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BBA - Bioenergetics (v.1710, #1)
Multiple Rieske proteins in prokaryotes: Where and why? by Dirk Schneider; Christian L. Schmidt (pp. 1-12).
Many microbial genomes have been sequenced in the recent years. Multiple genes encoding Rieske iron-sulfur proteins, which are subunits of cytochrome bc-type complexes or oxygenases, have been detected in many pro- and eukaryotic genomes. The diversity of substrates, co-substrates and reactions offers obvious explanations for the diversity of the low potential Rieske proteins associated with oxygenases, but the physiological significance of the multiple genes encoding high potential Rieske proteins associated with the cytochrome bc-type complexes remains elusive. For some organisms, investigations into the function of the later group of genes have been initiated. Here, we summarize recent finding on the characteristics and physiological functions of multiple high potential Rieske proteins in prokaryotes. We suggest that the existence of multiple high potential Rieske proteins in prokaryotes could be one way of allowing an organism to adapt their electron transfer chains to changing environmental conditions.
Keywords: Cyanobacteria; Sulfolobus; Gene family; Cytochrome bc; 1; Cytochrome b; 6; f
Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll–protein complexes in bean thylakoid membranes by Maciej Garstka; Anna Drożak; Małgorzata Rosiak; Jan Henk Venema; Borys Kierdaszuk; Ewa Simeonova; Philip R. van Hasselt; Jerzy Dobrucki; Agnieszka Mostowska (pp. 13-23).
Changes in chloroplast structure and rearrangement of chlorophyll–protein (CP) complexes were investigated in detached leaves of bean ( Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 °C and subsequent 3-h photoactivation under white light (200 μmol photons m−2 s−1) at 22 °C. Although, no change in chlorophyll (Chl) content and Chl a/ b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI–PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry ( Fv/ Fm) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.
Keywords: Abbreviations; CS; Chilling-sensitive; Chl; chlorophyll; CLSM; confocal laser scanning microscopy; CP; chlorophyll–protein; Em; emission; Ex; excitation; FFA; free fatty acids; PAM; modulated fluorometer; PAR; photosynthetic active radiationBean; Chloroplast and thylakoid structure; Chlorophyll; a; fluorescence; Confocal laser scanning microscopy; Dark-chilling stress; LHCI–PSI; LHCII-PSII
Simulated solar irradiation with enhanced UV-B adjust plastid- and thylakoid-associated polyamine changes for UV-B protection by C. Lütz; E. Navakoudis; H.K. Seidlitz; K. Kotzabasis (pp. 24-33).
Polyamines have been described to protect against numerous oxidative stresses in plants. Increasing UV-B radiation (280–315 nm) in the biosphere may also induce an increase in radical formation in tissues. This study employed the tobacco cultivars Bel B and Bel W3 to describe possible protective functions of polyamines against UV-B radiation in sun light simulators (GSF/Munich) with natural diurnal fluctuations of simulated UV-B. Polyamine measurements on a whole leaf basis in isolated chloroplasts and thylakoids were paralleled to photosynthetic and respiration rates, photosynthetic efficiency, leaf thickness and photosynthetic pigment compositions. The study revealed that an increase of polyamines, and especially of putrescine level in thylakoid membranes upon elevated UV-B exposure comprises one of the primary protective mechanisms in the photosynthetic apparatus of the tobacco variety Bel B against UV-B radiation. The tobacco cultivar Bel W3, sensitive to ozone, was also proved to be sensitive to UV-B. This sensitivity is attributed to its incapability to enhance putrescine level in thylakoid membranes. After prolongation of UV-B exposure, when endogenous plant balances are being gradually restored, due to secondary responses, (e.g., biosynthesis of carotenoids and of additional flavonoids) and the plant is adapting to the altered environmental conditions, then the polyamine level is being reduced. Thus, we can discriminate the UV-B induced stress period from a UV-B acclimation period.
Keywords: Abbreviations; PUT; putrescine; SPD; spermidine; SPM; spermine; PAR; photosynthetically active radiation; LHC; light harvesting complex; PSII; photosystem IIPutrescine; UV-B radiation; Spermidine; Spermine; Sun simulator chamber
Replacement or exclusion of the B-branch bacteriopheophytin in the purple bacterial reaction centre: The HB cofactor is not required for assembly or core function of the Rhodobacter sphaeroides complex by Ashley J. Watson; Paul K. Fyfe; Dmitrij Frolov; Marion C. Wakeham; Eliane Nabedryk; Rienk van Grondelle; Jacques Breton; Michael R. Jones (pp. 34-46).
All of the membrane-embedded cofactors of the purple bacterial reaction centre have well-defined functional or structural roles, with the exception of the bacteriopheophytin (HB) located approximately half-way across the membrane on the so-called inactive- or B-branch of cofactors. Sequence alignments indicate that this bacteriochlorin cofactor is a conserved feature of purple bacterial reaction centres, and a pheophytin is also found at this position in the Photosystem-II reaction centre. Possible structural or functional consequences of replacing the HB bacteriopheophytin by bacteriochlorophyll were investigated in the Rhodobacter sphaeroides reaction centre through mutagenesis of residue Leu L185 to His (LL185H). Results from absorbance spectroscopy indicated that the LL185H mutant assembled with a bacteriochlorophyll at the HB position, but this did not affect the capacity of the reaction centre to support photosynthetic growth, or change the kinetics of charge separation along the A-branch of cofactors. It was also found that mutation of residue Ala M149 to Trp (AM149W) caused the reaction centre to assemble without an HB bacteriochlorin, demonstrating that this cofactor is not required for correct assembly of the reaction centre. The absence of a cofactor at this position did not affect the capacity of the reaction centre to support photosynthetic growth, or the kinetics of A-branch electron transfer. A combination of X-ray crystallography and FTIR difference spectroscopy confirmed that the HB cofactor was absent in the AM149W mutant, and that this had not produced any significant disturbance of the adjacent ubiquinol reductase (QB) site. The data are discussed with respect to possible functional roles of the HB bacteriopheophytin, and we conclude that the reason(s) for conservation of a bacteriopheophytin cofactor at this position in purple bacterial reaction centres are likely to be different from those underlying conservation of a pheophytin at the analogous position in Photosystem-II.
