BBA - Bioenergetics (v.1807, #3)

2,2,6,6-tetramethylpiperidine (TEMP) and 2,2,6,6-tetramethyl-4-piperidinone (TEMPD) have earlier been used to quantify singlet oxygen produced by plant material. Both compounds were found to cause severe side effects on Photosystem II. Addition of TEMP or TEMPD to thylakoids immediately stabilized the reduced state of the QA electron acceptor and destabilized the reduced state of the QB acceptor, causing decrease in the driving force of forward electron transfer. Oxygen evolution, thermoluminescence and fluorescence measurements indicated that the number of functional PSII units decreased during incubation of thylakoids with TEMP or TEMPD. Singlet oxygen determinations in photosynthetic systems with piperidine derivatives should be interpreted with care.Display Omitted► The singlet oxygen sensor compounds TEMP and TEMPD inhibit oxygen evolution by Photosystem II. ► TEMP and TEMPD decrease the driving force of electron transfer from primary to secondary quinone of Photosystem II. ► It is questionable whether these compounds can be used for singlet oxygen measurement in plant material.
Keywords: Photosystem II; Photoinhibition; Singlet oxygen; Thermoluminescence; 2,2,6,6-tetramethylpiperidine; 2,2,6,6-tetramethyl-4-piperidinone;

A novel subfamily of mitochondrial dicarboxylate carriers from Drosophila melanogaster: Biochemical and computational studies by Domenico Iacopetta; Marianna Madeo; Gianluca Tasco; Chiara Carrisi; Rosita Curcio; Emanuela Martello; Rita Casadio; Loredana Capobianco; Vincenza Dolce (251-261).
The dicarboxylate carrier is an important member of the mitochondrial carrier family, which catalyzes an electroneutral exchange across the inner mitochondrial membrane of dicarboxylates for inorganic phosphate and certain sulfur-containing compounds. Screening of the Drosophila melanogaster genome revealed the presence of a mitochondrial carrier subfamily constituted by four potential homologs of mammalian and yeast mitochondrial dicarboxylate carriers designated as DmDic1p, DmDic2p, DmDic3p, and DmDic4p. In this paper, we report that DmDIC1 is broadly expressed at comparable levels in all development stages investigated whereas DmDIC3 and DmDIC4 are expressed only in the pupal stage, no transcripts are detectable for DmDIC2. All expressed proteins are localized in mitochondria. The transport activity of DmDic1-3-4 proteins has been investigated by reconstitution of recombinant purified protein into liposomes. DmDic1p is a typical dicarboxylate carrier showing similar substrate specificity and inhibitor sensitivity as mammalian and yeast mitochondrial dicarboxylate carriers. DmDic3p seems to be an atypical dicarboxylate carrier being able to transport only inorganic phosphate and certain sulfur-containing compounds. No transport activity was observed for DmDic4p. The biochemical results have been supported at molecular level by computing the protein structures and by structural alignments. All together these results indicate that D. melanogaster dicarboxylate carriers form a protein subfamily but the modifications in the amino acids sequences are indicative of specialized functions.► A novel subfamily of mitochondrial dicarboxylate carriers has been investigated. ► The kinetic constants Km and Vmax of DmDic1-3 proteins have been determined. ► A different transport specificity with respect to DmDic1-3-4p has been found. ► We supported the biochemical results at molecular level by computational studies. ► The expression of DIC genes in the developmental stages has been analyzed.
Keywords: Mitochondria; Proteomics; Drosophila melanogaster; Dicarboxylate carrier; CG8790; CG4323; CG11196; CG18363;

Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii by Aaron M. Collins; Christine Kirmaier; Dewey Holten; Robert E. Blankenship (262-269).
The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (QA and QB), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285 K, the lifetimes of the excited singlet state, P*, and the charge-separated state P+HA (where HA is the photoactive BPh) were found to be 3.2 ± 0.3 ps and 200 ± 20 ps, respectively. Overall charge separation P* → → P+QA occurred with ≥ 90% yield at 285 K. At 77 K, the P* lifetime was somewhat shorter and the P+HA lifetime was essentially unchanged. Poteniometric titrations gave a P 865/P 865 + midpoint potential of + 390 mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of + 85 and + 265 mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P+ suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.Display Omitted► Time-resolved optical studies of isolated reaction centers from Rfl. castenholzii. ► Free energy determination of reaction center cofactors. ► Heme arrangement in cytochrome complex presented. ► Comparisons are made to Cfl. aurantiacus and purple bacteria.
Keywords: Reaction center; Electron transfer; Cytochrome; Roseiflexus castenholzii;

