Showing papers on "Photosynthetic reaction centre published in 2005"

Towards complete cofactor arrangement in the 3.0 Å resolution structure of photosystem II

Abstract: Oxygenic photosynthesis in plants, algae and cyanobacteria is initiated at photosystem II, a homodimeric multisubunit protein-cofactor complex embedded in the thylakoid membrane. Photosystem II captures sunlight and powers the unique photo-induced oxidation of water to atmospheric oxygen. Crystallographic investigations of cyanobacterial photosystem II have provided several medium-resolution structures (3.8 to 3.2 A) that explain the general arrangement of the protein matrix and cofactors, but do not give a full picture of the complex. Here we describe the most complete cyanobacterial photosystem II structure obtained so far, showing locations of and interactions between 20 protein subunits and 77 cofactors per monomer. Assignment of 11 beta-carotenes yields insights into electron and energy transfer and photo-protection mechanisms in the reaction centre and antenna subunits. The high number of 14 integrally bound lipids reflects the structural and functional importance of these molecules for flexibility within and assembly of photosystem II. A lipophilic pathway is proposed for the diffusion of secondary plastoquinone that transfers redox equivalents from photosystem II to the photosynthetic chain. The structure provides information about the Mn4Ca cluster, where oxidation of water takes place. Our study uncovers near-atomic details necessary to understand the processes that convert light to chemical energy.

Two-step mechanism of photodamage to photosystem II: step 1 occurs at the oxygen-evolving complex and step 2 occurs at the photochemical reaction center.

Abstract: Under strong light, photosystem II (PSII) of oxygenic photosynthetic organisms is inactivated, and this phenomenon is called photoinhibition. In a widely accepted model, photoinhibition is induced by excess light energy, which is absorbed by chlorophyll but not utilized in photosynthesis. Using monochromatic light from the Okazaki Large Spectrograph and thylakoid membranes from Thermosynechococcus elongatus, we observed that UV and blue light inactivated the oxygen-evolving complex much faster than the photochemical reaction center of PSII. These observations suggested that the light-induced damage was associated with a UV- and blue light-absorbing center in the oxygen-evolving complex of PSII. The action spectrum of the primary event in photodamage to PSII revealed the strong effects of UV and blue light and differed considerably from the absorption spectra of chlorophyll and thylakoid membranes. By contrast to the photoinduced inactivation of the oxygen-evolving complex in untreated thylakoid membranes, red light efficiently induced inactivation of the PSII reaction center in Tris-treated thylakoid membranes, and the action spectrum resembled the absorption spectrum of chlorophyll. Our observations suggest that photodamage to PSII occurs in two steps. Step 1 is the light-induced inactivation of the oxygen-evolving complex. Step 2, occurring after step 1 is complete, is the inactivation of the PSII reaction center by light absorbed by chlorophyll. We confirmed our model by illumination of untreated thylakoid membranes with blue and UV light, which inactivated the oxygen-evolving complex, and then with red light, which inactivated the photochemical reaction center.

Photoinhibition of photosystem I

Abstract: Photosystem I (PSI) is a large pigment-protein complex consisting of about 18 different subunits in plants. Recently, the structure of PSI isolated from pea was solved by X-ray crystallography at a resolution of 4.4 A (Ben-Shem et al. 2003). This work has highlighted the structural similarities and differences between plant PSI and cyanobacterial PSI, where a 2.5 A structure was published earlier (Jordan et al. 2001). The subunits of PSI are traditionally divided into 14 core subun its—most of which are also found in cyanobacte ria—and four homologous light harvesting complex I (LHCI) subunits, which are specific to plants (Fig. 1). Under certain conditions, other LHCI subunits can be seen—at least in Arabidopsis (Klimmek et al. 2005). The PSI complex binds about 167 chlorophyll molecules (Ben-shem et al. 2003). A special pair of chlorophyll a molecules constitutes the P700 reaction center and

