσυστήματος ΙΙ
Alternative Title The role of polyamines in the management of photons and protons by the light harvesting antenna of photosystem II
Author Τσιάβος, Θεόδωρος Κ.
Thesis advisor Κοτζαμπάσης, Κυριάκος
Reviewer Ρουμπελάκη-Αγγελάκη, Καλλιόπη
Γανωτάκης, Δημήτριος
Abstract The plant light-harvesting complex of photosystem II (LHCII) functions not only to absorb and deliver solar energy to the photosystem II (PSII) reaction centers, but also to protect them from overexcitation. LHCII dissipates excess solar energy as thermal energy under high light conditions, in a process that is referred to as non-photochemical quenching (NPQ) (Horton et al., 1994). The molecular mechanism of high energy quenching (qE), the rapidly inducible and reversible component of NPQ which is triggered when ΔpH across the thylakoid membrane is increased, remains unresolved. The predominant hypothesis claims that an energy quenching center is formed from the interaction between specific xanthophyll and chlorophyll (Chl) molecules due to the ΔpH-induced changes in internal pigment organization of trimeric or monomeric proteins of LHCII (Horton et al., 2000; Pascal et al., 2005; Ruban et al., 2007; Ahn et al., 2008). This work is focused on the role of polyamines (PAs) in qE. The main PAs (putrescine, Put, spermidine, Spd and spermine, Spm) are found in vivo in chloroplast and bound to LHCII (Navakoudis et al., 2007), while upon illumination they accumulate in lumen (Ioannidis et al., 2012). In order to gradually focus on the mechanism in vitro a bottom-up bioenergetic approach was adopted. Initially, it was tested in detail the mode of interaction of all three PAs with isolated Chls as well as with their water-soluble analogue chlorophyllin (SCC). The results showed that all three PAs bind to the Mg ion of Chls and shift their coordination-state equilibrium. For the first time, spectroscopic data are presented for the axial ligation of Chl b. The coordination of Spm with Chl b has the most interesting features from all pigments tested. Spm induces reversible increases and decreases of the fluorescence yield of Chl b at about 661nm. Interestingly, equilibrium between a high-fluorescence yield conformation and a low yield is feasible by the interaction of Chl b and aminic ligands. Most probably Spm binds axially to the Mg of Chl b via its imino groups, as revealed by experiments with SCC showing that the mode of interaction is dependent upon their protonation state. Furthermore, new absorption data for the diagnostic region of 535nm, related to qE, are provided and discussed. Then, the interaction of PAs with isolated and purified complexes of the PSII antenna was studied. At physiological pH, PAs stimulate fluorescence quenching of both trimeric LHCII and monomeric LHCb complexes, mimicking the action of protons. Spm was the most potent quencher and induced aggregation of LHCII trimers, due to its highly cationic character. It seems that the mechanism of Spm-induced aggregation comprises a surface charge screening of LHCII until larger aggregates are formed. Spm-induced and qE-induced LHCII aggregation results in the same conformation as revealed by Raman spectroscopy. A possible interpretation of these results is that amines or/and amine-induced aggregation lead to deeper insertion of the Mg in the macrocycle and this quenches fluorescence of one Chl creating a sink that quenches the fluorescence of the LHCII. A putative site of this mechanism could be the domain that ligates Chl8 of LHCII, as it was shown by in silico analysis that it has similar structure to the domain of heme in myoglobin that modulates the positioning of the metal in heme. In the third part of this work, the antenna complexes were reconstituted into proteoliposomes in an attempt to mimic in vitro their in vivo conditions. Conductivity measurements showed that both monomeric and trimeric complexes increased the membrane permeability to protons when in a partly aggregated state. The Spm-induced aggregation of LHCII within the lipid phase was found to increase the non-specific proton permeation across the membrane. On the other hand, addition of Spm to already incorporated complexes reduced their permeability, indicating that LHCII may also serve as a specific channel to protons. All in all, a new model for the photoprotection of the photosynthetic apparatus by Spm is discussed.
Language Greek
Subject Chlorophyll
Lihgt harvestin complex
Non-photochemical quenching
Μη φωτοχημική διάχυση ενέργειας
Συναρμογή μετάλλου
Φωτοσυλλεκτικά σύμπλοκα
Issue date 2014-03-13
Collection   School/Department--School of Sciences and Engineering--Department of Biology--Doctoral theses
  Type of Work--Doctoral theses
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