Regional variation in mitochondrial DNA copy number in mouse brain by Satoshi Fuke; Mie Kubota-Sakashita; Takaoki Kasahara; Yasufumi Shigeyoshi; Tadafumi Kato (270-274).
Mitochondria have their own DNA (mitochondrial DNA [mtDNA]). Although mtDNA copy number is dependent on tissues and its decrease is associated with various neuromuscular diseases, detailed distribution of mtDNA copies in the brain remains uncertain. Using real-time quantitative PCR assay, we examined regional variation in mtDNA copy number in 39 brain regions of male mice. A significant regional difference in mtDNA copy number was observed (P  < 4.8 × 10− 35). High levels of mtDNA copies were found in the ventral tegmental area and substantia nigra, two major nuclei containing dopaminergic neurons. In contrast, cerebellar vermis and lobes had significantly lower copy numbers than other regions. Hippocampal dentate gyrus also had a relatively low mtDNA copy number. This study is the first quantitative analysis of regional variation in mtDNA copy number in mouse brain. Our findings are important for the physiological and pathophysiological studies of mtDNA in the brain.► There is a significant difference in mtDNA copy number among brain regions. ► VTA and SN contain highest mtDNA copy number. ► Lowest levels of mtDNA copies are observed in celebellar vermis and lobes.
Keywords: Cerebellum; Copy number; Dopamine; mtDNA; Substantia nigra; Ventral tegmental area;

Ion conductance pathways in potato tuber (Solanum tuberosum) inner mitochondrial membrane by Karolina Matkovic; Izabela Koszela-Piotrowska; Wieslawa Jarmuszkiewicz; Adam Szewczyk (275-285).
Single-ion channel activities were measured after reconstitution of potato tuber mitochondrial inner membranes into planar lipid bilayers. In addition to the recently described large-conductance Ca2+-activated potassium channel activity (Koszela-Piotrowska et al., 2009), the following mitochondrial ion conductance pathways were recorded: (i) an ATP-regulated potassium channel (mitoKATP channel) activity with a conductance of 164 ± 8 pS, (ii) a large-conductance Ca2+-insensitive iberiotoxin-sensitive potassium channel activity with a conductance of 312 pS ± 23, and (iii) a chloride 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS)-inhibited channel activity with a conductance of 117 pS ± 4. In isolated non-phosphorylating potato tuber mitochondria, individual and combined potassium channel activities caused significant (up to 14 mV) but not collapsing K+-influx-induced membrane potential depolarisation. Under phosphorylating conditions, the coupling parameters were unchanged in the presence of high K+ level, indicating that plant K+ channels function as energy-dissipating systems that are not able to divert energy from oxidative phosphorylation. A potato tuber K+ channel that is ATP-, 5-hydroxydecanonic acid-, glybenclamide-inhibited and diazoxide-stimulated caused low cation flux, modestly decreasing membrane potential (up to a few mV) and increasing respiration in non-phosphorylating mitochondria. Immunological analysis with antibodies raised against the mammalian plasma membrane ATP-regulated K+ channel identified a pore-forming subunit of the Kir-like family in potato tuber mitochondrial inner membrane. These results suggest that a mitoKATP channel similar to that of mammalian mitochondria is present in potato tuber mitochondria.► MitoBK channels, mitoKATP channel and mitoCl channel are detected in potato tuber mitochondria. ► Plant K channels function as energy-dissipating systems that do not decrease ATP synthesis yield. ► MitoKATP channel in potato tuber mitochondria is similar to that of mammalian mitochondria.
Keywords: Bioenergetics; Mitochondria; Ion channels; Potassium channels; Planar lipid membrane technique;

