BBA - Bioenergetics (v.1847, #2)

Membrane potential regulates mitochondrial ATP-diphosphohydrolase activity but is not involved in progesterone biosynthesis in human syncytiotrophoblast cells by Oscar Flores-Herrera; Sofia Olvera-Sánchez; Mercedes Esparza-Perusquía; Juan Pablo Pardo; Juan Luis Rendón; Guillermo Mendoza-Hernández; Federico Martínez (143-152).
ATP-diphosphohydrolase is associated with human syncytiotrophoblast mitochondria. The activity of this enzyme is implicated in the stimulation of oxygen uptake and progesterone synthesis. We reported previously that: (1) the detergent-solubilized ATP-diphosphohydrolase has low substrate specificity, and (2) purine and pyrimidine nucleosides, tri- or diphosphates, are fully dephosphorylated in the presence of calcium or magnesium (Flores-Herrera 1999, 2002). In this study we show that ATP-diphosphohydrolase hydrolyzes first the nucleoside triphosphate to nucleoside diphosphate, and then to nucleotide monophosphate, in the case of all tested nucleotides. The activation energies (E a) for ATP, GTP, UTP, and CTP were 6.06, 4.10, 6.25, and 5.26 kcal/mol, respectively; for ADP, GDP, UDP, and CDP, they were 4.67, 5.42, 5.43, and 6.22 kcal/mol, respectively. The corresponding Arrhenius plots indicated a single rate-limiting step for each hydrolyzed nucleoside, either tri- or diphosphate. In intact mitochondria, the ADP produced by ATP-diphosphohydrolase activity depolarized the membrane potential (ΔΨm) and stimulated oxygen uptake. Mitochondrial respiration showed the state-3/state-4 transition when ATP was added, suggesting that ATP-diphosphohydrolase and the F1F0-ATP synthase work in conjunction to avoid a futile cycle. Substrate selectivity of the ATP-diphosphohydrolase was modified by ΔΨm (i.e. ATP was preferred over GTP when the inner mitochondrial membrane was energized). In contrast, dissipation of ΔΨm by CCCP produced a loss of substrate specificity and so the ATP-diphosphohydrolase was able to hydrolyze ATP and GTP at the same rate. In intact mitochondria, ATP hydrolysis increased progesterone synthesis as compared with GTP. Although dissipation of ΔΨm by CCCP decreased progesterone synthesis, NADPH production restores steroidogenesis. Overall, our results suggest a novel physiological role for ΔΨm in steroidogenesis.
Keywords: ATP-diphosphohydrolase; Placental mitochondria; Mitochondrial bioenergetics; ATP hydrolysis; Progesterone synthesis;

Visible/UV absorption in PS II core complexes is dominated by the chl-a absorptions, which extend to ~ 700 nm. A broad 700–730 nm PS II core complex absorption in spinach has been assigned [1] to a charge transfer excitation between ChlD1 and ChlD2. Emission from this state, which peaks at 780 nm, has been seen [2] for both plant and cyanobacterial samples. We show that Thermosynechococcus vulcanus PS II core complexes have parallel absorbance in the 700–730 nm region and similar photochemical behaviour to that seen in spinach. This establishes the low energy charge transfer state as intrinsic to the native PS II reaction centre. High-sensitivity MCD measurements made in the 700–1700 nm region reveal additional electronic excitations at ~ 770 nm and ~ 1550 nm. The temperature and field dependence of MCD spectra establish that the system peaking near 1550 nm is a heme-to-Fe(III) charge transfer excitation. These transitions have not previously been observed for cyt b559 or cyt c550. The distinctive characteristics of the MCD signals seen at 770 nm allow us to assign absorption in this region to a dz 2  → dx2 − y2 transition of Mn(III) in the Ca-Mn4O5 cluster of the oxygen evolving centre. Current measurements were performed in the S1 state. Detailed analyses of this spectral region, especially in higher S states, promise to provide a new window on models of water oxidation.Display Omitted
Keywords: Photosystem II; Reaction centre; Oxygen evolving centre; Charge-transfer; Circular dichroism; Magnetic circular dichroism;

