BBA - Bioenergetics (v.1837, #8)

The site of regulation of light capture in Symbiodinium: Does the peridinin–chlorophyll a–protein detach to regulate light capture? by Atsuko Kanazawa; Gary J. Blanchard; Milán Szabó; Peter J. Ralph; David M. Kramer (1227-1234).
Dinoflagellates from the genus Symbiodinium form symbiotic associations with cnidarians including corals and anemones. The photosynthetic apparatuses of these dinoflagellates possess a unique photosynthetic antenna system incorporating the peridinin–chlorophyll a–protein (PCP). It has been proposed that the appearance of a PCP-specific 77 K fluorescence emission band around 672–675 nm indicates that high light treatment results in PCP dissociation from intrinsic membrane antenna complexes, blocking excitation transfer to the intrinsic membrane-bound antenna complexes, chlorophyll a–chlorophyll c 2–peridinin–protein-complex (acpPC) and associated photosystems (Reynolds et al., 2008 Proc Natl Acad Sci USA 105:13674–13678).We have tested this model using time-resolved fluorescence decay kinetics in conjunction with global fitting to compare the time-evolution of the PCP spectral bands before and after high light exposure. Our results show that no long-lived PCP fluorescence emission components appear either before or after high light treatment, indicating that the efficiency of excitation transfer from PCP to membrane antenna systems remains efficient and rapid even after exposure to high light. The apparent increased relative emission at around 675 nm was, instead, caused by strong preferential exciton quenching of the membrane antenna complexes associated with acpPC and reaction centers. This strong non-photochemical quenching (NPQ) is consistent with the activation of xanthophyll-associated quenching mechanisms and the generally-observed avoidance in nature of long-lived photoexcited states that can lead to oxidative damage. The acpPC component appears to be the most strongly quenched under high light exposure suggesting that it houses the photoprotective exciton quencher.
Keywords: Non-photochemical quenching; Peridinin–chlorophyll a–protein; Photoprotection; Photosynthesis;

Photoprotective sites in the violaxanthin–chlorophyll a binding Protein (VCP) from Nannochloropsis gaditana by Donatella Carbonera; Alessandro Agostini; Marilena Di Valentin; Caterina Gerotto; Stefania Basso; Giorgio Mario Giacometti; Tomas Morosinotto (1235-1246).
Violaxanthin–chlorophyll a binding protein (VCP) is the major light harvesting complex (LHC) of the Heterokonta Nannochloropsis gaditana. It binds chlorophyll a, violaxanthin and vaucheriaxanthin, the last in the form of 19′ deca/octanoate esters. Photosynthetic apparatus of algae belonging to this group have been poorly characterized in the past, but they are now receiving an increasing interest also because of their possible biotechnological application in biofuel production. In this work, isolated VCP proteins have been studied by means of advanced EPR techniques in order to prove the presence of the photoprotective mechanism based on the triplet–triplet energy transfer (TTET), occurring between chlorophyll and carotenoid molecules. This process has been observed before in several light harvesting complexes belonging to various photosynthetic organisms. We used Optically Detected Magnetic Resonance (ODMR) to identify the triplet states populated by photo-excitation, and describe the optical properties of the chromophores carrying the triplet states. In parallel, time-resolved EPR (TR-EPR) and pulse EPR have been employed to get insight into the TTET mechanism and reveal the structural features of the pigment sites involved in photoprotection. The analysis of the spectroscopic data shows a strong similarity among VCP, FCP from diatoms and LHC-II from higher plants. Although these antenna proteins have differentiated sequences and bind different pigments, results suggest that in all members of the LHC superfamily there is a protein core with a conserved structural organization, represented by two central carotenoids surrounded by five chlorophyll a molecules, which plays a fundamental photoprotective role in Chl triplet quenching through carotenoid triplet formation.
Keywords: Triplet state; VCP; Carotenoid; EPR; ODMR; Violaxanthin; Triplet–triplet energy transfer;

