BBA - Bioenergetics (v.1847, #6-7)

Toward an understanding of the function of Chlamydiales in plastid endosymbiosis by Steven G. Ball; Christophe Colleoni; Derifa Kadouche; Mathieu Ducatez; Maria-Cecilia Arias; Catherine Tirtiaux (495-504).
Plastid endosymbiosis defines a process through which a fully evolved cyanobacterial ancestor has transmitted to a eukaryotic phagotroph the hundreds of genes required to perform oxygenic photosynthesis, together with the membrane structures, and cellular compartment associated with this process. In this review, we will summarize the evidence pointing to an active role of Chlamydiales in metabolic integration of free living cyanobacteria, within the cytosol of the last common plant ancestor.
Keywords: Plastid endosymbiosis; Chlamydiales; Cyanobacteria; Photosynthesis evolution;

Like the evolutionary related F1FO ATP synthases and V1VO ATPases, the A1AO ATP synthases from archaea are multisubunit, membrane-bound transport machines that couple ion flow to the synthesis of ATP. Although the subunit composition is known for at least two species, nothing is known so far with respect to the function of individual subunits or amino acid residues. To pave the road for a functional analysis of A1AO ATP synthases, we have cloned the entire operon from Methanosarcina mazei into an expression vector and produced the enzyme in Escherichia coli. Inverted membrane vesicles of the recombinants catalyzed ATP synthesis driven by NADH oxidation as well as artificial driving forces. Δ μ ˜ H + as well as ΔpH were used as driving forces which is consistent with the inhibition of NADH-driven ATP synthesis by protonophores. Exchange of the conserved glutamate in subunit c led to a complete loss of ATP synthesis, proving that this residue is essential for H+ translocation. Exchange of two conserved arginine residues in subunit a has different effects on ATP synthesis. The role of these residues in ion translocation is discussed.
Keywords: Archaea; Energy conservation; Methanogens; Ion translocation; Site directed mutagenesis;

Both extremes of redox balance are known to cause cardiac injury, with mounting evidence revealing that the injury induced by both oxidative and reductive stress is oxidative in nature. During reductive stress, when electron acceptors are expected to be mostly reduced, some redox proteins can donate electrons to O2 instead, which increases reactive oxygen species (ROS) production. However, the high level of reducing equivalents also concomitantly enhances ROS scavenging systems involving redox couples such as NADPH/NADP+ and GSH/GSSG. Here our objective was to explore how reductive stress paradoxically increases net mitochondrial ROS production despite the concomitant enhancement of ROS scavenging systems. Using recombinant enzymes and isolated permeabilized cardiac mitochondria, we show that two normally antioxidant matrix NADPH reductases, glutathione reductase and thioredoxin reductase, generate H2O2 by leaking electrons from their reduced flavoprotein to O2 when electron flow is impaired by inhibitors or because of limited availability of their natural electron acceptors, GSSG and oxidized thioredoxin. The spillover of H2O2 under these conditions depends on H2O2 reduction by peroxiredoxin activity, which may regulate redox signaling in response to endogenous or exogenous factors. These findings may explain how ROS production during reductive stress overwhelms ROS scavenging capability, generating the net mitochondrial ROS spillover causing oxidative injury. These enzymes could potentially be targeted to increase cancer cell death or modulate H2O2-induced redox signaling to protect the heart against ischemia/reperfusion damage.
Keywords: Mitochondria; ROS production; Reductive stress;

