BBA - Bioenergetics (v.1709, #3)
Editorial Board (ii).
Remarkable substituent effect: β-aminosquamocin, a potent dual inhibitor of mitochondrial complexes I and III by Romain A. Duval; Erwan Poupon; Ulrich Brandt; Reynald Hocquemiller (191-194).
The introduction of a primary amine function on the terminal α,β-unsaturated lactone of squamocin 1, a common structural hallmark of annonaceous acetogenins, shifted this specific inhibitor of mitochondrial complex I into a potent dual inhibitor of complexes I and III. The mechanism of action of β-aminosquamocin 2, against these two respiratory targets, is studied and discussed in view of current structure–activity relationship knowledge in the acetogenin series.
Keywords: Annonaceous acetogenins; Squamocin; Cytotoxicity; Dual inhibitor; Complex I; Complex III;
Old and new data, new issues: The mitochondrial ΔΨ by Henry Tedeschi (195-202).
New and old data pertinent to the electrochemical potentials across the inner mitochondrial membrane are reviewed with the intent of reconciling the various findings in the light of new perspectives provided by more recent knowledge. A careful scrutiny of old data permits ruling out the presence of a significant metabolically dependent electrical membrane potential. Recent technological advances make it possible to test the proposed alternatives. These proposals recast the original idea, and the possible mechanisms that are emerging also invoke a protonmotive force. Our conclusions that ΔΨ is not involved in oxidative-phosphorylation finds parallel observations in Halobacterium halobium [H. Michel, D. Oesterhelt, Electrochemical proton gradient across the cell membrane of Halobacterium halobium: comparison of the light-induced increase with the increase of intracellular adenosine triphosphate under steady-state illumination, Biochemistry 19 (1980) 4615–4619] and thylakoid vesicles [D.R. Ort, R.A. Dilley, N.E. Good, Photophosphorylation as a function of illumination time II. Effects of permeant buffers, Biochim. Biophys. Acta 449 (1976) 108–129] in which light-induced ATP synthesis occurs in the absence of an apparent ΔΨ or ΔpH, suggesting the presence of mechanisms similar to the one proposed for mitochondria.
Keywords: Mitochondria; Membrane potential; Bioenergetic; Oxidative-phosphorylation;
Probing the FQR and NDH activities involved in cyclic electron transport around Photosystem I by the ‘afterglow’ luminescence by Michel Havaux; Dominique Rumeau; Jean-Marc Ducruet (203-213).
Far-red illumination of plant leaves for a few seconds induces a delayed luminescence rise, or afterglow, that can be measured with the thermoluminescence technique as a sharp band peaking at around 40–45 °C. The afterglow band is attributable to a heat-induced electron flow from the stroma to the plastoquinone pool and the PSII centers. Using various Arabidopsis and tobacco mutants, we show here that the electron fluxes reflected by the afterglow luminescence follow the pathways of cyclic electron transport around PSI. In tobacco, the afterglow signal relied mainly on the ferredoxin-quinone oxidoreductase (FQR) activity while the predominant pathway responsible for the afterglow in Arabidopsis involved the NAD(P)H dehydrogenase (NDH) complex. The peak temperature T m of the afterglow band varied markedly with the light conditions prevailing before the TL measurements, from around 30 °C to 45 °C in Arabidopsis. These photoinduced changes in T m followed the same kinetics and responded to the same light stimuli as the state 1–state 2 transitions. PSII-exciting light (leading to state 2) induced a downward shift while preillumination with far-red light (inducing state 1) caused an upward shift. However, the light-induced downshift was strongly inhibited in NDH-deficient Arabidopsis mutants and the upward shift was cancelled in plants durably acclimated to high light, which can perform normal state transitions. Taken together, our results suggest that the peak temperature of the afterglow band is indicative of regulatory processes affecting electron donation to the PQ pool which could involve phosphorylation of NDH. The afterglow thermoluminescence band provides a new and simple tool to investigate the cyclic electron transfer pathways and to study their regulation in vivo.