Mitochondrial hyperpolarization during chronic complex I inhibition is sustained by low activity of complex II, III, IV and V by Marleen Forkink; Ganesh R. Manjeri; Dania C. Liemburg-Apers; Esther Nibbeling; Maxime Blanchard; Aleksandra Wojtala; Jan A.M. Smeitink; Mariusz R. Wieckowski; Peter H.G.M. Willems; Werner J.H. Koopman (1247-1256).
The mitochondrial oxidative phosphorylation (OXPHOS) system consists of four electron transport chain (ETC) complexes (CI–CIV) and the FoF1-ATP synthase (CV), which sustain ATP generation via chemiosmotic coupling. The latter requires an inward-directed proton-motive force (PMF) across the mitochondrial inner membrane (MIM) consisting of a proton (ΔpH) and electrical charge (Δψ) gradient. CI actively participates in sustaining these gradients via trans-MIM proton pumping. Enigmatically, at the cellular level genetic or inhibitor-induced CI dysfunction has been associated with Δψ depolarization or hyperpolarization. The cellular mechanism of the latter is still incompletely understood. Here we demonstrate that chronic (24 h) CI inhibition in HEK293 cells induces a proton-based Δψ hyperpolarization in HEK293 cells without triggering reverse-mode action of CV or the adenine nucleotide translocase (ANT). Hyperpolarization was associated with low levels of CII-driven O2 consumption and prevented by co-inhibition of CII, CIII or CIV activity. In contrast, chronic CIII inhibition triggered CV reverse-mode action and induced Δψ depolarization. CI- and CIII-inhibition similarly reduced free matrix ATP levels and increased the cell's dependence on extracellular glucose to maintain cytosolic free ATP. Our findings support a model in which Δψ hyperpolarization in CI-inhibited cells results from low activity of CII, CIII and CIV, combined with reduced forward action of CV and ANT.
Keywords: ATeam; Live-cell microscopy; Respirometry; TMRM; SypHer;

Substrate water exchange in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae by Håkan Nilsson; Tomasz Krupnik; Joanna Kargul; Johannes Messinger (1257-1262).
The binding affinity of the two substrate–water molecules to the water-oxidizing Mn4CaO5 catalyst in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae was studied in the S2 and S3 states by the exchange of bound 16O-substrate against 18O-labeled water. The rate of this exchange was detected via the membrane-inlet mass spectrometric analysis of flash-induced oxygen evolution. For both redox states a fast and slow phase of water-exchange was resolved at the mixed labeled m/z 34 mass peak: k f  = 52 ± 8 s− 1 and k s  = 1.9 ± 0.3 s− 1 in the S2 state, and k f  = 42 ± 2 s− 1 and k slow  = 1.2 ± 0.3 s− 1 in S3, respectively. Overall these exchange rates are similar to those observed previously with preparations of other organisms. The most remarkable finding is a significantly slower exchange at the fast substrate–water site in the S2 state, which confirms beyond doubt that both substrate–water molecules are already bound in the S2 state. This leads to a very small change of the affinity for both the fast and the slowly exchanging substrates during the S2  → S3 transition. Implications for recent models for water-oxidation are briefly discussed.
Keywords: Cyanidioschyzon merolae; Photosystem II; Water oxidation; Oxygen evolution; Substrate–water exchange; Membrane-inlet mass spectrometry;

The architecture of Rhodobacter sphaeroides chromatophores by Simon Scheuring; Reinat Nevo; Lu-Ning Liu; Stéphanie Mangenot; Dana Charuvi; Thomas Boudier; Valerie Prima; Pierre Hubert; James N. Sturgis; Ziv Reich (1263-1270).
The chromatophores of Rhodobacter (Rb.) sphaeroides represent a minimal bio-energetic system, which efficiently converts light energy into usable chemical energy. Despite extensive studies, several issues pertaining to the morphology and molecular architecture of this elemental energy conversion system remain controversial or unknown. To tackle these issues, we combined electron microscope tomography, immuno-electron microscopy and atomic force microscopy. We found that the intracellular Rb. sphaeroides chromatophores form a continuous reticulum rather than existing as discrete vesicles. We also found that the cytochrome bc 1 complex localizes to fragile chromatophore regions, which most likely constitute the tubular structures that interconnect the vesicles in the reticulum. In contrast, the peripheral light-harvesting complex 2 (LH2) is preferentially hexagonally packed within the convex vesicular regions of the membrane network. Based on these observations, we propose that the bc 1 complexes are in the inter-vesicular regions and surrounded by reaction center (RC) core complexes, which in turn are bounded by arrays of peripheral antenna complexes. This arrangement affords rapid cycling of electrons between the core and bc 1 complexes while maintaining efficient excitation energy transfer from LH2 domains to the RCs.
Keywords: Electron tomography; Atomic force microscopy; Membrane structure; Reaction center; Light-harvesting complex;