The nature of the ions that are translocated by Escherichia coli and Paracoccus denitrificans complexes I was investigated. We observed that E. coli complex I was capable of proton translocation in the same direction to the established ΔΨ, showing that in the tested conditions, the coupling ion is the H+. Furthermore, Na+ transport to the opposite direction was also observed, and, although Na+ was not necessary for the catalytic or proton transport activities, its presence increased the latter. We also observed H+ translocation by P. denitrificans complex I, but in this case, H+ transport was not influenced by Na+ and also Na+ transport was not observed. We concluded that E. coli complex I has two energy coupling sites (one Na+ independent and the other Na+ dependent), as previously observed for Rhodothermus marinus complex I, whereas the coupling mechanism of P. denitrificans enzyme is completely Na+ independent. This work thus shows that complex I energy transduction by proton pumping and Na+/H+ antiporting is not exclusive of the R. marinus enzyme. Nevertheless, the Na+/H+ antiport activity seems not to be a general property of complex I, which may be correlated with the metabolic characteristics of the organisms.►We have proposed a model suggesting the existence of two coupling sites in R. marinus respiratory complex I, one operating as a proton pump and the other as a Na+/H+ antiporter. ►To test the universality of our model, we addressed the most studied bacterial complexes I, those from Escherichia coli and Paracoccus denitrificans. ►We observed that complex I from E. coli also presents the antiporter activity but the complex from P. denitrificans does not. ►We hypothesized a correlation between the type of quinone used as substrate and the presence of the antiporter activity.
Keywords: NADH dehydrogenase; NADH:quinone oxidoreductase; Respiratory chain; Sodium/proton antiporter;

Essential role of two tyrosines and two tryptophans on the photoprotection activity of the Orange Carotenoid Protein by Adjélé Wilson; Claire Punginelli; Mohea Couturier; François Perreau; Diana Kirilovsky (293-301).
Photosynthetic organisms have developed photoprotective mechanisms to protect themselves from lethal high light intensities. One of these mechanisms involves the dissipation of excess absorbed light energy into heat. In cyanobacteria, light activation of a soluble carotenoid protein, the Orange Carotenoid Protein (OCP), binding a keto carotenoid, is the key inducer of this mechanism. Blue-green light absorption triggers structural changes within the carotenoid and the protein, leading to the conversion of a dark orange form into a red active form. Here we report the role in photoconversion and photoprotection of individual conserved tyrosines and tryptophans surrounding the rings of the carotenoid. Our results demonstrate that the interaction between the keto group of the carotenoid and Tyr201 and Trp288 is essential for OCP photoactivity. In addition, these amino acids are responsible for carotenoid affinity and specificity. We have already demonstrated that the aromatic character of Tyr44 and Trp110 interacting with the hydroxyl ring is critical. Here we show that the replacement of Tyr44 by Ser affects the stability of the red form avoiding its accumulation at any temperature, while Trp110Ser is affected in the energy necessary to the orange to red conversion and in the interaction with the antenna. Collectively our data support the idea that the red form is essential for photoprotection but not sufficient. Specific conformational changes occurring in the protein seem to be critical to the events leading to energy dissipation.►In OCP, Y201 and W288 are essential for photoactivity and photoprotection. ►Hydrogen bonds between these amino acids and the carotenoid stabilize the orange form. ►Y201 and W288 are responsible for carotenoid affinity and specificity. ►Change of Y44 by S destabilizes the red form. ►Change of W110 by S increases the energy necessary to red conversion.
Keywords: Cyanobacteria; Non-photochemical quenching; Orange carotenoid protein; Photoprotection; Photosystem II; Synechocystis;

Functional and structural role of amino acid residues in the matrix α-helices, termini and cytosolic loops of the bovine mitochondrial oxoglutarate carrier by Daniela V. Miniero; Anna R. Cappello; Rosita Curcio; Anna Ludovico; Lucia Daddabbo; Italo Stipani; Alan J. Robinson; Edmund R.S. Kunji; Ferdinando Palmieri (302-310).
The mitochondrial oxoglutarate carrier belongs to the mitochondrial carrier family and exchanges oxoglutarate for malate and other dicarboxylates across the mitochondrial inner membrane. Here, single-cysteine mutant carriers were engineered for every residue in the amino- and carboxy-terminus, cytoplasmic loops, and matrix α-helices and their transport activity was measured in the presence and absence of sulfhydryl reagents. The analysis of the cytoplasmic side of the oxoglutarate carrier showed that the conserved and symmetric residues of the mitochondrial carrier motif [DE]XX[RK] localized at the C-terminal end of the even-numbered transmembrane α-helices are important for the function of the carrier, but the non-conserved cytoplasmic loops and termini are not. On the mitochondrial matrix side of the carrier most residues of the three matrix α-helices that are in the interface with the transmembrane α-helical bundle are important for function. Among these are the residues of the symmetric [ED]G motif present at the C-terminus of the matrix α-helices; the tyrosines of the symmetric YK motif at the N-terminus of the matrix α-helices; and the hydrophobic residues M147, I171 and I247. The functional role of these residues was assessed in the structural context of the homology model of OGC. Furthermore, in this study no evidence was found for the presence of a specific homo-dimerisation interface on the surface of the carrier consisting of conserved, asymmetric and transport-critical residues.► Identification of residues involved in the closing/opening of the carrier gate on the cytosolic side. ► Identification of residues involved in matrix a-helices movement during carrier conformational changes. ► The passage way for substrate translocation on the matrix side cannot be modified without affecting transport. ► Identification of three functionally crucial glutamines that probably interact with the carrier matrix gate. ► The oxoglutarate carrier most likely functions as a monomer
Keywords: Cysteine scanning mutagenesis; Mitochondria; Mitochondrial carrier; Oxoglutarate carrier; Membrane transport;