Structural differences of oxidized iron–sulfur and nickel–iron cofactors in O2-tolerant and O2-sensitive hydrogenases studied by X-ray absorption spectroscopy by Kajsa G.V. Sigfridsson; Nils Leidel; Oliver Sanganas; Petko Chernev; Oliver Lenz; Ki-Seok Yoon; Hirofumi Nishihara; Alison Parkin; Fraser A. Armstrong; Sébastien Dementin; Marc Rousset; Antonio L. De Lacey; Michael Haumann (162-170).
The class of [NiFe]-hydrogenases comprises oxygen-sensitive periplasmic (PH) and oxygen-tolerant membrane-bound (MBH) enzymes. For three PHs and four MBHs from six bacterial species, structural features of the nickel–iron active site of hydrogen turnover and of the iron–sulfur clusters functioning in electron transfer were determined using X-ray absorption spectroscopy (XAS). Fe-XAS indicated surplus oxidized iron and a lower number of ~ 2.7 Å Fe–Fe distances plus additional shorter and longer distances in the oxidized MBHs compared to the oxidized PHs. This supported a double-oxidized and modified proximal FeS cluster in all MBHs with an apparent trimer-plus-monomer arrangement of its four iron atoms, in agreement with crystal data showing a [4Fe3S] cluster instead of a [4Fe4S] cubane as in the PHs. Ni-XAS indicated coordination of the nickel by the thiol group sulfurs of four conserved cysteines and at least one iron–oxygen bond in both MBH and PH proteins. Structural differences of the oxidized inactive [NiFe] cofactor of MBHs in the Ni-B state compared to PHs in the Ni–A state included a ~ 0.05 Å longer Ni-O bond, a two times larger spread of the Ni–S bond lengths, and a ~ 0.1 Å shorter Ni–Fe distance. The modified proximal [4Fe3S] cluster, weaker binding of the Ni–Fe bridging oxygen species, and an altered localization of reduced oxygen species at the active site may each contribute to O2 tolerance.Display Omitted
Keywords: [NiFe]-hydrogenase; O2-tolerance; FeS cluster; [NiFe] active site; X-ray absorption spectroscopy;

The contributions of respiration and glycolysis to extracellular acid production by Shona A. Mookerjee; Renata L.S. Goncalves; Akos A. Gerencser; David G. Nicholls; Martin D. Brand (171-181).
The rate at which cells acidify the extracellular medium is frequently used to report glycolytic rate, with the implicit assumption that conversion of uncharged glucose or glycogen to lactate  + H+ is the only significant source of acidification. However, another potential source of extracellular protons is the production of CO2 during substrate oxidation: CO2 is hydrated to H2CO3, which then dissociates to HCO3  + H+.O2 consumption and pH were monitored in a popular platform for measuring extracellular acidification (the Seahorse XF Analyzer).We found that CO2 produced during respiration caused almost stoichiometric release of H+ into the medium. With C2C12 myoblasts given glucose, respiration-derived CO2 contributed 34% of the total extracellular acidification. When glucose was omitted or replaced by palmitate or pyruvate, this value was 67–100%. Analysis of primary cells, cancer cell lines, stem cell lines, and isolated synaptosomes revealed contributions of CO2-produced acidification that were usually substantial, ranging from 3% to 100% of the total acidification rate.Measurement of glycolytic rate using extracellular acidification requires differentiation between respiratory and glycolytic acid production.The data presented here demonstrate the importance of this correction when extracellular acidification is used for quantitative measurement of glycolytic flux to lactate. We describe a simple way to correct the measured extracellular acidification rate for respiratory acid production, using simultaneous measurement of oxygen consumption rate.Extracellular acidification is often assumed to result solely from glycolytic lactate production, but respiratory CO2 also contributes. We demonstrate that extracellular acidification by myoblasts given glucose is 66% glycolytic and 34% respiratory and describe a method to differentiate these sources.
Keywords: Oxygen consumption rate; Extracellular acidification rate; Bicarbonate; Carbon dioxide; Extracellular flux; Glycolysis;