Regulation of brain-type creatine kinase by AMP-activated protein kinase: Interaction, phosphorylation and ER localization by Sacnicte Ramírez Ríos; Frédéric Lamarche; Cécile Cottet-Rousselle; Anna Klaus; Roland Tuerk; Ramon Thali; Yolanda Auchli; René Brunisholz; Dietbert Neumann; Luc Barret; Malgorzata Tokarska-Schlattner; Uwe Schlattner (1271-1283).
AMP-activated protein kinase (AMPK) and cytosolic brain-type creatine kinase (BCK) cooperate under energy stress to compensate for loss of adenosine triphosphate (ATP) by either stimulating ATP-generating and inhibiting ATP-consuming pathways, or by direct ATP regeneration from phosphocreatine, respectively. Here we report on AMPK-dependent phosphorylation of BCK from different species identified by in vitro screening for AMPK substrates in mouse brain. Mass spectrometry, protein sequencing, and site-directed mutagenesis identified Ser6 as a relevant residue with one site phosphorylated per BCK dimer. Yeast two-hybrid analysis revealed interaction of active AMPK specifically with non-phosphorylated BCK. Pharmacological activation of AMPK mimicking energy stress led to BCK phosphorylation in astrocytes and fibroblasts, as evidenced with a highly specific phospho-Ser6 antibody. BCK phosphorylation at Ser6 did not affect its enzymatic activity, but led to the appearance of the phosphorylated enzyme at the endoplasmic reticulum (ER), close to the ER calcium pump, a location known for muscle-type cytosolic creatine kinase (CK) to support Ca2+-pumping.Display Omitted
Keywords: Protein kinase; Creatine kinase; Endoplasmic reticulum; Energy homeostasis; Protein phosphorylation; Subcellular localization;

Functional and molecular characterization of plastid terminal oxidase from rice (Oryza sativa) by Qiuju Yu; Kathleen Feilke; Anja Krieger-Liszkay; Peter Beyer (1284-1292).
The plastid terminal oxidase (PTOX) is a plastohydroquinone:oxygen oxidoreductase that shares structural similarities with alternative oxidases (AOX). Multiple roles have been attributed to PTOX, such as involvement in carotene desaturation, a safety valve function, participation in the processes of chlororespiration and setting the redox poise for cyclic electron transport. We have investigated a homogenously pure MBP fusion of PTOX. The protein forms a homo-tetrameric complex containing 2 Fe per monomer and is very specific for the plastoquinone head-group. The reaction kinetics were investigated in a soluble monophasic system using chemically reduced decyl-plastoquinone (DPQ) as the model substrate and, in addition, in a biphasic (liposomal) system in which DPQ was reduced with DT-diaphorase. While PTOX did not detectably produce reactive oxygen species in the monophasic system, their formation was observed by room temperature EPR in the biphasic system in a [DPQH2] and pH-dependent manner. This is probably the result of the higher concentration of DPQ achieved within the partial volume of the lipid bilayer and a higher Km observed with PTOX-membrane associates which is ≈ 47 mM compared to the monophasic system where a Km of ≈ 74 μM was determined. With liposomes and at the basic stromal pH of photosynthetically active chloroplasts, PTOX was antioxidant at low [DPQH2] gaining prooxidant properties with increasing quinol concentrations. It is concluded that in vivo, PTOX can act as a safety valve when the steady state [PQH2] is low while a certain amount of ROS is formed at high light intensities.
Keywords: Plastid terminal oxidase; PTOX; Reactive oxygen species; Photosynthesis; Carotene desaturation; Plastoquinone;