Skeletal muscle mitochondria of NDUFS4 −/− mice display normal maximal pyruvate oxidation and ATP production by Mohammad T. Alam; Ganesh R. Manjeri; Richard J. Rodenburg; Jan A.M. Smeitink; Richard A. Notebaart; Martijn Huynen; Peter H.G.M. Willems; Werner J.H. Koopman (526-533).
Mitochondrial ATP production is mediated by the oxidative phosphorylation (OXPHOS) system, which consists of four multi-subunit complexes (CI–CIV) and the FoF1-ATP synthase (CV). Mitochondrial disorders including Leigh Syndrome often involve CI dysfunction, the pathophysiological consequences of which still remain incompletely understood. Here we combined experimental and computational strategies to gain mechanistic insight into the energy metabolism of isolated skeletal muscle mitochondria from 5-week-old wild-type (WT) and CI-deficient NDUFS4 −/− (KO) mice. Enzyme activity measurements in KO mitochondria revealed a reduction of 79% in maximal CI activity (Vmax), which was paralleled by 45–72% increase in Vmax of CII, CIII, CIV and citrate synthase. Mathematical modeling of mitochondrial metabolism predicted that these Vmax changes do not affect the maximal rates of pyruvate (PYR) oxidation and ATP production in KO mitochondria. This prediction was empirically confirmed by flux measurements. In silico analysis further predicted that CI deficiency altered the concentration of intermediate metabolites, modestly increased mitochondrial NADH/NAD+ ratio and stimulated the lower half of the TCA cycle, including CII. Several of the predicted changes were previously observed in experimental models of CI-deficiency. Interestingly, model predictions further suggested that CI deficiency only has major metabolic consequences when its activity decreases below 90% of normal levels, compatible with a biochemical threshold effect. Taken together, our results suggest that mouse skeletal muscle mitochondria possess a substantial CI overcapacity, which minimizes the effects of CI dysfunction on mitochondrial metabolism in this otherwise early fatal mouse model.Display Omitted
Keywords: CI; IMS; MIM; MOM; ODE; ETC;

Molecular basis of chromatic adaptation in pennate diatom Phaeodactylum tricornutum by Miroslava Herbstová; David Bína; Peter Koník; Zdenko Gardian; František Vácha; Radek Litvín (534-543).
The remarkable adaptability of diatoms living in a highly variable environment assures their prominence among marine primary producers. The present study integrates biochemical, biophysical and genomic data to bring new insights into the molecular mechanism of chromatic adaptation of pennate diatoms in model species Phaeodactylum tricornutum, a marine eukaryote alga possessing the capability to shift its absorption up to ~ 700 nm as a consequence of incident light enhanced in the red component. Presence of these low energy spectral forms of Chl a is manifested by room temperature fluorescence emission maximum at 710 nm (F710). Here we report a successful isolation of the supramolecular protein complex emitting F710 and identify a member of the Fucoxanthin Chlorophyll a/c binding Protein family, Lhcf15, as its key building block. This red-shifted antenna complex of P. tricornutum appears to be functionally connected to photosystem II. Phylogenetic analyses do not support relation of Lhcf15 of P. tricornutum to other known red-shifted antenna proteins thus indicating a case of convergent evolutionary adaptation towards survival in shaded environments.
Keywords: Chromatic adaptation; Diatom; Phaeodactylum tricornutum; Light harvesting antenna; Red-shifted antenna complex; Heterokonta;

Emerging concepts in the therapy of mitochondrial disease by Carlo Viscomi; Emanuela Bottani; Massimo Zeviani (544-557).
Mitochondrial disorders are an important group of genetic conditions characterized by impaired oxidative phosphorylation. Mitochondrial disorders come with an impressive variability of symptoms, organ involvement, and clinical course, which considerably impact the quality of life and quite often shorten the lifespan expectancy. Although the last 20 years have witnessed an exponential increase in understanding the genetic and biochemical mechanisms leading to disease, this has not resulted in the development of effective therapeutic approaches, amenable of improving clinical course and outcome of these conditions to any significant extent. Therapeutic options for mitochondrial diseases still remain focused on supportive interventions aimed at relieving complications. However, new therapeutic strategies have recently been emerging, some of which have shown potential efficacy at the pre-clinical level. This review will present the state of the art on experimental therapy for mitochondrial disorders.
Keywords: Mitochondrion; Experimental therapy; Oxidative phosphorilation; Mitochondrial respiratory chain; Mitochondrial disease; Animal model;