Is coproporphyrin III a copper-acquisition compound in Paracoccus denitrificans? by Jani Anttila; Petri Heinonen; Timo Nenonen; Andrea Pino; Hideo Iwaï; Eeva Kauppi; Rabah Soliymani; Marc Baumann; Jani Saksi; Niina Suni; Tuomas Haltia (311-318).
Paracoccus denitrificans is a soil bacterium which can respire aerobically and also denitrify if oxygen is absent. Both processes are highly dependent on copper enzymes and copper is therefore likely to be an essential trace element for the bacterium. If copper is not easily available, a copper-acquisition mechanism would be highly beneficial. In this paper, we have addressed the question of whether Paracoccus secretes a copper-acquisition compound functionally analogous to that found in some methanotrophs. Bacteria were grown both in copper-containing and copper-deficient denitrification media, cells were removed by centrifugation and the supernatant was analysed using chromatography and spectroscopy. Bacterial growth yield in the absence of copper was 70–80% of that in the copper-containing medium. A notable difference between the two culture conditions was that spent copper-deficient medium was pigmented, whereas the copper-containing medium was not. Spectrophotometry indicated that a red compound with an absorption maximum at 405 nm was produced under copper-limited conditions. In addition to the strong 405 nm maximum, the visible spectrum of the purified red molecule had weaker maxima at 535 nm and 570 nm, features typical of metallated tetrapyrroles. Mass spectrometry showed that the purified pigment had a molecular mass of 716.18. Moreover, the fine structure of the mass spectrum suggested the presence of zinc and was consistent with the chemical formula of C36H36N4O8Zn. The presence of zinc was also demonstrated using inductively coupled plasma atomic emission spectroscopy. Fragmentation analysis with mass spectrometry showed the release of consecutive 59 Da fragments, assignable to four − CH2–COOH moieties. Thin layer chromatography as well as NMR analysis of the C-13/N-15 labelled red pigment suggested that it is predominantly zinc coproporphyrin III with a minor fraction of metal-free coproporphyrin III. We propose that in a copper-poor environment P. denitrificans secretes coproporphyrin III for copper chelation and subsequent uptake of the bound copper into the cell. Consistent with this idea, cell yields of copper-deficient cultures grown in the presence of 1 μM copper-coproporphyrin III were 90–95% of the yields of cultures grown in the normal copper-containing media. Coproporphyrin III may work as a copper-acquisition compound in P. denitrificans.► Denitrification and cell respiration depend strongly on copper enzymes. ► Does Cu-limited Paracoccus denitrificans produce copper-acquisition molecules? ► Under copper-poor conditions, the bacterium secretes a red pigment. ► This pigment is zinc coproporphyrin III with some metal-free coproporphyrin. ► The free coproporphyrin could act as a copper-acquisition compound.
Keywords: Copper protein; Chalkophore; Methanobactin; Copper metabolism; Soil chemistry;