Too much light? How β-carotene protects the photosystem II reaction centre

Abstract: The photosystem II reaction centre of all oxygenic organisms is subject to photodamage by high light i.e. photoinhibition. In this review I discuss the reasons for the inevitable and unpreventable oxidative damage that occurs in photosystem II and the way in which β-carotene bound to the reaction centre significantly mitigates this damage. Recent X-ray structures of the photosystem II core complex (reaction centre plus the inner antenna complexes) have revealed the binding sites of some of the carotenoids known to be bound to the complex. In the light of these X-ray structures and their known biophysical properties it is thus possible to identify the two β-carotenes present in the photosystem II reaction centre. The two carotenes are both bound to the D2 protein and this positioning is discussed in relation to their ability to act as quenchers of singlet oxygen, generated via the triplet state of the primary electron donor. It is proposed that their location on the D2 polypeptide means there is more oxidative damage to the D1 protein and that this underlies the fact that this latter protein is continuously re-synthesised, at a far greater rate than any other protein involved in photosynthesis. The relevance of a cycle of electrons around photosystem II, via cytochrome b559, in order to re-reduce the β-carotenes when they are oxidised and hence restore their ability to quench singlet oxygen, is also discussed.

Initial electron donor and acceptor in isolated Photosystem II reaction centers identified with femtosecond mid-IR spectroscopy

Abstract: Despite the apparent similarity between the plant Photosystem II reaction center (RC) and its purple bacterial counterpart, we show in this work that the mechanism of charge separation is very different for the two photosynthetic RCs. By using femtosecond visible-pump–mid-infrared probe spectroscopy in the region of the chlorophyll ester and keto modes, between 1,775 and 1,585 cm–1, with 150-fs time resolution, we show that the reduction of pheophytin occurs on a 0.6- to 0.8-ps time scale, whereas P+, the precursor state for water oxidation, is formed after ≈6 ps. We conclude therefore that in the Photosystem II RC the primary charge separation occurs between the “accessory chlorophyll” ChlD1 and the pheophytin on the so-called active branch.

Structure of cyanobacterial photosystem I.

Abstract: Photosystem I is one of the most fascinating membrane protein complexes for which a structure has been determined. It functions as a bio-solar energy converter, catalyzing one of the first steps of oxygenic photosynthesis. It captures the light of the sun by means of a large antenna system, consisting of chlorophylls and carotenoids, and transfers the energy to the center of the complex, driving the transmembrane electron transfer from plastoquinone to ferredoxin. Cyanobacterial Photosystem I is a trimer consisting of 36 proteins to which 381 cofactors are non-covalently attached. This review discusses the complex function of Photosystem I based on the structure of the complex at 2.5 A resolution as well as spectroscopic and biochemical data.

The effects of salt stress on photosynthetic electron transport and thylakoid membrane proteins in the cyanobacterium Spirulina platensis.

Abstract: The response of Spirulina (Arthrospira) platensis to high salt stress was investigated by incubating the cells in light of moderate intensity in the presence of 0.8 M NaCl. NaCl caused a decrease in photosystem II (PSII) mediated oxygen evolution activity and increase in photosystem I (PSI) activity and the amount of P700. Similarly maximal efficiency of PSII (Fv/Fm) and variable fluorescence (Fv/Fo) were also declined in salt-stressed cells. Western blot analysis reveal that the inhibition in PSII activity is due to a 40 % loss of a thylakoid membrane protein, known as D1, which is located in PSII reaction center. NaCl treatment of cells also resulted in the alterations of other thylakoid membrane proteins: most prominently, a dramatic diminishment of the 47-kDa chlorophyll protein (CP) and 94-kDa protein, and accumulation of a 17-kDa protein band were observed in SDS-PAGE. The changes in 47-kDa and 94-kDa proteins lead to the decreased energy transfer from light harvesting antenna to PSII, which was accompanied by alterations in the chlorophyll fluorescence emission spectra of whole cells and isolated thylakoids. Therefore we conclude that salt stress has various effects on photosynthetic electron transport activities due to the marked alterations in the composition of thylakoid membrane proteins.

