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BBA - Bioenergetics (v.1767, #2)
Chromatic photoacclimation, photosynthetic electron transport and oxygen evolution in the Chlorophyll d-containing oxyphotobacterium Acaryochloris marina by Rosalyn S. Gloag; Raymond J. Ritchie; Min Chen; Anthony W.D. Larkum; Rosanne G. Quinnell (pp. 127-135).
Changes in photosynthetic pigment ratios showed that the Chlorophyll d-dominated oxyphotobacterium Acaryochloris marina was able to photoacclimate to different light regimes. Chl d per cell were higher in cultures grown under low irradiance and red or green light compared to those found when grown under high white light, but phycocyanin/Chl d and carotenoid/Chl d indices under the corresponding conditions were lower. Chl a, considered an accessory pigment in this organism, decreased respective to Chl d in low irradiance and low intensity non-white light sources. Blue diode PAM ( Pulse Amplitude Modulation) fluorometry was able to be used to measure photosynthesis in Acaryochloris. Light response curves for Acaryochloris were created using both PAM and O2 electrode. A linear relationship was found between electron transport rate (ETR), measured using a PAM fluorometer, and oxygen evolution (net and gross photosynthesis). Gross photosynthesis and ETR were directly proportional to one another. The optimum light for white light (quartz halogen) was about 206±51 μmol m−2 s−1 (PAR) ( Photosynthetically Active Radiation), whereas for red light (red diodes) the optimum light was lower (109±27 μmol m−2 s−1 (PAR)). The maximum mean gross photosynthetic rate of Acaryochloris was 73±7 μmol mg Chl d−1 h−1. The gross photosynthesis/respiration ratio ( Pg/ R) of Acaryochloris under optimum conditions was about 4.02±1.69. The implications of our findings will be discussed in relation to how photosynthesis is regulated in Acaryochloris.
Keywords: Acaryochloris; Oxyphotobacteria; Photosynthesis; Chlorophyll; d; Photo adaptation; Oxygen electrode; PAM; Light saturation curve; Induction curve
VDAC1 serves as a mitochondrial binding site for hexokinase in oxidative muscles by Keltoum Anflous-Pharayra; Zong-Jin Cai; William J. Craigen (pp. 136-142).
Voltage-dependent anion channels (VDACs), also known as mitochondrial porins, are the main pathway for metabolites across the mitochondrial outer membrane and may serve as binding sites for kinases, including hexokinase. We determined that mitochondria-bound hexokinase activity is significantly reduced in oxidative muscles (heart and soleus) in vdac1− /− mice. The activity data were supported by western blot analysis using HK2 specific antibody. To gain more insight into the physiologic mean of the results with the activity data, VDAC deficient mice were subjected to glucose tolerance testing and exercise-induced stress, each of which involves tissue glucose uptake via different mechanisms. vdac1− /− mice exhibit impaired glucose tolerance whereas vdac3− /− mice have normal glucose tolerance and exercise capacity. Mice lacking both VDAC1 and VDAC3 ( vdac1− /− /vdac3− /−) have reduced exercise capacity together with impaired glucose tolerance. Therefore, we demonstrated a link between VDAC1 mediated mitochondria-bound hexokinase activity and the capacity for glucose clearance.
Keywords: Mitochondrial outer membrane; Muscle; Porin; Hexokinase; Glucose clearance; Exercise capacity
The role of Sdh4p Tyr-89 in ubiquinone reduction by the Saccharomyces cerevisiae succinate dehydrogenase by Yuri Silkin; Kayode S. Oyedotun; Bernard D. Lemire (pp. 143-150).
Succinate dehydrogenase (complex II or succinate:ubiquinone oxidoreductase) is a tetrameric, membrane-bound enzyme that catalyzes the oxidation of succinate and the reduction of ubiquinone in the mitochondrial respiratory chain. Two electrons from succinate are transferred one at a time through a flavin cofactor and a chain of iron–sulfur clusters to reduce ubiquinone to an ubisemiquinone intermediate and to ubiquinol. Residues that form the proximal quinone-binding site (QP) must recognize ubiquinone, stabilize the ubisemiquinone intermediate, and protonate the ubiquinone to ubiquinol, while minimizing the production of reactive oxygen species. We have investigated the role of the yeast Sdh4p Tyr-89, which forms a hydrogen bond with ubiquinone in the QP site. This tyrosine residue is conserved in all succinate:ubiquinone oxidoreductases studied to date. In the human SDH, mutation of this tyrosine to cysteine results in paraganglioma, tumors of the parasympathetic ganglia in the head and neck. We demonstrate that Tyr-89 is essential for ubiquinone reductase activity and that mutation of Tyr-89 to other residues does not increase the production of reactive oxygen species. Our results support a role for Tyr-89 in the protonation of ubiquinone and argue that the generation of reactive oxygen species is not causative of tumor formation.