Roles of PsbI and PsbM in photosystem II dimer formation and stability studied by deletion mutagenesis and X-ray crystallography by Keisuke Kawakami; Yasufumi Umena; Masako Iwai; Yousuke Kawabata; Masahiko Ikeuchi; Nobuo Kamiya; Jian-Ren Shen (319-325).
PsbM and PsbI are two low molecular weight subunits of photosystem II (PSII), with PsbM being located in the center, and PsbI in the periphery, of the PSII dimer. In order to study the functions of these two subunits from a structural point of view, we crystallized and analyzed the crystal structure of PSII dimers from two mutants lacking either PsbM or PsbI. Our results confirmed the location of these two subunits in the current crystal structure, as well as their absence in the respective mutants. The relative contents of PSII dimers were found to be decreased in both mutants, with a concomitant increase in the amount of PSII monomers, suggesting a destabilization of PSII dimers in both of the mutants. On the other hand, the accumulation level of the overall PSII complexes in the two mutants was similar to that in the wild-type strain. Treatment of purified PSII dimers with lauryldimethylamine N-oxide at an elevated temperature preferentially disintegrated the dimers from the PsbM deletion mutant into monomers and CP43-less monomers, whereas no significant degradation of the dimers was observed from the PsbI deletion mutant. These results indicate that although both PsbM and PsbI are required for the efficient formation and stability of PSII dimers in vivo, they have different roles, namely, PsbM is required directly for the formation of dimers and its absence led to the instability of the dimers accumulated. On the other hand, PsbI is required in the assembly process of PSII dimers in vivo; once the dimers are formed, PsbI was no longer required for its stability.► Location of PsbM and PsbI in PSII was confirmed. ► Deletion of both PsbM and PsbI decreased the stability of PSII dimer. ► PsbM and PsbI played different roles in the formation and stability of PSII dimers.
Keywords: Photosystem II; Mutant; Crystal structure; PsbM; PsbI; Oxygen evolution;

Regulation of LHCII aggregation by different thylakoid membrane lipids by Susann Schaller; Dariusz Latowski; Małgorzata Jemioła-Rzemińska; Ayad Dawood; Christian Wilhelm; Kazimierz Strzałka; Reimund Goss (326-335).
In the present study the influence of the lipid environment on the organization of the main light-harvesting complex of photosystem II (LHCII) was investigated by 77K fluorescence spectroscopy. Measurements were carried out with a lipid-depleted and highly aggregated LHCII which was supplemented with the different thylakoid membrane lipids. The results show that the thylakoid lipids are able to modulate the spectroscopic properties of the LHCII aggregates and that the extent of the lipid effect depends on both the lipid species and the lipid concentration. Addition of the neutral galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) seems to induce a modification of the disorganized structures of the lipid-depleted LHCII and to support the aggregated state of the complex. In contrast, we found that the anionic lipids sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) exert a strong disaggregating effect on the isolated LHCII. LHCII disaggregation was partly suppressed under a high proton concentration and in the presence of cations. The strongest suppression was visible at the lowest pH value (pH 5) and the highest Mg2+ concentration (40 mM) used in the present study. This suggests that the negative charge of the anionic lipids in conjunction with negatively charged domains of the LHCII proteins is responsible for the disaggregation. Additional measurements by photon correlation spectroscopy and sucrose gradient centrifugation, which were used to gain information about the size and molecular mass of the LHCII aggregates, confirmed the results of the fluorescence spectroscopy. LHCII treated with MGDG and DGDG formed an increased number of aggregates with large particle sizes in the μm-range, whereas the incubation with anionic lipids led to much smaller LHCII particles (around 40 nm in the case of PG) with a homogeneous distribution.►Systematic study of interaction between thylakoid lipids and LHCII. ►Antagonistic effect of neutral and anionic lipids. ►MGDG and DGDG support LHCII aggregation. ►SQDG and PG lead to strong LHCII disaggregation. ►Analysis of LHCII lipid particle sizes.
Keywords: Anionic lipid; Bilayer lipid; Light-harvesting complex; MGDG; Non-bilayer lipid; Protein structure; Thylakoid membrane;