Discoveries in photosynthesis

Abstract: Form the Series Editor, Preface A personal tribute to an eminent photosynthesis researcher, Martin D. Kamen (1913-2002) Biographies of the Editors Colour Plates Part I Editorials: Celebrating the Millennium - Historical Highlights of Photosynthesis Research, Part 1.- Part 2.- Part 3 .- Part II Overviews and Timelines: History of the word photosynthesis and evolution of its definition.- In one era and out the other.- Timeline of discoveries: Anoxygenic photosynthesis.- Discoveries in oxygenic photosynthesis (1727-2002): a perspective.- Part III Tributes: 'And whose bright presence'-an appreciation of Robert Hill and his reaction.- The Contributions of James Franck to photosynthetic research: a tribute.- Hydrogen metabolism of green algae: discovery and early research-a tribute to Hans Gaffron and his coworkers.- Samuel Ruben's contributions to research on photosynthesis and bacterial metabolism with radioactive carbon. Contributions of Henrik Lundegardh.-Part IV Excitation Energy Transfer: Photosynthetic exciton theory in the 1960s.- Excitation energy trapping in anoxygenic photosynthetic bacteria.- Fluorescence lifetime, yield, energy transfer and spectrum in photosynthesis, 1950-1960.- Visualization of excitation energy transfer processes in plants and algae.-Plastoquinone redox control of chloroplast thylakoid protein phosphorylation and distribution of excitation energy between photosystems: discovery, background, implications.- Excitation transfer between photosynthetic units: the 1964 experiment.-Part V Reaction Centers: Research on photosynthetic reaction centers from 1932 to 1987.- Chlorophyll chemistry before and after crystals of photosynthetic reaction centers.- Electron donors and acceptors in the initial steps of photosynthesis in purple bacteria: a personal account.- My daily constitutional in Martinsried.- The two-electron gate in photosynthetic bacteria.- Steps on the way to building-blocks, 3-D crystalsand X-ray structural analysis of photosystem I and II of water-oxidizing photosynthesis. a personal account.- The identification of the Photosystem II reaction center: a personal story.- The isolated Photosystem II reaction center: first attempts to directly measure the kinetics of primary charge separation.- Discovery of pheophytin function in the photosynthetic energy conversion as the primary electron acceptor of Photosystem II.- Engine of life and big bang of evolution.- Role of bicarbonate at the acceptor side of Photosystem II.- Unraveling the PS I reaction center: a long history or the sum of many efforts.- Photosystem I reaction center: past and future.- P 430: a retrospective, 1971-2001.- Part VI Oxygen Evolution: Apparatus and mechanism of photosynthetic oxygen evolution: a personal perspective.- Period-four oscillations of the flash-induced oxygen formation in photosynthesis.- Period four oscillations in chlorophyll a fluorescence- Chloride and calcium in Photosystem II: from effects to enigma.- The bicarbonate effect, oxygen evolution, and the shadow of Otto Warburg.- Early indications for manganese oxidation state changes during photosynthetic oxygen production.-Part VII Light-harvesting and Pigment-protein Complexes: Purple bacterial light-harvesting complexes: From dreams to structures.- The Fenna-Matthews-Olson protein.- Physical separation of chlorophyll-protein complexes.-How the chlorophyll-proteins got their names.- Phycobiliproteins and phycobilisomes: the early observations.- Part VIII Electron Transport and ATP Synthesis: Discovery and characterization of electron transfer proteins in the photosynthetic bacteria.- Membrane-anchored cytochrome c as an electron carrier in photosynthesis and respiration: past, present and future of an unexpected discovery.- The Q-cycle:a personal perspective.- The isolation of functional cytochrome b6-f complex: From lucky encounter to rewardingexperience.- Ironies in photosynthetic electron transport

Interprotein electron transfer from cytochrome c2 to photosynthetic reaction center: tunneling across an aqueous interface.

Abstract: Interprotein electron transfer (ET) reactions play an important role in biological energy conversion processes. One of these reactions, the ET between cytochrome c2 (cyt) and reaction center from photosynthetic bacteria, is the focus of this theoretical study. The changes in the ET rate constant at fixed distances during the association process were calculated as the cyt moved from the electrostatically stabilized encounter complex to the bound state having short range van der Waals contacts in the tunneling region. Multiple conformations of the protein were generated by molecular dynamics simulations including explicit water molecules. For each of these conformations, the ET rate was calculated by using the Pathways model. The ET rate increased smoothly as the cyt approached from the encounter complex to the bound state, with a tunneling decay factor β = 1.1 A-1. This relatively efficient coupling between redox centers is due to the ability of interfacial water molecules to form multiple strong hydrogen bonding pathways connecting tunneling pathways on the surfaces of the two proteins. The ET rate determined for the encounter complex ensemble of states is only about a factor of 100 slower than that of the bound state (τ = 100 μs, compared with 1 μs), because of fluctuations of the cyt within the encounter complex ensemble through configurations having strong tunneling pathways. The ET rate for the encounter complex is in agreement with rates observed in mutant reaction centers modified to remove shortrange hydrophobic interactions, suggesting that in this case, ET occurs within the solvent-separated, electrostatically stabilized encounter complex.

