Studies on well coupled Photosystem I-enriched subchloroplast vesicles. Content and redox properties of electron-transfer components

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. II. Structure and Functions

In Part I, by using 31P-NMR spectroscopy, we have shown that isolated granum and stroma thylakoid membranes (TMs), in addition to the bilayer, display two isotropic phases and an inverted hexagonal (HII) phase; saturation transfer experiments and selective effects of lipase and thermal treatments have shown that these phases arise from distinct, yet interconnectable structural entities. To obtain information on the functional roles and origin of the different lipid phases, here we performed spectroscopic measurements and inspected the ultrastructure of these TM fragments. Circular dichroism, 77 K fluorescence emission spectroscopy, and variable chlorophyll-a fluorescence measurements revealed only minor lipase- or thermally induced changes in the photosynthetic machinery. Electrochromic absorbance transients showed that the TM fragments were re-sealed, and the vesicles largely retained their impermeabilities after lipase treatments-in line with the low susceptibility of the bilayer against the same treatment, as reflected by our 31P-NMR spectroscopy. Signatures of HII-phase could not be discerned with small-angle X-ray scattering-but traces of HII structures, without long-range order, were found by freeze-fracture electron microscopy (FF-EM) and cryo-electron tomography (CET). EM and CET images also revealed the presence of small vesicles and fusion of membrane particles, which might account for one of the isotropic phases. Interaction of VDE (violaxanthin de-epoxidase, detected by Western blot technique in both membrane fragments) with TM lipids might account for the other isotropic phase. In general, non-bilayer lipids are proposed to play role in the self-assembly of the highly organized yet dynamic TM network in chloroplasts.

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon's energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet-singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5 ps. We further observed spectral equilibration component in the core complex with a 3-4 ps lifetime between the bulk Chl states and a state absorbing at 700 nm. Trapping in the core complex occurred with a 20 ps lifetime, which in the supercomplex split into two lifetimes, 16 ps and 67-75 ps. The experimental data could be modelled with two alternative models resulting in equally good fits-a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3-4 ps component is ascribed to equilibration with a "red" core antenna pool absorbing at 700 nm. Conversely, if these low-energy states are identified with the P700 reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.

Characterization of the structural changes and photochemical activity of photosystem I under Al(3+) effect

The photochemical activity of photosystem I (PSI) as affected by Al(3+) was investigated in thylakoid membranes and PSI submembrane fractions isolated from spinach. Biophysical and biochemical techniques such as oxygen uptake, light induced absorbance changes at 820nm, chlorophyll fluorescence emission, SDS-polyacrylamide gel electrophoresis, and FTIR spectroscopy have been used to analyze the sites and action modes of this cation on the PSI complex. Our results showed that Al(3+) above 3mM induces changes in the redox state of P700 reflected by an increase of P700 photooxidation phase and a delay of the slower rate of P700 re-reduction which reveals that Al(3+) exerted an inhibitory action at the donor side of PSI especially at plastocyanin (PC). Furthermore, results of P700 photooxidation monitored in the presence of DCMU with or without MV suggested that the same range of Al(3+) concentrations impairs the photochemical reaction centers (RC) of PSI, as shown by the decline in the amount of active population of P700, and disrupts the charge separation between P700 and the primary electron acceptor A0 leading to the inhibition of electron transfer at the acceptor side of PSI. These inhibitory actions were also accompanied by an impairment of the energy transfer from light harvesting complex (LHCI) to RC of PSI, following the disconnection of LHCI antenna as illustrated by an enhancement of chlorophyll fluorescence emission spectra at low temperature (77K). The above results coincided with FTIR measurements that indicated a conformational change of the protein secondary structures in PSI complex where 25% of α-helix was converted into β-sheet, β-antiparallel and turn structures. These structural changes in PSI complex proteins are closely related with the alteration photochemical activity of PSI including the inhibition of the electron transport through both acceptor and donor sides of PSI.

Heat- and light-induced detachment of the light-harvesting antenna complexes of photosystem I in isolated stroma thylakoid membranes

The multisubunit pigment-protein complex of photosystem I (PSI) consists of a core and peripheral light-harvesting antenna (LHCI). PSI is thought to be a rather rigid system and very little is known about its structural and functional flexibility. Recent data, however, suggest LHCI detachment from the PSI supercomplex upon heat and light treatments. Furthermore, it was suggested that the splitting off of LHCI acts as a safety valve for PSI core upon photoinhibition (Alboresi et al., 2009). In this work we analyzed the heat- and light-induced reorganizations in isolated PSI vesicles (stroma membrane vesicles enriched in PSI). Using differential scanning calorimetry we revealed a stepwise disassembly of PSI supercomplex above 50°C. Circular dichroism, sucrose gradient centrifugation and 77K fluorescence experiments identified the sequence of events of PSI destabilization: 3min heating at 60°C or 40min white light illumination at 25°C resulted in pronounced Lhca1/4 detachment from the PSI supercomplex, which is then followed by the degradation of Lhca2/3. The similarity of the main structural effects due to heat and light treatments supports the notion that thermo-optic mechanism, structural changes induced by ultrafast local thermal transients, which has earlier been shown to be responsible for structural changes in the antenna system of photosystem II, can also regulate the assembly and functioning of PSI antenna.