Keywords: Mitochondria; Yeast; Succinate:ubiquinone oxidoreductase; Complex II
Replacement of the methionine axial ligand to the primary electron acceptor A0 slows the A0− reoxidation dynamics in Photosystem I by V.M. Ramesh; Krzysztof Gibasiewicz; Su Lin; Scott E. Bingham; Andrew N. Webber (pp. 151-160).
The recent crystal structure of photosystem I (PSI) from Thermosynechococcus elongatus shows two nearly symmetric branches of electron transfer cofactors including the primary electron donor, P700, and a sequence of electron acceptors, A, A0 and A1, bound to the PsaA and PsaB heterodimer. The central magnesium atoms of each of the putative primary electron acceptor chlorophylls, A0, are unusually coordinated by the sulfur atom of methionine 688 of PsaA and 668 of PsaB, respectively. We [Ramesh et al. (2004a) Biochemistry 43:1369–1375] have shown that the replacement of either methionine with histidine in the PSI of the unicellular green alga Chlamydomonas reinhardtii resulted in accumulation of A0− (in 300-ps time scale), suggesting that both the PsaA and PsaB branches are active. This is in contrast to cyanobacterial PSI where studies with methionine-to-leucine mutants show that electron transfer occurs predominantly along the PsaA branch. In this contribution we report that the change of methionine to either leucine or serine leads to a similar accumulation of A0− on both the PsaA and the PsaB branch of PSI from C. reinhardtii, as we reported earlier for histidine mutants. More importantly, we further demonstrate that for all the mutants under study, accumulation of A0− is transient, and that reoxidation of A0− occurs within 1–2 ns, two orders of magnitude slower than in wild type PSI, most likely via slow electron transfer to A1. This illustrates an indispensable role of methionine as an axial ligand to the primary acceptor A0 in optimizing the rate of charge stabilization in PSI. A simple energetic model for this reaction is proposed. Our findings support the model of equivalent electron transfer along both cofactor branches in Photosystem I.
Keywords: Abbreviations; A; accessory chlorophyll in Photosystem I; A; 0; primary electron acceptor in photosystem I (chlorophyll; a; monomer); A; 1; phylloquinone secondary electron acceptor in photosystem I; Chls; chlorophylls; DAS; decay associated spectra; ETC; electron transport chain; F; A; , F; B; and F; X; iron sulfur centers of photosystem I; ND; non-decaying; PMS; phenazine methosulphate; PSI; photosystem I; P; 700; primary electron donor of photosystem IA; 0; mutant; Femtosecond spectroscopy; Energy transfer; Electron transfer; Photosystem I; Chlamydomonas reinhardtii
Inhibition of respiration and nitrate assimilation enhances photohydrogen evolution under low oxygen concentrations in Synechocystis sp. PCC 6803 by Franziska Gutthann; Melanie Egert; Alexandra Marques; Jens Appel (pp. 161-169).
In cyanobacterial membranes photosynthetic light reaction and respiration are intertwined. It was shown that the single hydrogenase of Synechocystis sp. PCC 6803 is connected to the light reaction. We conducted measurements of hydrogenase activity, fermentative hydrogen evolution and photohydrogen production of deletion mutants of respiratory electron transport complexes. All single, double and triple mutants of the three terminal respiratory oxidases and the ndhB-mutant without a functional complex I were studied. After activating the hydrogenase by applying anaerobic conditions in the dark hydrogen production was measured at the onset of light. Under these conditions respiratory capacity and amount of photohydrogen produced were found to be inversely correlated. Especially the absence of the quinol oxidase induced an increased hydrogenase activity and an increased production of hydrogen in the light compared to wild type cells. Our results support that the hydrogenase as well as the quinol oxidase function as electron valves under low oxygen concentrations. When the activities of photosystem II and I (PSII and PSI) are not in equilibrium or in case that the light reaction is working at a higher pace than the dark reaction, the hydrogenase is necessary to prevent an acceptor side limitation of PSI, and the quinol oxidase to prevent an overreduction of the plastoquinone pool (acceptor side of PSII). Besides oxygen, nitrate assimilation was found to be an important electron sink. Inhibition of nitrate reductase resulted in an increased fermentative hydrogen production as well as higher amounts of photohydrogen.
Keywords: Hydrogenase; Cyanobacteria; Fermentation; Nitrate reductase; Cytochrome c oxidases; Quinol oxidase
Cell surface oxygen consumption: A major contributor to cellular oxygen consumption in glycolytic cancer cell lines by Patries M. Herst; Michael V. Berridge (pp. 170-177).