Bidirectional electron transfer in photosystem I: determination of two distances between P700+ and A1- in spin-correlated radical pairs

Abstract: The spin-correlated radical pair [P700+A1-] gives rise to a characteristic “out-of-phase” electron spin−echo signal. The electron spin−echo envelope modulation (ESEEM) of these signals has been studied in thylakoids prepared from the wild-type strain of Chlamydomonas reinhardtii and in two site-directed mutants, in which the methionine residue which acts as the axial ligand to the chlorin electron acceptor A0 has been substituted with a histidine either on the PsaA (PsaA-M684H) or the PsaB (PsaB-M664H) reaction center subunits. The analysis of the time domain ESEEM provides information about the spin−spin interaction in the [P700+A1-] radical pair, and the values of the dipolar (D) and the exchange (J) interaction can be extracted. From the distance dependence of the dipolar coupling term, the distance between the unpaired electron spin density clouds of the primary donor P700+ and the phyllosemiquinone A1- can be determined. The [P700+A1-] ESEEM spectrum obtained in wild-type thylakoids can be reconstruc...

Control of Quinone Redox Potentials in Photosystem II: Electron Transfer and Photoprotection

Abstract: In O2-evolving complex Photosystem II (PSII), an unimpeded transfer of electrons from the primary quinone (QA) to the secondary quinone (QB) is essential for the efficiency of photosynthesis. Recent PSII crystal structures revealed the protein environment of the QA/B binding sites. We calculated the plastoquinone (QA/B) redox potentials (Em) for one-electron reduction with a full account of the PSII protein environment. We found two different H-bond patterns involving QA and D2-Thr217, resulting in an upshift of Em(QA) by 100 mV if the H bond between QA and Thr is present. The formation of this H bond to QA may be the origin of a photoprotection mechanism, which is under debate. At the QB side, the formation of a H bond between D2-Ser264 and QB depends on the protonation state of D1-His252. QB adopts the high-potential form if the H bond to Ser is present. Conservation of this residue and H-bond pattern for QB sites among bacterial photosynthetic reaction centers (bRC) and PSII strongly indicates their es...

The yeast split-ubiquitin system to study chloroplast membrane protein interactions

Abstract: Each photosynthetic complex within the thylakoid membrane consists of several different subunits. During formation of these complexes, numerous regulatory factors are required for the coordinated transport and assembly of the subunits. Interactions between transport/assembly factors and their specific polypeptides occur in a membraneous environment and are usually transient and short-lived. Thus, a detailed analysis of the underlying molecular mechanisms by biochemical techniques is often difficult to perform. Here, we report on the suitability of a genetic system, i.e. the yeast split-ubiquitin system, to investigate protein–protein interactions of thylakoid membrane proteins. The data confirm the previously established binding of the cpSec-translocase subunits, cpSecY and cpSecE, and the interaction of the cpSec-translocase from Arabidopsis thaliana with Alb3, a factor required for the insertion of the light-harvesting chlorophyll-binding proteins into the thylakoid membrane. In addition, the proposed interaction between D1, the reaction center protein of photosystem II and the soluble periplasmic PratA factor from Synechocystis sp. PCC 6803 was verified. A more comprehensive analysis of Alb3-interacting proteins revealed that Alb3 is able to form dimers or oligomers. Interestingly, Alb3 was also shown to bind to the PSII proteins D1, D2 and CP43, to the PSI reaction center protein PSI-A and the ATP synthase subunit CF0III, suggesting an important role of Alb3 in the assembly of photosynthetic thylakoid membrane complexes.

Bidirectional Electron Transfer in Photosystem I: Direct Evidence from High-Frequency Time-Resolved EPR Spectroscopy

Abstract: Efficient charge separation occurring within membrane-bound reaction center proteins is the most important step of photosynthetic solar energy conversion. All reaction centers are classified into two types, I and II. X-ray crystal structures reveal that both types bind two symmetric membrane-spanning branches of potential electron-transfer cofactors. Determination of the functional roles of these pairs of branches is of fundamental importance. While it is established that in type II reaction centers only one branch functions in electron transfer, we present the first direct spectroscopic evidence that both cofactor branches are active in the type I reaction center, photosystem I.