Ferredoxins function as electron carrier in a wide range of metabolic and regulatory reactions. It is not clear yet, whether the multiplicity of ferredoxin proteins is also reflected in functional multiplicity in photosynthetic organisms. We addressed the biological function of the bacterial-type ferredoxin, Fed7 in the cyanobacterium Synechocystis sp. PCC 6803. The expression of fed7 is induced under low CO2 conditions and further enhanced by additional high light treatment. These conditions are considered as promoting photooxidative stress, and prompted us to investigate the biological function of Fed7 under these conditions. Loss of Fed7 did not inhibit growth of the mutant strain Δfed7 but significantly modulated photosynthesis parameters when the mutant was grown under low CO2 and high light conditions. Characteristics of the Δfed7 mutant included elevated chlorophyll and photosystem I levels as well as reduced abundance and activity of photosystem II. Transcriptional profiling of the mutant under low CO2 conditions demonstrated changes in gene regulation of the carbon concentrating mechanism and photoprotective mechanisms such as the Flv2/4 electron valve, the PSII dimer stabilizing protein Sll0218, and chlorophyll biosynthesis. We conclude that the function of Fed7 is connected to coping with photooxidative stress, possibly by constituting a redox-responsive regulatory element in photoprotection. In photosynthetic eukaryotes domains homologous to Fed7 are exclusively found in chloroplast DnaJ-like proteins that are likely involved in remodeling of regulator protein complexes. It is conceivable that the regulatory function of Fed7 evolved in cyanobacteria and was recruited by Viridiplantae as the controller for the chloroplast DnaJ-like proteins.
Keywords: Bacterial-type ferredoxin; Photooxidative stress; Regulation; Chloroplast DnaJ-like protein; Synechocystis;

The dynamic complex of cytochrome c 6 and cytochrome f studied with paramagnetic NMR spectroscopy by Irene Díaz-Moreno; Rinske Hulsker; Pavol Skubak; Johannes M. Foerster; Davide Cavazzini; Michelina G. Finiguerra; Antonio Díaz-Quintana; Blas Moreno-Beltrán; Gian-Luigi Rossi; G. Matthias Ullmann; Navraj S. Pannu; Miguel A. De la Rosa; Marcellus Ubbink (1305-1315).
The rapid transfer of electrons in the photosynthetic redox chain is achieved by the formation of short-lived complexes of cytochrome b 6 f with the electron transfer proteins plastocyanin and cytochrome c 6. A balance must exist between fast intermolecular electron transfer and rapid dissociation, which requires the formation of a complex that has limited specificity. The interaction of the soluble fragment of cytochrome f and cytochrome c 6 from the cyanobacterium Nostoc sp. PCC 7119 was studied using NMR spectroscopy and X-ray diffraction. The crystal structures of wild type, M58H and M58C cytochrome c 6 were determined. The M58C variant is an excellent low potential mimic of the wild type protein and was used in chemical shift perturbation and paramagnetic relaxation NMR experiments to characterize the complex with cytochrome f. The interaction is highly dynamic and can be described as a pure encounter complex, with no dominant stereospecific complex. Ensemble docking calculations and Monte-Carlo simulations suggest a model in which charge–charge interactions pre-orient cytochrome c 6 with its haem edge toward cytochrome f to form an ensemble of orientations with extensive contacts between the hydrophobic patches on both cytochromes, bringing the two haem groups sufficiently close to allow for rapid electron transfer. This model of complex formation allows for a gradual increase and decrease of the hydrophobic interactions during association and dissociation, thus avoiding a high transition state barrier that would slow down the dissociation process.Display Omitted
Keywords: Electron transfer; Photosynthesis; Crystallography; Paramagnetic relaxation enhancement; Monte-Carlo modelling; Protein interaction;

Photosystem II (PSII) is a membrane-bound protein complex that oxidizes water to produce energized protons, which are used to built up a proton gradient across the thylakoidal membrane in the leafs of plants. This light-driven reaction is catalyzed by withdrawing electrons from the Mn4CaO5-cluster (Mn-cluster) in four discrete oxidation steps [S1  − (S4  / S0)] characterized in the Kok-cycle. In order to understand in detail the proton release events and the subsequent translocation of such energized protons, the protonation pattern of the Mn-cluster need to be elucidated. The new high-resolution PSII crystal structure from Umena, Kawakami, Shen, and Kamiya is an excellent basis to make progress in solving this problem. Following our previous work on oxidation and protonation states of the Mn-cluster, in this work, quantum chemical/electrostatic calculations were performed in order to estimate the pKa of different protons of relevant groups and atoms of the Mn-cluster such as W2, O4, O5 and His337. In broad agreement with previous experimental and theoretical work, our data suggest that W2 and His337 are likely to be in hydroxyl and neutral form, respectively, O5 and O4 to be unprotonated. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.Display Omitted
Keywords: PSII; Mn-cluster; Protonation of the OEC; Redox states; pKa;

