Reaction centers in lipids

Specific functional role of physiologically important lipids of the photosynthetic membrane (phosphatidylcholine, cardiolipin and phosphatidylglycerol) was investigated on the thermodynamic and kinetic requirements of the charge movements in bacterial reaction centers. The major effect of these lipids is to increase the stabilization of the separated caharges induced by light excitation during the photosynthetic energy conversion. It can be achieved by (1) changing the redox midpoint potential of the QA/QA-and QB/QB-redox couples, which results in the increase of the free energy gap that drives the QA– to QB electron transfer or (2) by changing the quinone binding/unbinding equilibrium. This study provides evidence that from kinetic point of view the P+QA-QB → P+QAQB-charge transfer is mainly driven by the change in the enthalpy in LDAO and PC, whereas the entropy contribution is larger if negatively charged lipids are introduced

A stresszfehérjék és lipidek membrán kapcsolt homeosztázisa = Coupled homeostasis of membrane lipids and stress proteins

1. Megállapítottuk, hogy az IbpA/B fehérje in vivo különböző mértékben asszociálódik az E. coli membrán(ok)hoz. Langmuir-Blodget monolayer kísérletekben igazoltuk az IbpAB membránlipid kötését. Jellemeztük az E. coli IbpAB- mutánst mind molekuláris, mind fiziológiás szinten. Megállapítottuk, hogy az IbpAB- mutáns törzsek rendelkeznek „membrán-fenotípussal”, a vad típustól markánsan eltérő lipid zsírsavösszetétellel, valamint megváltozott fluiditással és permeabilitással. A lipidomikai kísérletek rávilágítottak a fluiditásváltozás kompenzációs jellegére, a homeosztatikus membrán adaptáció elvének megfelelően. A membránlipidek és hősokkfehérjék kapcsolatának egy másik dimenziójára kívántunk rávilágítani a vad típusú és mutáns törzsek globális „transcriptome” analízisével. A wt és az IbpAB mutánsok mRNS profilja hatalmas eltérést mutat, a változások gének egész sorát érintik, amelyek megértése további kísérleteket kíván. 2. Munkánk során elsőként igazoltuk Synechocystisben a DnaK2 fehérje szubfrakciójának tilakoid asszociációját. A DnaK2 részleges hiánya megváltoztatta a tilakoid membrán lipid- és zsírsavösszetételét, valamint a membrán fizikai állapotát. A membránváltozások befolyásolják a tilakoid membrán működését, ami eltérő hő- illetve UV-B stresszérzékenységben is megnyilvánul. A DnaK2 protein részleges hiánya UV károsodás után a PSII “repair” részleges inhibícióját eredményezte. Ily módon kimutattuk egy új, membránasszociált DnaK-függő stresszvédő mechanizmus létezését. | 1. In E. Coli the small heat shock protein IbpAB interacts with membranes in vivo and in vitro and rigidifides it which counterbalances heat-induced fluidization, indicating a protective role of this Hsp in the heat-shocked membrane. IbpAB- cells have an increased membrane permeability but higher outer membrane transition temperature and better survival at heat challenge. The improved survival is linked to the altered membrane phenotype. These include unique fatty acid changes and fluidization in the hydrophobic core of the membranes, which are associated with better survival upon heat challenge. 2. We have shown that in Synechocystis PCC6803 DnaK2 chaperon interacts with the thylakoid membrane, especially with the phosphatidyl glycerol (PG) lipid of the thylakoid. In DnaK2- cells the fatty acid composition of the thylakoid membrane differed from that of the wild type resulting in altered physico-chemical properties. We exposed cells to UV-B damage and concluded, that lack of the DnaK2 leads to partial inhibition of PSII repair and the rate of forward electron transport between QA and QB quinone electron acceptors is slowed down. PG plays an important role in binding extrinsic proteins required for a functional Mn cluster on the donor side of PSII. The binding of DnaK2 may take place trough binding specifically to PG and/or with the PSII complex. This interaction leads directly or indirectly to change in membrane fluidity which influences PSII electron transport

Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria.

In this paper we propose an energy dissipation mechanism that is completely reliant on changes in the aggregation state of the phycobilisome light-harvesting antenna components. All photosynthetic organisms regulate the efficiency of excitation energy transfer (EET) to fit light energy supply to biochemical demands. Not many do this to the extent required of desert crust cyanobacteria. Following predawn dew deposition, they harvest light energy with maximum efficiency until desiccating in the early morning hours. In the desiccated state, absorbed energy is completely quenched. Time and spectrally resolved fluorescence emission measurements of the desiccated desert crust Leptolyngbya ohadii strain identified (i) reduced EET between phycobilisome components, (ii) shorter fluorescence lifetimes, and (iii) red shift in the emission spectra, compared with the hydrated state. These changes coincide with a loss of the ordered phycobilisome structure, evident from small-angle neutron and X-ray scattering and cryo-transmission electron microscopy data. Based on these observations we propose a model where in the hydrated state the organized rod structure of the phycobilisome supports directional EET to reaction centers with minimal losses due to thermal dissipation. In the desiccated state this structure is lost, giving way to more random aggregates. The resulting EET path will exhibit increased coupling to the environment and enhanced quenching