Keywords: Arabidopsis; Cyclic electron transport; Ferredoxin-plastoquinone oxidoreductase; NAD(P)H dehydrogenase, Photosystem I; Thermoluminescence;
The topology of superoxide production by complex III and glycerol 3-phosphate dehydrogenase in Drosophila mitochondria by Satomi Miwa; Martin D. Brand (214-219).
The topology of superoxide generation by sn-glycerol 3-phosphate dehydrogenase and complex III in intact Drosophila mitochondria was studied using aconitase inactivation to measure superoxide production in the matrix, and hydrogen peroxide formation in the presence of superoxide dismutase to measure superoxide production from both sides of the membrane. Aconitase inactivation was calibrated using the known rate of matrix superoxide production from complex I. Glycerol phosphate dehydrogenase generated superoxide about equally to each side of the membrane, whereas centre o of complex III in the presence of antimycin A generated superoxide about 30% on the cytosolic side and 70% on the matrix side.
Keywords: Drosophila; Mitochondria; Superoxide; Glycerol phosphate dehydrogenase; Complex III;
Characterization of the core complex of Rubrivivax gelatinosus in a mutant devoid of the LH2 antenna by Jean-Luc Ranck; Frédéric Halgand; Olivier Laprévote; Françoise Reiss-Husson (220-230).
The core complex of purple bacteria is a supramolecular assembly consisting of an array of light-harvesting LH1 antenna organized around the reaction center. It has been isolated and characterized in this work using a Rubrivivax gelatinosus mutant lacking the peripheral LH2 antenna. The purification did not modify the organization of the complex as shown by comparison with the intact membranes of the mutant. The protein components consisted exclusively of the reaction center, the associated tetraheme cyt c and the LH1 αβ subunits; no other protein which could play the role of pufX could be detected. The complex migrated as a single band in a sucrose gradient, and as a monomer in a native Blue gel electrophoresis. Comparison of its absorbance spectrum with those of the isolated RC and of the LH1 antenna as well as measurements of the bacteriochlorophyll/tetraheme cyt c ratio indicated that the mean number of LH1 subunits per RC-cyt c is near 16. The polypeptides of the LH1 antenna were shown to present several modifications. The α one was formylated at its N-terminal residue and the N-terminal methionine of β was cleaved, as already observed for other Rubrivivax gelatinosus strains. Both modifications occurred possibly by post-translational processing. Furthermore the α polypeptides were heterogeneous, some of them having lost the 15 last residues of their C-terminus. This truncation of the hydrophobic C-terminal extension is similar to that observed previously for the α polypeptide of the Rubrivivax gelatinosus LH2 antenna and is probably due to proteolysis or to instability of this extension.
Keywords: Light-harvesting antenna; Reaction center-LH1 core complex; Purple bacteria; Rubrivivax gelatinosus;
Structural and functional changes in heart mitochondria from sucrose-fed hypertriglyceridemic rats by Karla Carvajal; Mohammed El Hafidi; Alvaro Marin-Hernández; Rafael Moreno-Sánchez (231-239).