Regulation of electron transport in C3 plant chloroplasts in situ and in silico: Short-term effects of atmospheric CO2 and O2 by Ilya V. Kuvykin; Vasily V. Ptushenko; Alexey V. Vershubskii; Alexander N. Tikhonov (336-347).
In this work, we have investigated the effects of atmospheric CO2 and O2 on induction events in Hibiscus rosa-sinensis leaves. These effects manifest themselves as multiphase kinetics of P700 redox transitions and non-monotonous changes in chlorophyll fluorescence. Depletion of CO2 and O2 in air causes a decrease in linear electron flux (LEF) and dramatic lowering of P700 + level. This is explained by the impediment to electron efflux from photosystem 1 (PS1) at low acceptor capacity. With the release of the acceptor deficit, the rate of LEF significantly increases. We have found that oxygen promotes the outflow of electrons from PS1, providing the rise of P700 + level. The effect of oxygen as an alternative electron acceptor becomes apparent at low and ambient concentrations of atmospheric CO2 (≤0.06–0.07%). A decrease in LEF at low CO2 is accompanied by a significant (about 3-fold) rise of non-photochemical quenching (NPQ) of chlorophyll fluorescence. Such an increase in NPQ can be explained by more significant acidification of the thylakoid lumen. This occurs due to lessening the proton flux through the ATP synthases caused by a decrease in the ATP consumption in the Bassham–Benson–Calvin (BBC) cycle. pH-dependent mechanisms of electron transport control have been described within the frames of our mathematical model. The model describes the reciprocal changes in LEF and NPQ and predicts the redistribution of electron fluxes on the acceptor side of PS1. In particular, the contribution of cyclic electron flow around PS1 (CEF1) and water–water cycle gradually decays during the induction phase. This result is consistent with experimental data indicating that under the steady-state conditions the contribution of CEF1 to photosynthetic electron transport in Hibiscus rosa-sinensis is insignificant (≤ 10%).Display Omitted► Depletion of atmospheric CO2 and O2 causes a decrease in linear electron flux (LEF). ► Oxygen promotes the outflow of electrons from photosystem 1. ► Non-photochemical quenching (NPQ) of chlorophyll fluorescence increases at low LEF. ► Effects of CO2 and O2 on electron transport are controlled by the lumen and stroma pH. ► Our mathematical model explains inverse changes in LEF and NPQ at low CO2 and O2.
Keywords: Photosynthesis; Electron and proton transport; Regulation; EPR; PAM-fluorimetry; Mathematical modeling;

We present a mathematical model for the functioning of proton-pumping cytochrome c oxidase, consisting of cyclic conversions between 26 enzyme states. The model is based on the mechanism of oxygen reduction and linked proton translocation postulated by Wikström and Verkhovsky (2007). It enables the calculation of the steady-state turnover rates and enzyme-state populations as functions of the cytochrome c reduction state, oxygen concentration, membrane potential, and pH on either side of the inner mitochondrial membrane. We use the model to explain the enigmatic decrease in oxygen affinity of the enzyme that has been observed in mitochondria when the proton-motive force is increased. The importance of the 26 transitions in the mechanism of cytochrome oxidase for the functional properties of cytochrome oxidase is compared through Metabolic Control Analysis. The control of the K M value is distributed mainly between the steps in the mechanism that involve electrogenic proton movements, with both positive and negative contributions. Positive contributions derive from the same steps that control enzyme turnover rate in the model. Limitations and possible further applications of the model are discussed.► A kinetic model for cytochrome oxidase including proton translocation is given. ► The increased KM for oxygen observed at high protonmotive force is explained. ► The control distribution of turnover, Vmax and KM for O2 between reaction steps is quantified. ► Electrogenic H+ movements dominate control of turnover, Vmax and KM.
Keywords: Cytochrome c oxidase; Kinetic model; Membrane potential; Oxygen affinity; Mitochondria; Metabolic control analysis;

Dual localization of plant glutamate receptor AtGLR3.4 to plastids and plasmamembrane by Enrico Teardo; Elide Formentin; Anna Segalla; Giorgio Mario Giacometti; Oriano Marin; Manuela Zanetti; Fiorella Lo Schiavo; Mario Zoratti; Ildikò Szabò (359-367).
Bioinformatic approaches have allowed the identification in Arabidopsis thaliana of twenty genes encoding for homologues of animal ionotropic glutamate receptors (iGLRs). Some of these putative receptor proteins, grouped into three subfamilies, have been located to the plasmamembrane, but their possible location in organelles has not been investigated so far. In the present work we provide multiple evidence for the plastid localization of a glutamate receptor, AtGLR3.4, in Arabidopsis and tobacco. Biochemical analysis was performed using an antibody shown to specifically recognize both the native protein in Arabidopsis and the recombinant AtGLR3.4 fused to YFP expressed in tobacco. Western blots indicate the presence of AtGLR3.4 in both the plasmamembrane and in chloroplasts. In agreement, in transformed Arabidopsis cultured cells as well as in agroinfiltrated tobacco leaves, AtGLR3.4::YFP is detected both at the plasmamembrane and at the plastid level by confocal microscopy. The photosynthetic phenotype of mutant plants lacking AtGLR3.4 was also investigated. These results identify for the first time a dual localization of a glutamate receptor, revealing its presence in plastids and chloroplasts and opening the way to functional studies.►Intracellular location of a putative ionotropic glutamate receptor ►AtGLR3.4 glutamate receptor is located to both chloroplasts and plasmamembrane ►AtGLR3.4 is expressed in green tissues but not in the root ►AtGLR3.4 is located to chloroplast envelope but not to thylakoids ►Mutants lacking AtGLR3.4 shows a slight decrease in photosynthetic efficiency
Keywords: Glutamate receptor; Chloroplast; Arabidopsis; Confocal microscopy; Photosynthesis;