Directly probing redox-linked quinones in photosystem II membrane fragments via UV resonance Raman scattering by Jun Chen; Mingdong Yao; Cynthia V. Pagba; Yang Zheng; Liping Fei; Zhaochi Feng; Bridgette A. Barry (558-564).
In photosynthesis, photosystem II (PSII) harvests sunlight with bound pigments to oxidize water and reduce quinone to quinol, which serves as electron and proton mediators for solar-to-chemical energy conversion. At least two types of quinone cofactors in PSII are redox-linked: QA, and QB. Here, we for the first time apply 257-nm ultraviolet resonance Raman (UVRR) spectroscopy to acquire the molecular vibrations of plastoquinone (PQ) in PSII membranes. Owing to the resonance enhancement effect, the vibrational signal of PQ in PSII membranes is prominent. A strong band at 1661 cm− 1 is assigned to ring C=C/C=O symmetric stretch mode (ν8a mode) of PQ, and a weak band at 469 cm− 1 to ring stretch mode. By using a pump-probe difference UVRR method and a sample jet technique, the signals of QA and QB can be distinguished. A frequency difference of 1.4 cm− 1 in ν8a vibrational mode between QA and QB is observed, corresponding to ~ 86 mV redox potential difference imposed by their protein environment. In addition, there are other PQs in the PSII membranes. A negligible anharmonicity effect on their combination band at 2130 cm− 1 suggests that the ‘other PQs’ are situated in a hydrophobic environment. The detection of the ‘other PQs’ might be consistent with the view that another functional PQ cofactor (not QA or QB) exists in PSII. This UVRR approach will be useful to the study of quinone molecules in photosynthesis or other biological systems.Display Omitted
Keywords: Carbonyl bond; Photosynthesis; Proton transfer; Electron transfer; Lipid membrane; Protein dynamics;

Oxidation of plastohydroquinone by photosystem II and by dioxygen in leaves by Agu Laisk; Hillar Eichelmann; Vello Oja (565-575).
In sunflower leaves linear electron flow LEF = 4 O2 evolution rate was measured at 20 ppm O2 in N2. PSII charge separation rate CSRII  =  a II∙ PAD∙(F m  −  F) /  F m, where a II is excitation partitioning to PSII, PAD is photon absorption density, F m and F are maximum and actual fluorescence yields. Under 630 nm LED + 720 nm far-red light (FRL), LEF was equal to CSRII with a II  = 0.51 to 0.58. After FRL was turned off, plastoquinol (PQH2) accumulated, but LEF decreased more than accountable by F increase, indicating PQH2-oxidizing cyclic electron flow in PSII (CEFII). CEFII was faster under conditions requiring more ATP, consistent with CEFII being coupled with proton translocation. We propose that PQH2 bound to the QC site is oxidized, one e moving to P680+, the other e to Cyt b 559. From Cyt b 559 the e reduces QB at the QB site, forming PQH2. About 10–15% electrons may cycle, causing misses in the period-4 flash O2 evolution and lower quantum yield of photosynthesis under stress. We also measured concentration dependence of PQH2 oxidation by dioxygen, as indicated by post-illumination decrease of Chl fluorescence yield. After light was turned off, F rapidly decreased from F m to 0.2 F v, but further decrease to F 0 was slow and O2 concentration dependent. The rate constant of PQH2 oxidation, determined from this slow phase, was 0.054 s− 1 at 270 μM (21%) O2, decreasing with K m(O2) of 60 μM (4.6%) O2. This eliminates the interference of O2 in the measurements of CEFII.
Keywords: Leaves; Photosystem II; Cyclic electron transport;

The site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S0 to S4) before O2 is evolved. It consists of a Mn4CaO5-cluster close to a redox-active tyrosine residue (YZ). Cl is also required for enzyme activity. By using EPR spectroscopy it has been shown that both Ca2+/Sr2+ exchange and Cl/I exchange perturb the proportions of centers showing high (S  = 5/2) and low spin (S  = 1/2) forms of the S2-state. The S3-state was also found to be heterogeneous with: i) a S  = 3 form that is detectable by EPR and not sensitive to near-infrared light; and ii) a form that is not EPR visible but in which Mn photochemistry occurs resulting in the formation of a (S2YZ )′ split EPR signal upon near-infrared illumination. In Sr/Cl-PSII, the high spin (S  = 5/2) form of S2 shows a marked heterogeneity with a g  = 4.3 form generated at low temperature that converts to a relaxed form at g  = 4.9 at higher temperatures. The high spin g  = 4.9 form can then progress to the EPR detectable form of S3 at temperatures as low as 180 K whereas the low spin (S  = 1/2) S2-state can only advance to the S3 state at temperatures ≥ 235 K. Both of the two S2 configurations and the two S3 configurations are each shown to be in equilibrium at ≥ 235 K but not at 198 K. Since both S2 configurations are formed at 198 K, they likely arise from two specific populations of S1. The existence of heterogeneous populations in S1, S2 and S3 states may be related to the structural flexibility associated with the positioning of the oxygen O5 within the cluster highlighted in computational approaches and which has been linked to substrate exchange. These data are discussed in the context of recent in silico studies of the electron transfer pathways between the S2-state(s) and the S3-state(s).Display Omitted
Keywords: Photosystem II; Oxygen evolution; Mn4CaO5 cluster; Spin state; EPR;