In the heart of sugar-induced hypertriglyceridemic (HTG) rats, cardiac performance is impaired with glucose as fuel, but not with fatty acids. Accordingly, the glycolytic flux and the transfer of energy diminish in the HTG heart, in comparison to control heart. To further explore the biochemical nature of such alteration in the HTG heart, the components of the non-glycolytic energy systems involved were evaluated. Total creatine kinase (CK) activity in the myocardial tissue was depressed by 30% in the HTG heart whereas the activity of the mitochondrial CK (mitCK) isoenzyme fraction that is functionally associated with oxidative phosphorylation decreased in isolated HTG heart mitochondria by 45%. Adenylate kinase (AK) was 20% lower in the HTG heart. In contrast, respiratory rates with 2-oxoglutarate (2-OG) and pyruvate/malate (pyr) were significantly higher in HTG heart mitochondria than in control mitochondria. 2-OG dehydrogenase activity was also higher in HTG mitochondria. Respiration with succinate was similar in both groups. Content of cytochromes b, c + c 1 and a + a 3, and cytochrome c oxidase activity, were also similar in the two kinds of mitochondria. A larger content of saturated and monounsaturated fatty acids was found in the HTG mitochondrial membranes with no changes in phospholipids composition or cholesterol content. Mitochondrial membranes from HTG hearts were more rigid, which correlated with the generation of higher membrane potentials. As the mitochondrial function was preserved or even enhanced in the HTG heart, these results indicated that deficiency in energy transfer was associated with impairment in mitCK and AK. This situation brought about uncoupling between the site of ATP production and the site of ATP consumption (contractile machinery), in spite of compensatory increase in mitochondrial oxidative capacity and membrane potential generation.
Keywords: Creatine kinase; Energy metabolism; Heart mitochondria; Hypertriglyceridemia; Oxidative phosphorylation;
Kinetics of charge translocation in the passive downhill uptake mode of the Na+/H+ antiporter NhaA of Escherichia coli by D. Zuber; R. Krause; M. Venturi; E. Padan; E. Bamberg; K. Fendler (240-250).
The Na+/H+ antiporter NhaA is the main Na+ extrusion system in E. coli. Using direct current measurements combined with a solid supported membrane (SSM), we obtained electrical data of the function of NhaA purified and reconstituted in liposomes. These measurements demonstrate NhaA's electrogenicity, its specificity for Li+ and Na+ and its pronounced pH dependence in the range pH 6.5–8.5. The mutant G338S, in contrast, presents a pH independent profile, as reported previously. A complete right-side-out orientation of the NhaA antiporter within the proteoliposomal membrane was determined using a NhaA-specific antibody based ELISA assay. This allowed for the first time the investigation of NhaA in the passive downhill uptake mode corresponding to the transport of Na+ from the periplasmic to the cytoplasmic side of the membrane. In this mode, the transporter has kinetic properties differing significantly from those of the previously investigated efflux mode. The apparent K m values were 11 mM for Na+ and 7.3 mM for Li+ at basic pH and 180 mM for Na+ and 50 mM for Li+ at neutral pH. The data demonstrate that in the passive downhill uptake mode pH regulation of the carrier affects both apparent K m as well as turnover (V max).
Keywords: NhaA; Sodium proton antiporter; Uptake mode; Proteoliposome; Solid supported membrane; Electrical;
Photosynthesis and negative entropy production by Robert C. Jennings; Enrico Engelmann; Flavio Garlaschi; Anna Paola Casazza; Giuseppe Zucchelli (251-255).
The widely held view that the maximum efficiency of a photosynthetic pigment system is given by the Carnot cycle expression (1 − T/T r) for energy transfer from a hot bath (radiation at temperature T r) to a cold bath (pigment system at temperature T) is critically examined and demonstrated to be inaccurate when the entropy changes associated with the microscopic process of photon absorption and photochemistry at the level of single photosystems are considered. This is because entropy losses due to excited state generation and relaxation are extremely small (ΔS ≪ T/T r) and are essentially associated with the absorption-fluorescence Stokes shift. Total entropy changes associated with primary photochemistry for single photosystems are shown to depend critically on the thermodynamic efficiency of the process. This principle is applied to the case of primary photochemistry of the isolated core of higher plant photosystem I and photosystem II, which are demonstrated to have maximal thermodynamic efficiencies of ξ > 0.98 and ξ > 0.92 respectively, and which, in principle, function with negative entropy production. It is demonstrated that for the case of ξ > (1 − T/T r) entropy production is always negative and only becomes positive when ξ < (1 − T/T r).
Keywords: Carnot cycle; Entropy; Fluorescence lifetime; Photosynthesis; Photosystem I core; Photosystem II core;
Bioenergetics Cumulative Contents (256-257).