Fine structure of granal thylakoid membrane organization using cryo electron tomography by Roman Kouřil; Gert T. Oostergetel; Egbert J. Boekema (368-374).
The architecture of grana membranes from spinach chloroplasts was studied by cryo electron tomography. Tomographic reconstructions of ice-embedded isolated grana stacks enabled to resolve features of photosystem II (PSII) in the native membrane and to assign the absolute orientation of individual membranes of granal thylakoid discs. Averaging of 3D sub-volumes containing PSII complexes provided a 3D structure of the PSII complex at 40 Å resolution. Comparison with a recently proposed pseudo-atomic model of the PSII supercomplex revealed the presence of unknown protein densities right on top of 4 light harvesting complex II (LHCII) trimers at the lumenal side of the membrane. The positions of individual dimeric PSII cores within an entire membrane layer indicates that about 23% supercomplexes must be of smaller size than full C2S2M2 supercomplexes, to avoid overlap.Display Omitted► Cryo electron tomography of ice-embedded isolated grana membrane stacks. ► Tomographic reconstruction revealed densities of Photosystem II core complex. ► 3D structure of the Photosystem II was solved at 40 Å resolution. ► Novel protein densities were revealed at the lumenal side of the Photosystem II.
Keywords: Thylakoid membrane; Photosystem II; Supercomplex; Electron microscopy; Cryo tomography;

Regulation of photosynthetic electron transport by Jean-David Rochaix (375-383).
The photosynthetic electron transport chain consists of photosystem II, the cytochrome b 6 f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO2 assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO2 levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled "Regulation of Electron Transport in Chloroplasts".► Linear and cyclic electron flow are required for efficient photosynthetic activity. ► The photosynthetic apparatus constantly adapts to changes in light conditions. ► Besides providing energy the photosynthetic apparatus acts as a light sensor. ► Excess absorbed light energy is dissipated through non-photochemical quenching. ► State transitions equilibrate light excitation energy between the two photosystems.
Keywords: Electron transport; Linear electron flow; Cyclic electron flow; Photosystem II; Photosystem I; Light-harvesting system; Cytochrome b 6 f complex; Thylakoid protein phosphorylation; State transitions;

Having long been debated, it is only in the last few years that a concensus has emerged that the cyclic flow of electrons around Photosystem I plays an important and general role in the photosynthesis of higher plants. Two major pathways of cyclic flow have been identified, involving either a complex termed NDH or mediated via a pathway involving a protein PGR5 and two functions have been described—to generate ATP and to provide a pH gradient inducing non-photochemical quenching. The best evidence for the occurrence of the two pathways comes from measurements under stress conditions—high light, drought and extreme temperatures. In this review, the possible relative functions and importance of the two pathways is discussed as well as evidence as to how the flow through these pathways is regulated. Our growing knowledge of the proteins involved in cyclic electron flow will, in the future, enable us to understand better the occurrence and diversity of cyclic electron transport pathways. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.► Cyclic electron flow around Photosystem I has been shown to play an essential role in photosynthesis. ► Cyclic flow makes ATP and protects plants from stress by triggering non photochemical quenching. ► Two distinct pathways of cyclic flow exist, the PGR5 and the NDH pathways. ► Regulation of cyclic flow probably occurs though competition for oxidising ferredoxin.
Keywords: Photosynthesis; Stress; High light; Drought; Heat; Chilling; Redox regulation;