Cytochrome bd from Escherichia coli catalyzes peroxynitrite decomposition by Vitaliy B. Borisov; Elena Forte; Sergey A. Siletsky; Paolo Sarti; Alessandro Giuffrè (182-188).
Cytochrome bd is a prokaryotic respiratory quinol oxidase phylogenetically unrelated to heme-copper oxidases, that was found to promote virulence in some bacterial pathogens. Cytochrome bd from Escherichia coli was previously reported to contribute not only to proton motive force generation, but also to bacterial resistance to nitric oxide (NO•) and hydrogen peroxide (H2O2). Here, we investigated the interaction of the purified enzyme with peroxynitrite (ONOO), another harmful reactive species produced by the host to kill invading microorganisms. We found that addition of ONOO to cytochrome bd in turnover with ascorbate and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) causes the irreversible inhibition of a small (≤ 15%) protein fraction, due to the NO• generated from ONOO and not to ONOO itself. Consistently, addition of ONOO to cells of the E. coli strain GO105/pTK1, expressing cytochrome bd as the only terminal oxidase, caused only a minor (≤ 5%) irreversible inhibition of O2 consumption, without measurable release of NO•. Furthermore, by directly monitoring the kinetics of ONOO decomposition by stopped-flow absorption spectroscopy, it was found that the purified E. coli cytochrome bd in turnover with O2 is able to metabolize ONOO with an apparent turnover rate as high as ~ 10 mol ONOO (mol enzyme)− 1  s− 1 at 25 °C. To the best of our knowledge, this is the first time that the kinetics of ONOO decomposition by a terminal oxidase has been investigated. These results strongly suggest a protective role of cytochrome bd against ONOO damage.
Keywords: Immune response; Nitrosative and oxidative stress; Reactive nitrogen species; Peroxynitrite; Cytochrome bd; Escherichia coli;

Assembly of functional photosystem complexes in Rhodobacter sphaeroides incorporating carotenoids from the spirilloxanthin pathway by Shuang C. Chi; David J. Mothersole; Preston Dilbeck; Dariusz M. Niedzwiedzki; Hao Zhang; Pu Qian; Cvetelin Vasilev; Katie J. Grayson; Philip J. Jackson; Elizabeth C. Martin; Ying Li; Dewey Holten; C. Neil Hunter (189-201).
Carotenoids protect the photosynthetic apparatus against harmful radicals arising from the presence of both light and oxygen. They also act as accessory pigments for harvesting solar energy, and are required for stable assembly of many light-harvesting complexes. In the phototrophic bacterium Rhodobacter (Rba.) sphaeroides phytoene desaturase (CrtI) catalyses three sequential desaturations of the colourless carotenoid phytoene, extending the number of conjugated carbon–carbon double bonds, N, from three to nine and producing the yellow carotenoid neurosporene; subsequent modifications produce the yellow/red carotenoids spheroidene/spheroidenone (N  = 10/11). Genomic crtI replacements were used to swap the native three-step Rba. sphaeroides CrtI for the four-step Pantoea agglomerans enzyme, which re-routed carotenoid biosynthesis and culminated in the production of 2,2′-diketo-spirilloxanthin under semi-aerobic conditions. The new carotenoid pathway was elucidated using a combination of HPLC and mass spectrometry. Premature termination of this new pathway by inactivating crtC or crtD produced strains with lycopene or rhodopin as major carotenoids. All of the spirilloxanthin series carotenoids are accepted by the assembly pathways for LH2 and RC–LH1–PufX complexes. The efficiency of carotenoid-to-bacteriochlorophyll energy transfer for 2,2′-diketo-spirilloxanthin (15 conjugated C=C bonds; N  = 15) in LH2 complexes is low, at 35%. High energy transfer efficiencies were obtained for neurosporene (N  = 9; 94%), spheroidene (N  = 10; 96%) and spheroidenone (N  = 11; 95%), whereas intermediate values were measured for lycopene (N  = 11; 64%), rhodopin (N  = 11; 62%) and spirilloxanthin (N  = 13; 39%). The variety and stability of these novel Rba. sphaeroides antenna complexes make them useful experimental models for investigating the energy transfer dynamics of carotenoids in bacterial photosynthesis.Display Omitted
Keywords: Bacterial photosynthesis; Light harvesting; Carotenoid; Membrane protein; Antenna; Synthetic biology;