The time course of ATP synthesis, oxygen consumption, and change in the membrane potential in Paracoccus denitrificans inside-out plasma membrane vesicles was traced. ATP synthesis initiated by the addition of a limited amount of either ADP or inorganic phosphate proceeded up to very low residual concentrations of the limiting substrate. Accumulated ATP did not decrease the rate of its synthesis initiated by the addition of ADP. The amount of residual ADP determined at State 4 respiration was independent of ten-fold variation of Pi or the presence of ATP. The pH-dependence of K m for Pi could not be fitted to a simple phosphoric acid dissociation curve. Partial inhibition of respiration resulted in a decrease in the rate of ATP synthesis without affecting the ATP/ADP reached at State 4. At pH 8.0, hydrolysis of ATP accumulated at State 4 was induced by a low concentration of an uncoupler, whereas complete uncoupling results in rapid inactivation of ATPase. At pH 7.0, no reversal of the ATP synthase reaction by the uncoupler was seen. The data show that ATP/ADP × Pi ratio maintained at State 4 is not in equilibrium with respiratory-generated driving force. Possible mechanisms of kinetic control and unidirectional operation of the Fo·F1-ATP synthase are discussed.
Keywords: Oxidative phosphorylation; Fo·F1-ATP synthase; Respiratory control; Paracoccus denitrificans;

Unsuspected task for an old team: Succinate, fumarate and other Krebs cycle acids in metabolic remodeling by Paule Bénit; Eric Letouzé; Malgorzata Rak; Laetitia Aubry; Nelly Burnichon; Judith Favier; Anne-Paule Gimenez-Roqueplo; Pierre Rustin (1330-1337).
Seventy years from the formalization of the Krebs cycle as the central metabolic turntable sustaining the cell respiratory process, key functions of several of its intermediates, especially succinate and fumarate, have been recently uncovered. The presumably immutable organization of the cycle has been challenged by a number of observations, and the variable subcellular location of a number of its constitutive protein components is now well recognized, although yet unexplained. Nonetheless, the most striking observations have been made in the recent period while investigating human diseases, especially a set of specific cancers, revealing the crucial role of Krebs cycle intermediates as factors affecting genes methylation and thus cell remodeling. We review here the recent advances and persisting incognita about the role of Krebs cycle acids in diverse aspects of cellular life and human pathology.
Keywords: Mitochondria; Tricarboxylic acid cycle; Dioxygenase; HIF1α; Histone;

Apoptosis in neuronal tissue is an efficient mechanism which contributes to both normal cell development and pathological cell death. The present study explores the effects of extracellular ADP on low [K+]-induced apoptosis in rat cerebellar granule cells. ADP, released into the extracellular space in brain by multiple mechanisms, can interact with its receptor or be converted, through the actions of ectoenzymes, to adenosine. The findings reported in this paper demonstrate that ADP inhibits the proapoptotic stimulus supposedly via: i) inhibition of ROS production during early stages of apoptosis, an effect mediated by its interaction with cell receptor/s. This conclusion is validated by the increase in SOD and catalase activities as well as by the GSSG/GSH ratio value decrease, in conjunction with the drop of ROS level and the prevention of the ADP protective effect by pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS), a novel functionally selective antagonist of purine receptor; ii) safeguard of the functionality of the mitochondrial adenine nucleotide-1 translocator (ANT-1), which is early impaired during apoptosis. This effect is mediated by its plausible internalization into cell occurring as such or after its hydrolysis, by means of plasma membrane nucleotide metabolizing enzymes, and resynthesis into the cell. Moreover, the findings that ADP also protects ANT-1 from the toxic action of the two Alzheimer's disease peptides, i.e. Aβ1–42 and NH2htau, which are known to be produced in apoptotic cerebellar neurons, further corroborate the molecular mechanism of neuroprotection by ADP, herein proposed.
Keywords: ADP; Apoptosis; Protection; Antioxidant enzyme; Adenine nucleotide translocator; Mitochondria;