Temperature dependence of resonance Raman spectra of carotenoids

To understand the mechanism of the photoprotective and antioxidative functions of carotenoids, it is essential to have a profound knowledge of their excited electronic and vibronic states. In the present study we investigate the most powerful antioxidants: β-carotene and lutein by means of resonance Raman spectroscopy. The aim was to study in detail their Raman spectra in solution at room temperature and their changes as a function of temperature. To measure the spectra in their natural environment pyridine has been used as a solvent. It has been chosen because of its polarizability (n=1.5092) which is close to that of membrane lipids and proteins. The temperature dependence of the most intensive ν(1) band in the range from 77 K to 295 K at 514.5 nm excitation has been obtained. It was found that in pyridine the CC stretching frequency, its intensity, line shape, and line width are very sensitive to the temperature (the sensitivity being different for the two studied carotenoids). The observed linear temperature dependence of the CC stretching frequency is explained by a mechanism involving changes of the vibronic coupling and the extent of π-electron delocalization. The different behavior of the temperature-induced broadening of the ν(1) band and its intensity for the two studied carotenoids can be associated with the different nature of their solid matrices: glassy for β-carotene and crystalline-like for lutein, owing to their different chemical structures.

Resonance Raman spectroscopy of carotenoids in Photosystem I particles

Low-temperature resonance Raman (RR) spectroscopy was used for the first time to study the spectral properties, binding sites and composition of major carotenoids in spinach Photosystem I (PSI) particles. Excitation was provided by an argon ion laser at 457.9, 476.5, 488, 496.5, 502 and 514.5 nm. Raman spectra contained the four known groups of bands characteristic for carotenoids (called from nu(1) to nu4). Upon 514.5, 496.5 and 476.5 nm excitations, the nu(1)-nu(3) frequencies coincided with those established for lutein. Spectrum upon 502-nm excitation could be assigned to originate from violaxanthin, at 488 nm to 9-cis neoxanthin, and at 457.9 nm to beta-carotene and 9-cis neoxanthin. The overall configuration and composition of these bound carotenoid molecules in Photosystem I particles were compared with the composition of pigment extracts from the same PSI particles dissolved in pyridine, as well as to configuration in the main chlorophyll a/b light-harvesting protein complex of photosystem II. The absorption transitions for lutein, violaxanthin and 9-cis neoxanthin in spinach photosystem I particles are characterized, and the binding sites of lutein and neoxanthin are discussed. Resonance Raman data suggest that beta-carotene molecules are also present in all-trans and, probably, in 9-cis configurations.

Variable thermal dissipation in a Photosystem I submembrane fraction

Photoacoustic spectroscopy was used to study the thermal deactivation processes in a Photosystem I submembrane fraction isolated from spinach. A large part of the thermal dissipation was variable. The yield of this variable thermal emission depended on the redox state of the Photosystem. It increased with the measuring modulated light intensity coinciding with the gradual closure of the reaction centers. Thermal deactivation was maximal when the reaction centers were closed by a saturating illumination. Extrapolation of the data at zero light intensity indicated that the yield of non-variable thermal emission represented about 37% of the maximal emission. The presence of methylviologen as artificial electron acceptor decreased the yield of variable thermal emission whereas inhibition following heat stress treatments increased it. The significance of the variable and non-variable components of thermal dissipation is discussed and the measured energy storage is suggested to originate from the reduction of the plastoquinone pool during cyclic electron transport around Photosystem I.

On the question of the identity of cytochrome b-560 in thylakoid stromal membranes

Stromal membranes enriched in PS I contain a low potential cytochrome with a reduced α-band peak close to 560 nm. The identity of this cytochrome component has been ascribed either to a low potential form of the Photosystem II cytochrome b-559 or to a different cytochrome with a reduced α-band of 560 nm. The half-bandwidth of the 560 nm component in stromal membranes is identical to that of purified cytochrome b-559. Western blots show that the stromal membranes contain an amount of PS II cytochrome b-559 α-subunit that is more than sufficient to account for the cytochrome b-560 detected spectrophotometrically in these membranes. These immunochemical data and the similarity of (i) the spectral peaks, and (ii) the redox properties of low potential PS II cytochrome b-559 and the b-560 component, suggest that the simplest inference is that the cytochrome b-560 protein in stromal membranes is identical to the PS II cytochrome b-559.