Cardiolipin is a key determinant for mtDNA stability and segregation during mitochondrial stress by Luis Alberto Luévano-Martínez; Maria Fernanda Forni; Valquiria Tiago dos Santos; Nadja C. Souza-Pinto; Alicia J. Kowaltowski (587-598).
Mitochondria play a key role in adaptation during stressing situations. Cardiolipin, the main anionic phospholipid in mitochondrial membranes, is expected to be a determinant in this adaptive mechanism since it modulates the activity of most membrane proteins. Here, we used Saccharomyces cerevisiae subjected to conditions that affect mitochondrial metabolism as a model to determine the possible role of cardiolipin in stress adaptation. Interestingly, we found that thermal stress promotes a 30% increase in the cardiolipin content and modifies the physical state of mitochondrial membranes. These changes have effects on mtDNA stability, adapting cells to thermal stress. Conversely, this effect is cardiolipin-dependent since a cardiolipin synthase-null mutant strain is unable to adapt to thermal stress as observed by a 60% increase of cells lacking mtDNA (ρ0). Interestingly, we found that the loss of cardiolipin specifically affects the segregation of mtDNA to daughter cells, leading to a respiratory deficient phenotype after replication. We also provide evidence that mtDNA physically interacts with cardiolipin both in S. cerevisiae and in mammalian mitochondria. Overall, our results demonstrate that the mitochondrial lipid cardiolipin is a key determinant in the maintenance of mtDNA stability and segregation.
Keywords: Phospholipid; Mitochondrion; Mitochondrial DNA; Membrane plasticity;

Cardiolipin (CL) and phosphatidylglycerol (PG) are the main anionic phospholipids present in the Eukarya and Bacteria domains. They participate in energy transduction by activating and stabilizing the components of the oxidative phosphorylation machinery. Experimental evidence shows that they are synthesized by two different mechanisms which indicate that both pathways evolved convergently. Former studies on the lipid composition of archaeal membranes showed the absence of CL in these organisms, consequently, restricting it to the Eukarya and Bacteria domains. Interestingly, recent studies have demonstrated that both CL and PG are present as constitutive components of membranes of the haloarchaea group. However, this scenario complicates the analysis of the evolutionary origin of this biosynthetic pathway. Here I suggest that a phospholipid biosynthetic pathway in Eukarya probably arose from a chimeric event between bacterial and archaeal enzymes during the endosymbiosis event. Phylogenetic analyses support the different evolutionary origin of the enzymes comprising this pathway in bacteria and Eukarya. Based on protein domain analyses, orthologous proteins in the Archaea domain were identified. An integrative analysis of the proteins found demonstrates that CL biosynthesis in major clades of the Eukarya domain originated by chimerism between the bacteria and archaea pathways. Moreover, primary and secondary endosymbiontic events in plants and chromoalveolata respectively, reshaped this pathway again. The implications and advantages that these new enzymatic orders conferred to the Eukarya domain are discussed.
Keywords: Cardiolipin; Eukarya; Archaea; Bacteria; Chimerism; Evolution;