Protein chaperones mediating copper insertion into the CuA site of the aa 3-type cytochrome c oxidase of Paracoccus denitrificans by Banaja Priyadarshini Dash; Melanie Alles; Freya Alena Bundschuh; Oliver-M.H. Richter; Bernd Ludwig (202-211).
The biogenesis of the mitochondrial cytochrome c oxidase is a complex process involving the stepwise assembly of its multiple subunits encoded by two genetic systems. Moreover, several chaperones are required to recruit and insert the redox-active metal centers into subunits I and II, two a-type hemes and a total of three copper ions, two of which form the CuA center located in a hydrophilic domain of subunit II. The copper-binding Sco protein(s) have been implicated with the metallation of this site in various model organisms.Here we analyze the role of the two Sco homologues termed ScoA and ScoB, along with two other copper chaperones, on the biogenesis of the cytochrome c oxidase in the bacterium Paracoccus denitrificans by deleting each of the four genes individually or pairwise, followed by assessing the functionality of the assembled oxidase both in intact membranes and in the purified enzyme complex. Copper starvation leads to a drastic decrease of oxidase activity in membranes from strains involving the scoB deletion. This loss is shown to be of dual origin, (i) a severe drop in steady-state oxidase levels in membranes, and (ii) a diminished enzymatic activity of the remaining oxidase complex, traced back to a lower copper content, specifically in the CuA site of the enzyme. Neither of the other proteins addressed here, ScoA or the two PCu proteins, exhibit a direct effect on the metallation of the CuA site in P. denitrificans, but are discussed as potential interaction partners of ScoB.
Keywords: Respiratory chain; Oxidase biogenesis; CuA center; Copper chaperone; Sco1;

The inhibitor methyl viologen (MV) has been widely used in photosynthesis to study oxidative stress. Its effects on electron transfer kinetics in Synechocystis sp. PCC6803 cells were studied to characterize its electron-accepting properties. For the first hundreds of flashes following MV addition at submillimolar concentrations, the kinetics of NADPH formation were hardly modified (less than 15% decrease in signal amplitude) with a significant signal decrease only observed after more flashes or continuous illumination. The dependence of the P700 photooxidation kinetics on the MV concentration exhibited a saturation effect at 0.3 mM MV, a concentration which inhibits the recombination reactions in photosystem I. The kinetics of NADPH formation and decay under continuous light with MV at 0.3 mM showed that MV induces the oxidation of the NADP pool in darkness and that the yield of linear electron transfer decreased by only 50% after 1.5–2 photosystem-I turnovers. The unexpectedly poor efficiency of MV in inhibiting NADPH formation was corroborated by in vitro flash-induced absorption experiments with purified photosystem-I, ferredoxin and ferredoxin-NADP+-oxidoreductase. These experiments showed that the second-order rate constants of MV reduction are 20 to 40-fold smaller than the competing rate constants involved in reduction of ferredoxin and ferredoxin-NADP+-oxidoreductase. The present study shows that MV, which accepts electrons in vivo both at the level of photosystem-I and ferredoxin, can be used at submillimolar concentrations to inhibit recombination reactions in photosystem-I with only a moderate decrease in the efficiency of fast reactions involved in linear electron transfer and possibly cyclic electron transfer.
Keywords: Paraquat; NADP photoreduction; P700 photooxidation kinetics; Photosystem I; Ferredoxin; Ferredoxin-NADP+-oxidoreductase;