Heat-stress stimulation of oxygen uptake by Photosystem I involves the reduction of superoxide radicals by specific electron donors

A Photosystem I submembrane fraction isolated from spinach was used to study the mechanism of heat-stress stimulation of oxygen uptake by the photosystem. Various artificial electron donors were shown to generate electron transport reactions with various degrees of thermally induced stimulation. A strong stimulation was observed with durohydroquinone as electron donor with a maximal effect at 50 °C. The degree of stimulation obtained was independent from the redox potential of the electron donors and from their oxidation site because the enzyme superoxide dismutase fully inhibited the stimulation. Instead, it is proposed that thermal stress causes the release of membrane bound superoxide dismutase from the thylakoids thus allowing the reduced form of electron donors with specific properties to reduce O2 (-) radicals to H2O2 besides the usual disproportionation of O2 (-) into O2 and H2O2.

Specific release of a 9-kDa extrinsic polypeptide of photosystem I from spinach chloroplasts by salt washing

The newly reported 9-kDa polypeptide in photosystem I [(1991) FEBS Lett. 280, 332-334] is an extrinsic component located on the lumenal side of the thylakoid membrane. This subunit can be solubilized with high salt buffer and does not bind any cofactors. The photosystem I electron transfer chain remains intact and functional in the absence of this component as characterized by the photoreduction of NADP+.

Copper tolerance in Silene cucubalus : Subcellular distribution of copper and its effects on chloroplasts and plastocyanin synthesis

The uptake, translocation and subcellular distribution of copper as well as its effect on chloroplasts and plastocyanin synthesis were studied in a copper-sensitive and a copper-tolerant population of Silene cucubalus (L.) Wib. As a function of time, the copper concentration in roots of tolerant plants increased more slowly than that in roots of sensitive ones. Translocation to the shoot occurred more rapidly in tolerant plants than in sensitive ones. Although it was accumulated in leaf cells, copper was not accumulated in the chloroplasts of either sensitive or tolerant plants. Chlorophyll content was not affected by copper in tolerant plants, whereas sensitive plants became chlorotic. Plastocyanin synthesis was not enhanced as a result of high copper concentrations and no difference in plastocyanin content between tolerant and sensitive plants was detected. Measurements of copper in purified cell walls revealed that storage of the metal in cell-wall material does not play an important role in tolerance mechanism. Uptake characteristics, distribution and cytoplasmic detoxification of copper are discussed.

Efficiency of hydrogen photoproduction by photosystem I-enriched subchloroplast vesicles combined with Proteus mirabilis cells. Effects of some exogenous electron donors

High rates of hydrogen photoproduction are obtained when glutaraldehyde-fixed Photosystem I-enriched vesicles (Photosystem II-depleted) are added to hydrogenase-containing cells of Proteus mirabilis in the presence of the mediator methylviologen and a suitable electron donating system. This donor system includes ascorbate, dithioerythritol (DTE) and the mediator tetramethylphenylene-diamine (TMPD) and reduces the photosynthetic electron transfer chain at the level of plastocyanin. Both DTE and ascorbate are required for hydrogen photoproduction, DTE being the ultimate electron donor and ascorbate only having a catalytic function. Whereas the aerobic photoreduction of methylviologen is similar in the presence of DTE, ascorbate or both, under anaerobic conditions only combination of both compounds results in a high and stable amount of reduced methylviologen that can be utilized by the hydrogenase. It is concluded that oxidation reactions of reduced methylviologen, competing with the hydrogenase, rather than methylviologen photoreduction, limit hydrogen photoproduction in the presence of either DTE or ascorbate. These oxidation reactions are suggested to involve back reactions to the oxidized form(s) of ascorbate and DTE but backflow to the photosynthetic electron transfer chain (i.e. cyclic electron transfer) can not be excluded.

The kinetics of P515 in relation to the lipid composition of the thylakoid membrane

Flash-induced P515 absorbance changes have been studied in dark-adapted chloroplasts isolated from spinach plants grown under two different light intensities. The slow component (reaction 2), normally present in the P515 response of chloroplasts isolated from plants grown at an intensity of 60 W X m-2, was largely reduced in chloroplasts isolated from plants grown at an intensity of 6 W X m-2. This reduction of the slow component in the P515 response appeared to be coincident with an alteration in the lipid composition of the thylakoid membrane. Mainly the ratio monogalactosyldiacylglycerol to digalactosyldiacylglycerol appeared to be altered. In thylakoids from plants grown at 6 W X m-2, the ratio was approximately 35% lower than that of plants grown at 60 W X m-2. The amount of both cytochrome b563 and cytochrome f was largely reduced in chloroplasts isolated from plants grown at low light intensity. These results may indicate a possible correlation between structural organization of the thylakoid membrane and the kinetics of the flash-induced P515 response.