Antenna complexes protect Photosystem I from Photoinhibition

Background Photosystems are composed of two moieties, a reaction center and a peripheral antenna system. In photosynthetic eukaryotes the latter system is composed of proteins belonging to Lhc family. An increasing set of evidences demonstrated how these polypeptides play a relevant physiological function in both light harvesting and photoprotection. Despite the sequence similarity between antenna proteins associated with the two Photosystems, present knowledge on their physiological role is mostly limited to complexes associated to Photosystem II. Results In this work we analyzed the physiological role of Photosystem I antenna system in Arabidopsis thaliana both in vivo and in vitro. Plants depleted in individual antenna polypeptides showed a reduced capacity for photoprotection and an increased production of reactive oxygen species upon high light exposure. In vitro experiments on isolated complexes confirmed that depletion of antenna proteins reduced the resistance of isolated Photosystem I particles to high light and that the antenna is effective in photoprotection only upon the interaction with the core complex. Conclusions We show that antenna proteins play a dual role in Arabidopsis thaliana Photosystem I photoprotection: first, a Photosystem I with an intact antenna system is more resistant to high light because of a reduced production of reactive oxygen species and, second, antenna chlorophyll-proteins are the first target of high light damages. When photoprotection mechanisms become insufficient, the antenna chlorophyll proteins act as fuses: LHCI chlorophylls are degraded while the reaction center photochemical activity is maintained. Differences with respect to photoprotection strategy in Photosystem II, where the reaction center is the first target of photoinhibition, are discussed

Iron Coordination in Photosystem II: Interaction between Bicarbonate and the Q B Pocket Studied by Fourier Transform Infrared Spectroscopy

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Fourier transform infrared (FTIR) spectroscopy

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Transfert d'électrons et désexcitation de l'énergie chez les organismes photosynthétiques

AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

Vibrational spectroscopy to study the properties of redox-active tyrosines in photosystem II and other proteins

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Bicarbonate binding to the non-heme iron of photosystem II, investigated by Fourier transform infrared difference spectroscopy and 13C-labeled bicarbonate.

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Molecular origin of the pH dependence of tyrosine D oxidation kinetics and radical stability in photosystem II

AbstractA role for redox-active tyrosines has been demonstrated in many important biological processes, including water oxidation carried out by photosystem II (PSII) of oxygenic photosynthesis. The rates of tyrosine oxidation and reduction and the Tyr/Tyr reduction potential are undoubtedly controlled by the immediate environment of the tyrosine, with the coupling of electron and proton transfer, a critical component of the kinetic and redox behavior. It has been demonstrated by Faller et al. that the rate of oxidation of tyrosine D (TyrD) at room temperature and the extent of TyrD oxidation at cryogenic temperatures, following flash excitation, dramatically increase as a function of pH with a pKa of ≈7.6 [Faller et al. 2001 Proc. Natl. Acad. Sci. USA 98, 14368–14373; Faller et al. 2001 Biochemistry 41, 12914–12920]. In this work, we investigated, using FTIR difference spectroscopy, the mechanistic reasons behind this large pH dependence. These studies were carried out on Mn-depleted PSII core complexes isolated from Synechocystis sp. PCC 6803, WT unlabeled and labeled with 13C6-, or 13C1(4)-labeled tyrosine, as well as on the D2-Gln164Glu mutant. The main conclusions of this work are that the pH-induced changes involve the reduced TyrD state and not the oxidized TyrD state and that TyrD does not exist in the tyrosinate form between pH 6 and 10. We can also exclude a change in the protonation state of D2-His189 as being responsible for the large pH dependence of TyrD oxidation. Indeed, our data are consistent with D2-His189 being neutral both in the TyrD and TyrD states in the whole pH6-10 range. We show that the interactions between reduced TyrD and D2-His189 are modulated by the pH. At pH greater than 7.5, the ν(CO) mode frequency of TyrD indicates that TyrD is involved in a strong hydrogen bond, as a hydrogen bond donor only, in a fraction of the PSII centers. At pH below 7.5, the hydrogen-bonding interaction formed by TyrD is weaker and TyrD could be also involved as a hydrogen bond acceptor, according to calculations performed by Takahashi and Noguchi [J. Phys. Chem. B 2007 111, 13833–13844]. The involvement of TyrD in this strong hydrogen-bonding interaction correlates with the ability to oxidize TyrD at cryogenic temperatures and rapidly at room temperature. A strong hydrogen-bonding interaction is also observed at pH 6 in the D2-Gln164Glu mutant, showing that the residue at position D2-164 regulates the properties of TyrD. The IR data point to the role of a protonatable group(s) (with a pKa of ≈7) other than D2-His189 and TyrD, in modifying the characteristics of the TyrD hydrogen-bonding interactions, and hence its oxidation properties. It remains to be determined whether the strong hydrogen-bonding interaction involves D2-His189 and if TyrD oxidation involves the same proton transfer route at low and at high pH

Fourier Transform Infrared Difference Study of Tyrosine D Oxidation and Plastoquinone Q A Reduction in Photosystem II

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