The 2nd electron transfer in reaction center of photosynthetic bacterium Rhodobacter sphaeroides is a two step process in which protonation of QB precedes interquinone electron transfer. The thermal activation and pH dependence of the overall rate constants of different RC variants were measured and compared in solvents of water (H2O) and heavy water (D2O). The electron transfer variants where the electron transfer is rate limiting (wild type and M17DN, L210DN and H173EQ mutants) do not show solvent isotope effect and the significant decrease of the rate constant of the second electron transfer in these mutants is due to lowering the operational pK a of QB /QBH: 4.5 (native), 3.9 (L210DN), 3.7 (M17DN) and 3.1 (H173EQ) at pH 7. On the other hand, the proton transfer variants where the proton transfer is rate limiting demonstrate solvent isotope effect of pH-independent moderate magnitude (2.11 ± 0.26 (WT + Ni2 +), 2.16 ± 0.35 (WT + Cd2 +) and 2.34 ± 0.44 (L210DN/M17DN)) or pH-dependent large magnitude (5.7 at pH 4 (L213DN)). Upon deuteration, the free energy and the enthalpy of activation increase in all proton transfer variants by about 1 kcal/mol and the entropy of activation becomes negligible in L210DN/M17DN mutant. The results are interpreted as manifestation of equilibrium and kinetic solvent isotope effects and the structural, energetic and kinetic possibility of alternate proton delivery pathways are discussed.Display Omitted
Keywords: Bacterial photosynthesis; Reaction center protein; Flash-induced proton delivery; Protonation mutant; Solvent isotope effect;

Spermine selectively inhibits high-conductance, but not low-conductance calcium-induced permeability transition pore by Pia A. Elustondo; Alexander Negoda; Constance L. Kane; Daniel A. Kane; Evgeny V. Pavlov (231-240).
The permeability transition pore (PTP) is a large channel of the mitochondrial inner membrane, the opening of which is the central event in many types of stress-induced cell death. PTP opening is induced by elevated concentrations of mitochondrial calcium. It has been demonstrated that spermine and other polyamines can delay calcium-induced swelling of isolated mitochondria, suggesting their role as inhibitors of the mitochondrial PTP. Here we further investigated the mechanism by which spermine inhibits the calcium-induced, cyclosporine A (CSA) — sensitive PTP by using three indicators: 1) calcium release from the mitochondria detected with calcium green, 2) mitochondrial membrane depolarization using TMRM, and 3) mitochondrial swelling by measuring light absorbance. We found that despite calcium release and membrane depolarization, indicative of PTP activation, mitochondria underwent only partial swelling in the presence of spermine. This was in striking contrast to the high-amplitude swelling detected in control mitochondria and in mitochondria treated with the PTP inhibitor CSA. We conclude that spermine selectively prevents opening of the high-conductance state, while allowing activation of the lower conductance state of the PTP. We propose that the existence of lower conductance, stress-induced PTP might play an important physiological role, as it is expected to allow the release of toxic levels of calcium, while keeping important molecules (e.g., NAD) within the mitochondrial matrix.
Keywords: Mitochondrial permeability transition pore; Spermine; Calcium;

Mapping energy transfer channels in fucoxanthin–chlorophyll protein complex by Andrius Gelzinis; Vytautas Butkus; Egidijus Songaila; Ramūnas Augulis; Andrew Gall; Claudia Büchel; Bruno Robert; Darius Abramavicius; Donatas Zigmantas; Leonas Valkunas (241-247).
Fucoxanthin–chlorophyll protein (FCP) is the key molecular complex performing the light-harvesting function in diatoms, which, being a major group of algae, are responsible for up to one quarter of the total primary production on Earth. These photosynthetic organisms contain an unusually large amount of the carotenoid fucoxanthin, which absorbs the light in the blue–green spectral region and transfers the captured excitation energy to the FCP-bound chlorophylls. Due to the large number of fucoxanthins, the excitation energy transfer cascades in these complexes are particularly tangled. In this work we present the two-color two-dimensional electronic spectroscopy experiments on FCP. Analysis of the data using the modified decay associated spectra permits a detailed mapping of the excitation frequency dependent energy transfer flow with a femtosecond time resolution.Display Omitted
Keywords: Diatoms; Light-harvesting; Two-dimensional spectroscopy;