Oxygen consumption for bioenergetic purposes has long been thought to be the prerogative of mitochondria. Nevertheless, mitochondrial gene knockout (ρ0) cells that are defective in mitochondrial respiration require oxygen for growth and consume oxygen at the cell surface via trans-plasma membrane electron transport (tPMET). This raises the possibility that cell surface oxygen consumption may support glycolytic energy metabolism by reoxidising cytosolic NADH to facilitate continued glycolysis. In this paper we determined the extent of cell surface oxygen consumption in a panel of 19 cancer cell lines. Non-mitochondrial (myxothiazol-resistant) oxygen consumption was demonstrated to consist of at least two components, cell surface oxygen consumption (inhibited by extracellular NADH) and basal oxygen consumption (insensitive to both myxothiazol and NADH). The extent of cell surface oxygen consumption varied considerably between parental cell lines from 1% to 80% of total oxygen consumption rates. In addition, cell surface oxygen consumption was found to be associated with low levels of superoxide production and to contribute significantly (up to 25%) to extracellular acidification in HL60ρ0 cells. In summary, cell surface oxygen consumption contributes significantly to total cellular oxygen consumption, not only in ρ0 cells but also in mitochondrially competent tumour cell lines with glycolytic metabolism.
Keywords: Cell surface oxygen consumption; Aerobic glycolysis; ρ; 0; cellsAbbreviations; DPI; diphenyleneiodonium; FCCP; carbonyl cyanide para-trifluoromethoxyphenylhydrazone; NOX; family of inducible NADPH-oxidases of phagocytes; NQO1; NADH:ubiquinone oxidoreductase; PMA; phorbol myristate acetate; PMS; 1-methoxy-5-methyl phenazinium methylsulphate; SOD; superoxide dismutase; tPMET; trans-plasma membrane electron transport; WST-1; 2-(4-iodophenyl)-3-(4-nitrophenyl) -5- (2,4-disulfophenyl) -2H- tetrazolium monosodium salt
Functional properties of type I and type II cytochromes c3 from Desulfovibrio africanus by Catarina M. Paquete; Patrícia M. Pereira; Teresa Catarino; David. L. Turner; Ricardo O. Louro; António V. Xavier (pp. 178-188).
Type I cytochrome c3 is a key protein in the bioenergetic metabolism of Desulfovibrio spp., mediating electron transfer between periplasmic hydrogenase and multihaem cytochromes associated with membrane bound complexes, such as type II cytochrome c3. This work presents the NMR assignment of the haem substituents in type I cytochrome c3 isolated from Desulfovibrio africanus and the thermodynamic and kinetic characterisation of type I and type II cytochromes c3 belonging to the same organism. It is shown that the redox properties of the two proteins allow electrons to be transferred between them in the physiologically relevant direction with the release of energised protons close to the membrane where they can be used by the ATP synthase.
Keywords: Abbreviations; SRB; Sulphate reducing bacteria; TpI; c; 3; Type I cytochrome; c; 3; TpII; c; 3; Type II cytochrome; c; 3; D.; Desulfovibrio; Dsm.; Desulfomicrobium; NMR; Nuclear Magnetic ResonanceCytochrome c3; Desulfovibrio africanus; Electron transfer rates; Multicentre protein; Thermodynamic characterisation
The NT-26 cytochrome c552 and its role in arsenite oxidation by Joanne M. Santini; Ulrike Kappler; Seamus A. Ward; Michael J. Honeychurch; Rachel N. vanden Hoven; Paul V. Bernhardt (pp. 189-196).
Arsenite oxidation by the facultative chemolithoautotroph NT-26 involves a periplasmic arsenite oxidase. This enzyme is the first component of an electron transport chain which leads to reduction of oxygen to water and the generation of ATP. Involved in this pathway is a periplasmic c-type cytochrome that can act as an electron acceptor to the arsenite oxidase. We identified the gene that encodes this protein downstream of the arsenite oxidase genes ( aroBA). This protein, a cytochrome c552, is similar to a number of c-type cytochromes from the α- Proteobacteria and mitochondria. It was therefore not surprising that horse heart cytochrome c could also serve, in vitro, as an alternative electron acceptor for the arsenite oxidase. Purification and characterisation of the c552 revealed the presence of a single heme per protein and that the heme redox potential is similar to that of mitochondrial c-type cytochromes. Expression studies revealed that synthesis of the cytochrome c gene was not dependent on arsenite as was found to be the case for expression of aroBA.
Keywords: Arsenite oxidation; Metabolism; Cytochrome; Redox potential