Redox potentials of chlorophylls in the photosystem II reaction center.

Abstract: Water oxidation generating atmospheric oxygen occurs in photosystem II (PSII), a large protein−pigment complex located in the thylakoid membrane. The recent crystal structures at 3.2 and 3.5 A resolutions provide novel details on amino acid side chains, especially in the D1/D2 subunits. We calculated the redox potentials for one-electron oxidation of the chlorophyll a (Chla) molecules in PSII, considering the protein environment in atomic detail. The calculated redox potentials for the dimer Chla (PD1/D2) and accessory Chla (ChlD1/D2) were 1.11−1.30 V relative to the normal hydrogen electrode at pH 7, which is high enough for water oxidation. The D1/D2 proteins and their cofactors contribute approximately 390 mV to the enormous upshift of 470 mV compared to the redox potential of monomeric Chla in dimethylformamide. The other subunits are responsible for the remaining 80 mV. The high redox potentials of the two accessory Chla ChlD1/D2 suggests that they also participate in the charge separation process.

Charge recombination fluorescence in photosystem I reaction centers from Chlamydomonas reinhardtii.

Abstract: The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Muller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Muller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii.

The structure and function of the cytochrome c2: reaction center electron transfer complex from Rhodobacter sphaeroides

Abstract: In the photosynthetic bacterium, Rhodobacter sphaeroides, the mobile electron carrier, cytochrome c2 (cyt c2) transfers an electron from reduced heme to the photooxidized bacteriochlorophyll dimer in the membrane bound reaction center (RC) as part of the light induced cyclic electron transfer chain. A complex between these two proteins that is active in electron transfer has been crystallized and its structure determined by X-ray diffraction. The structure of the cyt:RC complex shows the cyt c2 (cyt c2) positioned at the center of the periplasmic surface of the RC. The exposed heme edge from cyt c2 is in close tunneling contact with the electron acceptor through an intervening bridging residue, Tyr L162 located on the RC surface directly above the bacteriochlorophyll dimer. The binding interface between the two proteins can be divided into two regions: a short-range interaction domain and a long-range interaction domain. The short-range domain includes residues immediately surrounding the tunneling contact region around the heme and Tyr L162 that display close intermolecular contacts optimized for electron transfer. These include a small number of hydrophobic interactions, hydrogen bonds and a pi-cation interaction. The long-range interaction domain consists of solvated complementary charged residues; positively charged residues from the cyt and negatively charged residues from the RC that provide long range electrostatic interactions that can steer the two proteins into position for rapid association.

Magnetic field dependence of photo-CIDNP MAS NMR on photosynthetic reaction centers of Rhodobacter sphaeroides WT.

Abstract: Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in frozen and quinone depleted photosynthetic reaction centers of the purple bacteria Rhodobacter sphaeroides wild type (WT) by 13C solid-state NMR at three different magnetic fields. All light-induced signals appear to be emissive at all three fields. At 4.7 T (200 MHz proton frequency), the strongest enhancement of NMR signals is observed, which is more than 10 000 above the Boltzmann polarization. At higher fields, the enhancement factor decreases. At 17.6 T, the enhancement factor is about 60. The field dependence of the enhancement appears to be the same for all nuclei. The observed field dependence is in line with simulations that assume two competing mechanisms of polarization transfer from electrons to nuclei, three-spin mixing (TSM) and differential decay (DD). These simulations indicate a ratio of the electron spin density on the special pair cofactors is 3:2 in favor of the L-BChl during the radical cation state. The...

Conformational regulation of charge recombination reactions in a photosynthetic bacterial reaction center.

Abstract: In bright light the photosynthetic reaction center (RC) of Rhodobacter sphaeroides stabilizes the P(+)(870).Q(-)(A) charge-separated state and thereby minimizes the potentially harmful effects of light saturation. Using X-ray diffraction we report a conformational change that occurs within the cytoplasmic domain of this RC in response to prolonged illumination with bright light. Our observations suggest a novel structural mechanism for the regulation of electron transfer reactions in photosynthesis.