Direct energy transfer from the major antenna to the photosystem II core complexes in the absence of minor antennae in liposomes by Ruixue Sun; Kun Liu; Lianqing Dong; Yuling Wu; Harald Paulsen; Chunhong Yang (248-261).
Minor antennae of photosystem (PS) II, located between the PSII core complex and the major antenna (LHCII), are important components for the structural and functional integrity of PSII supercomplexes. In order to study the functional significance of minor antennae in the energetic coupling between LHCII and the PSII core, characteristics of PSII–LHCII proteoliposomes, with or without minor antennae, were investigated. Two types of PSII preparations containing different antenna compositions were isolated from pea: 1) the PSII preparation composed of the PSII core complex, all of the minor antennae, and a small amount of major antennae (MCC); and 2) the purified PSII dimeric core complexes without periphery antenna (CC). They were incorporated, together with LHCII, into liposomes composed of thylakoid membrane lipids. The spectroscopic and functional characteristics were measured. 77 K fluorescence emission spectra revealed an increased spectral weight of fluorescence from PSII reaction center in the CC–LHCII proteoliposomes, implying energetic coupling between LHCII and CC in the proteoliposomes lacking minor antennae. This result was further confirmed by chlorophyll a fluorescence induction kinetics. The incorporation of LHCII together with CC markedly increased the antenna cross-section of the PSII core complex. The 2,6-dichlorophenolindophenol photoreduction measurement implied that the lack of minor antennae in PSII supercomplexes did not block the energy transfer from LHCII to the PSII core complex. In conclusion, it is possible, in liposomes, that LHCII transfer energy directly to the PSII core complex, in the absence of minor antennae.
Keywords: Proteoliposome; Protein–protein interaction; Photosystem II; Light-harvesting complex; Minor antenna;

Carotenoid triplet states in photosystem II: Coupling with low-energy states of the core complex by Stefano Santabarbara; Alessandro Agostini; Anna Paola Casazza; Giuseppe Zucchelli; Donatella Carbonera (262-275).
The photo-excited triplet states of carotenoids, sensitised by triplet–triplet energy transfer from the chlorophyll triplet states, have been investigated in the isolated Photosystem II (PSII) core complex and PSII–LHCII (Light Harvesting Complex II) supercomplex by Optically Detected Magnetic Resonance techniques, using both fluorescence (FDMR) and absorption (ADMR) detection. The absence of Photosystem I allows us to reach the full assignment of the carotenoid triplet states populated in PSII under steady state illumination at low temperature. Five carotenoid triplet (3Car) populations were identified in PSII–LHCII, and four in the PSII core complex. Thus, four 3Car populations are attributed to β-carotene molecules bound to the core complex. All of them show associated fluorescence emission maxima which are relatively red-shifted with respect to the bulk emission of both the PSII–LHCII and the isolated core complexes. In particular the two populations characterised by Zero Field Splitting parameters |D| = 0.0370–0.0373 cm− 1/|E| = 0.00373–0.00375 cm− 1 and |D| = 0.0381–0.0385 cm− 1/|E| = 0.00393–0.00389 cm− 1, are coupled by singlet energy transfer with chlorophylls which have a red-shifted emission peaking at 705 nm. This observation supports previous suggestions that pointed towards the presence of long-wavelength chlorophyll spectral forms in the PSII core complex. The fifth 3Car component is observed only in the PSII–LHCII supercomplex and is then assigned to the peripheral light harvesting system.Display Omitted
Keywords: Photosystem II; Photosystem II core complex; Carotenoid; Triplet state; Triplet state quenching; Optically Detected Magnetic Resonance;