Anisotropic Circular Dichroism of Light-Harvesting Complex II in Oriented Lipid Bilayers: Theory Meets Experiment

Anisotropic circular dichroism (ACD) spectroscopy of macroscopically aligned molecules reveals additional information about their excited states that is lost in the CD of randomly oriented solutions. ACD spectra of light-harvesting complex II (LHCII)-the main peripheral antenna of photosystem II in plants-in oriented lipid bilayers were recorded from the far-UV to the visible wavelength region. ACD spectra show a drastically enhanced magnitude and level of detail compared to the isotropic CD spectra, resolving a greater number of bands and weak optical transitions. Exciton calculations show that the spectral features in the chlorophyll Q y region are well-reproduced by an existing Hamiltonian for LHCII, providing further evidence for the identity of energy sinks at chlorophylls a603 and a610 in the stromal layer and chlorophylls a604 and a613 in the lumina] layer. We propose ACD spectroscopy to be a valuable tool linking the three-dimensional structure and the photophysical properties of pigment-protein complexes

In situ high-resolution structure of the baseplate antenna complex in <i>Chlorobaculum tepidum</i>

Photosynthetic antenna systems enable organisms harvesting light and transfer the energy to the photosynthetic reaction centre, where the conversion to chemical energy takes place. One of the most complex antenna systems, the chlorosome, found in the photosynthetic green sulfur bacterium Chlorobaculum (Cba.) tepidum contains a baseplate, which is a scaffolding super-structure, formed by the protein CsmA and bacteriochlorophyll a. Here we present the first high-resolution structure of the CsmA baseplate using intact fully functional, light-harvesting organelles from Cba. tepidum, following a hybrid approach combining five complementary methods: solid-state NMR spectroscopy, cryo-electron microscopy, isotropic and anisotropic circular dichroism and linear dichroism. The structure calculation was facilitated through development of new software, GASyCS for efficient geometry optimization of highly symmetric oligomeric structures. We show that the baseplate is composed of rods of repeated dimers of the strongly amphipathic CsmA with pigments sandwiched within the dimer at the hydrophobic side of the helix

Thermal Effects and Structural Changes of Photosynthetic Reaction Centers Characterized by Wide Frequency Band Hydrophone: Effects of Carotenoids and Terbutryn.

Photothermal characteristics and light-induced structural (volume) changes of carotenoid-containing and noncontaining photosynthetic reaction centers (RCs) were investigated by wide frequency band hydrophone. We found that the presence of carotenoid either does not play considerable role in the light-induced conformational movements, or these rearrangements are too slow for inducing a photoacoustic (PA) signal. The kinetic component with a few tens of microseconds, exhibited by the carotenoid-less RCs, appears to be similar to that of triplet state lifetimes, identified by other methods. The binding of terbutryn to the acceptor side is shown to affect the dynamics of the RC. Our results do not confirm large displacements or volume changes induced by the charge movements and by the charge relaxation processes in the RCs in few hundreds of microseconds time scale that accompanies the electron transfer between the primary and secondary electron acceptor quinones

Pigment Interactions in Light-Harvesting Complex II in Different Molecular Environments.

Extraction of plant light-harvesting complex II (LHCII) from the native thylakoid membrane or from aggregates by the use of surfactants brings about significant changes in the excitonic circular dichroism (CD) spectrum and fluorescence quantum yield. To elucidate the cause of these changes, e.g. trimer-trimer contacts or surfactant-induced structural perturbations, we compared the CD spectra and fluorescence kinetics of LHCII aggregates, artificial and native LHCII-lipid membranes, and LHCII solubilized in different detergents or trapped in polymer gel. By this means we were able to identify CD spectral changes specific to LHCII-LHCII interactions - at (-)437 nm and (+)484 nm, and changes specific to the interaction with the detergent n-dodecyl-beta-maltoside (beta-DM) or membrane lipids - at (+)447 nm and ( )494 nm. The latter change is attributed to the conformational change of the LHCII-bound carotenoid neoxanthin, by analyzing the CD spectra of neoxanthin-deficient plant thylakoid membranes. The neoxanthin-specific band at ( )494 nm was not pronounced in LHCII in detergent-free gels or solubilized in the alpha isomer of DM but was present when LHCII was reconstituted in membranes composed of phosphatidylcholine or plant thylakoid lipids, indicating that the conformation of neoxanthin is sensitive to the molecular environment. Neither the aggrega-tion-specific CD bands, nor the surfactant-specific bands were positively associated with the onset of fluorescence quenching, which could be triggered without invoking such spec-tral changes. Significant quenching was not active in reconstituted LHCII proteoliposomes, while a high degree of energetic connectivity, depending on the lipid:protein ratio, in these membranes allows for efficient light harvesting