Formation of highly organized intracellular structure and energy metabolism in cardiac muscle cells during postnatal development of rat heart by Tiia Anmann; Minna Varikmaa; Natalja Timohhina; Kersti Tepp; Igor Shevchuk; Vladimir Chekulayev; Valdur Saks; Tuuli Kaambre (1350-1361).
Adult cardiomyocytes have highly organized intracellular structure and energy metabolism whose formation during postnatal development is still largely unclear. Our previous results together with the data from the literature suggest that cytoskeletal proteins, particularly βII-tubulin, are involved in the formation of complexes between mitochondria and energy consumption sites. The aim of this study was to examine the arrangement of intracellular architecture parallel to the alterations in regulation of mitochondrial respiration in rat cardiomyocytes during postnatal development, from 1 day to 6 months.Respirometric measurements were performed to study the developmental alterations of mitochondrial function. Changes in the mitochondrial arrangement and cytoarchitecture of βII- and αIV-tubulin were examined by confocal microscopy.Our results show that functional maturation of oxidative phosphorylation in mitochondria is completed much earlier than efficient feedback regulation is established between mitochondria and ATPases via creatine kinase system. These changes are accompanied by significant remodeling of regular intermyofibrillar mitochondrial arrays aligned along the bundles of βII-tubulin. Additionally, we demonstrate that formation of regular arrangement of mitochondria is not sufficient per se to provide adult-like efficiency in metabolic feed-back regulation, but organized tubulin networks and reduction in mitochondrial outer membrane permeability for ADP are necessary as well. In conclusion, cardiomyocytes in rat heart become mature on the level of intracellular architecture and energy metabolism at the age of 3 months.
Keywords: Mitochondria; Bioenergetics; Cardiomyocyte; Development; Tubulin; Intracellular architecture;

Evidence that histidine forms a coordination bond to the A0A and A0B chlorophylls and a second H-bond to the A1A and A1B phylloquinones in M688HPsaA and M668HPsaB variants of Synechocystis sp. PCC 6803 by Junlei Sun; Sijie Hao; Matthew Radle; Wu Xu; Ivan Shelaev; Victor Nadtochenko; Vladimir Shuvalov; Alexey Semenov; Heather Gordon; Art van der Est; John H. Golbeck (1362-1375).
The axial ligands of the acceptor chlorophylls, A0A and A0B, in Photosystem I are the Met sulfur atoms of M688PsaA and M668PsaB. To determine the role of the Met, His variants were generated in Synechocystis sp. PCC 6803. Molecular dynamics simulations on M688HPsaA show that there exist low energy conformations with the His coordinated to A0A and possibly H-bonded to A1A. Transient EPR studies on M688HPsaA indicate a more symmetrical electron spin distribution in the A1A phyllosemiquinone ring consistent with the presence of an H-bond to the C1 carbonyl. Ultrafast optical studies on the variants show that the 150 fs charge separation between P700 and A0 remains unaffected. Studies on the ns timescale show that 57% of the electrons are transferred from A0A to A1A in M688HPsaA and 48% from A0B to A1B in M668HPsaB; the remainder recombine with P700 + with 1/e times of 25 ns and 37 ns, respectively. Those electrons that reach A1A and A1B in the branch carrying the mutation are not transferred to FX, but recombine with P700 + with 1/e times of ~ 15 μs and ~ 5 μs, respectively. Hence, the His is coordinated to A0 in all populations, but in a second population, the His may be additionally H-bonded to A1. Electron transfer from A0 to A1 occurs only in the latter, but the higher redox potentials of A0 and A1 as a result of the stronger coordination bond to A0 and the proposed second H-bond to A1 preclude electron transfer to the Fe/S clusters.Display Omitted
Keywords: Chlorophyll a; Photosynthesis; Photosystem I; A0; A1; Phylloquinone;