Cytb 559 in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Roles for Cytb 559 remain elusive. In Thermosynechococcus elongatus, taking advantage of the robustness of the PSII variant with PsbA3 as the D1 subunit (WT*3), 4 mutants were designed hoping to get mutants nevertheless the obligatory phototrophy of this cyanobacterium. In two of them, an axial histidine ligand of the haem-iron was substituted for either a methionine, PsbE/H23M, which could be potentially a ligand or for an alanine, PsbE/H23A, which cannot. In the other mutants, PsbE/Y19F and PsbE/T26P, the environment around PsbE/H23 was expected to be modified. From EPR, MALDI-TOF and O2 evolution activity measurements, the following results were obtained: Whereas the PsbE/H23M and PsbE/H23A mutants assemble only an apo-Cytb 559 the steady-state level of active PSII was comparable to that in WT*3. The lack of the haem or, in PsbE/T26P, conversion of the high-potential into a lower potential form, slowed-down the recovery rate of the O2 activity after high-light illumination but did not affect the photoinhibition rate. This resulted in the following order for the steady-state level of active PSII centers under high-light conditions: PsbE/H23M ≈ PsbE/H23A < < PsbE/Y19F ≤ PsbE/T26P ≤ WT*3. These data show i) that the haem has no structural role provided that PsbE and PsbF are present, ii) a lack of correlation between the rate of photoinhibition and the E m of the haem and iii) that the holo-Cytb 559 favors the recovery of a functional enzyme upon photoinhibition.
Keywords: Photosystem II; Cytb 559; Haem axial ligands; Photoinhibition;

An irradiation density dependent energy relaxation in plant photosystem II antenna assembly by Wenming Tian; Jun Chen; Liezheng Deng; Mingdong Yao; Heping Yang; Yang Zheng; Rongrong Cui; Guohe Sha (286-293).
Plant photosystem II (PSII) is a multicomponent pigment-protein complex that harvests sunlight via pigments photoexcitation, and converts light energy into chemical energy. Against high light induced photodamage, excess light absorption of antenna pigments triggers the operation of photoprotection mechanism in plant PSII. Non-photochemical energy relaxation as a major photoprotection way is essentially correlated to the excess light absorption. Here we investigate the energy relaxation of plant PSII complexes with varying incident light density, by performing steady-state and transient chlorophyll fluorescence measurements of the grana membranes (called as BBY), functional moiety PSII reaction center and isolated light-harvesting complex LHCII under excess light irradiation. Based on the chlorophyll fluorescence decays of these samples, it is found that an irradiation density dependent energy relaxation occurs in the LHCII assemblies, especially in the antenna assembly of PSII supercomplexes in grana membrane, when irradiation increases to somewhat higher density levels. Correspondingly, the average chlorophyll fluorescence lifetime of the highly isolated BBY fragments gradually decreases from ~ 1680 to ~ 1360 ps with increasing the irradiation density from 6.1 × 109 to 5.5 × 1010 photon cm− 2 pulse− 1. Analysis of the relation of fluorescence decay change to the aggregation extent of LHCIIs suggests that a dense arrangement of trimeric LHCIIs is likely the structural base for the occurrence of this irradiation density dependent energy relaxation. Once altering the irradiation density, this energy relaxation is quickly reversible, implying that it may play an important role in photoprotection of plant PSII.Display Omitted
Keywords: Photosystem II; LHCII; Chlorophyll fluorescence; Non-photochemical quenching; Photoprotection; Energy relaxation;

Comparison of nano-sized Mn oxides with the Mn cluster of photosystem II as catalysts for water oxidation by Mohammad Mahdi Najafpour; Mohadeseh Zarei Ghobadi; Behzad Haghighi; Tatsuya Tomo; Jian-Ren Shen; Suleyman I. Allakhverdiev (294-306).
“Back to Nature” is a promising way to solve the problems that we face today, such as air pollution and shortage of energy supply based on conventional fossil fuels. A Mn cluster inside photosystem II catalyzes light-induced water-splitting leading to the generation of protons, electrons and oxygen in photosynthetic organisms, and has been considered as a good model for the synthesis of new artificial water-oxidizing catalysts. Herein, we surveyed the structural and functional details of this cluster and its surrounding environment. Then, we review the mechanistic findings concerning the cluster and compare this biological catalyst with nano-sized Mn oxides, which are among the best artificial Mn-based water-oxidizing catalysts.Display Omitted
Keywords: Model complex; Nano-sized manganese oxides; Photosystem II; Water oxidation; Water-oxidizing complex;