Keywords: Abbreviations; B; A; first electron acceptor; B*; accessory bacteriochlorophyll singlet excited state; B; A; , B; B; monomeric bacteriochlorophyll cofactors; BChl; bacteriochlorophyll; Bcl; .; Blastochloris; BPhe; bacteriopheophytin; FTIR; Fourier transform infrared; H; A; , H; B; bacteriopheophytin cofactors; LDAO; lauryl dimethylamine oxide; P; primary donor of electrons; P; A; , P; B; primary donor bacteriochlorophylls; P*; primary donor singlet excited state; Q; A; Q; B; , ubiquinone cofactors; Q; 3; ubiquinone-3; Rb; .; Rhodobacter; SADS; Species Associated Difference Spectra; Tch; .; ThermochromatiumReaction centre; Photosynthesis; Bacteriochlorophyll; Bacteriopheophytin; Electron transfer
Analysis of the kinetics of the membrane potential generated by cytochrome c oxidase upon single electron injection by D.M. Medvedev; E.S. Medvedev; A.I. Kotelnikov; A.A. Stuchebrukhov (pp. 47-56).
In a recent work from this group (Popovic, D. M.; Stuchebrukhov A. A. FEBS Lett. 2004, 566, 126), a model of proton pumping by cytochrome c oxidase (CcO) was proposed. The key element of the model is His291 (bovine notation), a histidine ligand to enzyme's CuB redox center, which plays the role of the pump element. The model assumes that upon electron transfer between heme a and the binuclear catalytic center of the enzyme, two sequential proton transfers occur: First, a proton from Glu242 is transferred to an unprotonated His291, then a second proton, after reprotonation of Glu242 from the negative side of the membrane, is transferred to a hydroxyl group in the binuclear center, a water molecule is formed, and the first proton, due to proton–proton repulsion, is expelled from His291 to the positive side of the membrane, resulting in a pumping event. In the process the free energy of water formation (i.e., reduction of oxygen) is transformed into a proton gradient across the membrane. The model possesses specific kinetic features. It assumes, for example, that upon electron transfer the first proton is transferred to the proton-loading site of the pump, His291, and not to the catalytic center of the enzyme. Here, we analyze the kinetic properties of the proposed model, and calculate the time dependence of the membrane potential generated by CcO upon a single electron injection into the enzyme. These data are directly compared with recent experimental measurements of the membrane potential generated by CcO. Specifically, F to O, and O to E transitions will be discussed. Several enzymes from different organisms (bovine, two bacterial enzymes, and several mutants) are compared and discussed in detail. The kinetic description, however, is phenomenological, and does not include explicitly the nature of the groups involved in proton translocation, except in terms of their position depth within the membrane; thus, the kinetic equations developed here are in fact describe a generic model, similar, e.g., to that proposed earlier by Peter Rich (P.R. Rich, Towards an understanding of the chemistry of oxygen reduction and proton translocation in the iron-copper respiratory oxidases. Aust. J. Plant Physiol. 22 (1995) 479–486), and which is based on the idea of displacement of the pumped protons by the chemical ones.
Keywords: Kinetic; Membrane potential; Cytochrome; c
Localization of coenzyme Q10 in the center of a deuterated lipid membrane by neutron diffraction by Thomas Hauß; Silvia Dante; Thomas H. Haines; Norbert A. Dencher (pp. 57-62).
Quinones (e.g., coenzyme Q, CoQ10) are best known as carriers of electrons and protons during oxidative phosphorylation and photosynthesis. A myriad of mostly more indirect physical methods, including fluorescence spectroscopy, electron-spin resonance, and nuclear magnetic resonance, has been used to localize CoQ10 within lipid membranes. They have yielded equivocal and sometimes contradictory results. Seeking unambiguous evidence for the localization of ubiquinone within lipid bilayers, we have employed neutron diffraction. CoQ10 was incorporated into stacked bilayers of perdeuterated dimyristoyl phosphatidyl choline doped with dimyristoyl phosphatidyl serine containing perdeuterated chains in the natural fluid-crystalline state. Our data show CoQ10 at the center of the hydrophobic core parallel to the membrane plane and not, as might be expected, parallel to the lipid chains. This localization is of importance for its function as a redox shuttle between the respiratory complexes and, taken together with our recent result that squalane is in the bilayer center, may be interpreted to show that all natural polyisoprene chains lie in the bilayer center. Thus ubiquinone, in addition to its free radical scavenging and its well-known role in oxidative phosphorylation as a carrier of electrons and protons, might also act as an inhibitor of transmembrane proton leaks.
Keywords: Coenzyme Q; 10; Ubiquinone; Neutron diffraction; Lipid membrane; Electron transfer; Proton permeability