Light-harvesting II antenna trimers connect energetically the entire photosynthetic machinery — including both photosystems II and I by Michele Grieco; Marjaana Suorsa; Anjana Jajoo; Mikko Tikkanen; Eva-Mari Aro (607-619).
In plant chloroplasts, the two photosystems (PSII and PSI) are enriched in different thylakoid domains and, according to the established view, are regarded as energetically segregated from each other. A specific fraction of the light harvesting complex II (LHCII) has been postulated to get phosphorylated by the STN7 kinase and subsequently to migrate from PSII to PSI as part of a process called ‘state transition’. Nevertheless, the thylakoid membrane incorporates a large excess of LHCII not present in the isolatable PSII–LHCII and PSI–LHCII complexes. Moreover, LHCII phosphorylation is not limited to a specific LHCII pool and “state 2” condition, but is found in all thylakoid domains in any constant light condition. Here, using a targeted solubilization of pigment–protein complexes from different thylakoid domains, we demonstrate that even a minor detachment of LHCII leads to markedly increased fluorescence emission from LHCII and PSII both in grana core and non-appressed thylakoid membranes and the effect of the detergent to detach LHCII is enhanced in the absence of LHCII phosphorylation. These findings provide evidence that PSII and PSI are energy traps embedded in the same energetically connected LHCII lake. In the lake, PSI and LHCII are energetically connected even in the absence of LHCII phosphorylation, yet the phosphorylation enhances the interaction required for efficient energy transfer to PSI in the grana margin regions.
Keywords: Thylakoid membrane; Lateral heterogeneity; Extra LHCII; Energy spillover; State transitions; Protein phosphorylation;

Uncoupling, metabolic inhibition and induction of mitochondrial permeability transition in rat liver mitochondria caused by the major long-chain hydroxyl monocarboxylic fatty acids accumulating in LCHAD deficiency by Fernanda Hermes Hickmann; Cristiane Cecatto; Daniele Kleemann; Wagner Oliveira Monteiro; Roger Frigério Castilho; Alexandre Umpierrez Amaral; Moacir Wajner (620-628).
Patients with long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiency commonly present liver dysfunction whose pathogenesis is unknown. We studied the effects of long-chain 3-hydroxylated fatty acids (LCHFA) that accumulate in LCHAD deficiency on liver bioenergetics using mitochondrial preparations from young rats. We provide strong evidence that 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, the monocarboxylic acids that are found at the highest tissue concentrations in this disorder, act as metabolic inhibitors and uncouplers of oxidative phosphorylation. These conclusions are based on the findings that these fatty acids decreased ADP-stimulated (state 3) and uncoupled respiration, mitochondrial membrane potential and NAD(P)H content, and, in contrast, increased resting (state 4) respiration. We also verified that 3HTA and 3HPA markedly reduced Ca2 + retention capacity and induced swelling in Ca2 +-loaded mitochondria. These effects were mediated by mitochondrial permeability transition (MPT) induction since they were totally prevented by the classical MPT inhibitors cyclosporin A and ADP, as well as by ruthenium red, a Ca2 + uptake blocker. Taken together, our data demonstrate that the major monocarboxylic LCHFA accumulating in LCHAD deficiency disrupt energy mitochondrial homeostasis in the liver. It is proposed that this pathomechanism may explain at least in part the hepatic alterations characteristic of the affected patients.
Keywords: Long-chain 3-hydroxy-acyl-CoA dehydrogenase deficiency; Liver mitochondrial bioenergetics; Mitochondrial permeability transition; Calcium;

Respective effects of oxygen and energy substrate deprivation on beta cell viability by Sandrine Lablanche; Cécile Cottet-Rousselle; Laurent Argaud; Camille Laporte; Frédéric Lamarche; Marie-Jeanne Richard; Thierry Berney; Pierre-Yves Benhamou; Eric Fontaine (629-639).
Deficit in oxygen and energetic substrates delivery is a key factor in islet loss during islet transplantation. Permeability transition pore (PTP) is a mitochondrial channel involved in cell death. We have studied the respective effects of oxygen and energy substrate deprivation on beta cell viability as well as the involvement of oxidative stress and PTP opening. Energy substrate deprivation for 1 h followed by incubation in normal conditions led to a cyclosporin A (CsA)-sensitive-PTP-opening in INS-1 cells and human islets. Such a procedure dramatically decreased INS-1 cells viability except when transient removal of energy substrates was performed in anoxia, in the presence of antioxidant N-acetylcysteine (NAC) or when CsA or metformin inhibited PTP opening. Superoxide production increased during removal of energy substrates and increased again when normal energy substrates were restored. NAC, anoxia or metformin prevented the two phases of oxidative stress while CsA prevented the second one only. Hypoxia or anoxia alone did not induce oxidative stress, PTP opening or cell death. In conclusion, energy substrate deprivation leads to an oxidative stress followed by PTP opening, triggering beta cell death. Pharmacological prevention of PTP opening during islet transplantation may be a suitable option to improve islet survival and graft success.
Keywords: Permeability transition; Apoptosis; Beta cell; Ischemia–reperfusion injury;

Functional characteristics of spirilloxanthin and keto-bearing Analogues in light-harvesting LH2 complexes from Rhodobacter sphaeroides with a genetically modified carotenoid synthesis pathway by Dariusz M. Niedzwiedzki; Preston L. Dilbeck; Qun Tang; David J. Mothersole; Elizabeth C. Martin; David F. Bocian; Dewey Holten; C. Neil Hunter (640-655).
Light-harvesting 2 (LH2) complexes from a genetically modified strain of the purple photosynthetic bacterium Rhodobacter (Rba.) sphaeroides were studied using static and ultrafast optical methods and resonance Raman spectroscopy. Carotenoid synthesis in the Rba. sphaeroides strain was engineered to redirect carotenoid production away from spheroidene into the spirilloxanthin synthesis pathway. The strain assembles LH2 antennas with substantial amounts of spirilloxanthin (total double-bond conjugation length N  = 13) if grown anaerobically and of keto-bearing long-chain analogs [2-ketoanhydrorhodovibrin (N  = 13), 2-ketospirilloxanthin (N  = 14) and 2,2′-diketospirilloxanthin (N  = 15)] if grown semi-aerobically (with ratios that depend on growth conditions). We present the photophysical, electronic, and vibrational properties of these carotenoids, both isolated in organic media and assembled within LH2 complexes. Measurements of excited-state energy transfer to the array of excitonically coupled bacteriochlorophyll a molecules (B850) show that the mean lifetime of the first singlet excited state (S1) of the long-chain (N  ≥ 13) carotenoids does not change appreciably between organic media and the protein environment. In each case, the S1 state appears to lie lower in energy than that of B850. The energy-transfer yield is ~ 0.4 in LH2 (from the strain grown aerobically or semi-aerobically), which is less than half that achieved for LH2 that contains short-chain (N  ≤ 11) analogues. Collectively, the results suggest that the S1 excited state of the long-chain (N  ≥ 13) carotenoids participates little if at all in carotenoid-to-BChl a energy transfer, which occurs predominantly via the carotenoid S2 excited state in these antennas.Display Omitted
Keywords: LH2; Carotenoid; Energy transfer; Spirilloxanthin; Diketospirilloxanthin; Keto-carotenoid;

A computational mechanistic model of superoxide (O 2 •−) formation in the mitochondrial electron transport chain (ETC) was developed to facilitate the quantitative analysis of factors controlling mitochondrial O 2 •− production and assist in the interpretation of experimental studies. The model takes into account all individual electron transfer reactions in Complexes I and III. The model accounts for multiple, often seemingly contradictory observations on the effects of ΔΨ and ΔpH, and for the effects of multiple substrate and inhibitor conditions, including differential effects of Complex III inhibitors antimycin A, myxothiazol and stigmatellin. Simulation results confirm that, in addition to O 2 •− formation in Complex III and at the flavin site of Complex I, the quinone binding site of Complex I is an additional superoxide generating site that accounts for experimental observations on O 2 •− production during reverse electron transfer. However, our simulation results predict that, when cytochrome c oxidase is inhibited during oxidation of succinate, ROS production at this site is eliminated and almost all superoxide in Complex I is generated by reduced FMN, even when the redox pressure for reverse electron transfer from succinate is strong. In addition, the model indicates that conflicting literature data on the kinetics of electron transfer in Complex III involving the iron–sulfur protein-cytochrome bL complex can be resolved in favor of a dissociation of the protein only after electron transfer to cytochrome bH. The model predictions can be helpful in understanding factors driving mitochondrial superoxide formation in intact cells and tissues.
Keywords: Respiratory chain; Superoxide; Semiquinone; Membrane potential; Computational model; Inhibitory analysis;