BBA - Bioenergetics (v.1777, #11)

Function of plastoquinone in heat stress reactions of plants by Natallia L. Pshybytko; Jerzy Kruk; Liudmila F. Kabashnikova; Kazimierz Strzalka (1393-1399).
The effect of high temperature treatment (40 °C, 3 h, illumination at 100 μmol m− 2 s− 1) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of QA reoxidation by Tyrz ox in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following QA reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with QA reoxidized through recombination with the S2/S3 state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from QA to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions.
Keywords: Heat shock; Photosynthetic electron flow; Plastoquinone pool; Redox state; Reoxidation of QA ; Photosystem II;

Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium, Acaryochloris marina by Matthias Schenderlein; Marianne Çetin; James Barber; Alison Telfer; Eberhard Schlodder (1400-1408).
Absorbance difference spectroscopy and redox titrations have been applied to investigate the properties of photosystem I from the chlorophyll d containing cyanobacterium Acaryochloris marina. At room temperature, the (P740+  − P740) and (FA/B  − FA/B) absorbance difference spectra were recorded in the range between 300 and 1000 nm while at cryogenic temperatures, (P740+A1  − P740A1) and (3P740 − P740) absorbance difference spectra have been measured. Spectroscopic and kinetic evidence is presented that the cofactors involved in the electron transfer from the reduced secondary electron acceptor, phylloquinone (A1 ), to the terminal electron acceptor and their structural arrangement are virtually identical to those of chlorophyll a containing photosystem I. The oxidation potential of the primary electron donor P740 of photosystem I has been reinvestigated. We find a midpoint potential of 450 ± 10 mV in photosystem I-enriched membrane fractions as well as in thylakoids which is very similar to that found for P700 in chlorophyll a dominated organisms. In addition, the extinction difference coefficient for the oxidation of the primary donor has been determined and a value of 45,000 ± 4000 M− 1 cm− 1 at 740 nm was obtained. Based on this value the ratio of P740 to chlorophyll is calculated to be 1:~ 200 chlorophyll d in thylakoid membranes. The consequences of our findings for the energetics in photosystem I of A. marina are discussed as well as the pigment stoichiometry and spectral characteristics of P740.
Keywords: Photosystem I; Chlorophyll d; P740; Charge separation; Acaryochloris marina;

Oligomeric BAX induces mitochondrial permeability transition and complete cytochrome c release without oxidative stress by Tsyregma Li; Tatiana Brustovetsky; Bruno Antonsson; Nickolay Brustovetsky (1409-1421).
In the present study, we investigated the mechanism of cytochrome c release from isolated brain mitochondria induced by recombinant oligomeric BAX (BAXoligo). We found that BAXoligo caused a complete release of cytochrome c in a concentration- and time-dependent manner. The release was similar to those induced by alamethicin, which causes maximal mitochondrial swelling and eliminates barrier properties of the OMM. BAXoligo also produced large amplitude mitochondrial swelling as judged by light scattering assay and transmission electron microscopy. In addition, BAXoligo resulted in a strong mitochondrial depolarization. ATP or a combination of cyclosporin A and ADP, inhibitors of the mPT, suppressed BAXoligo-induced mitochondrial swelling and depolarization as well as cytochrome c release but did not influence BAXoligo insertion into the OMM. Both BAXoligo- and alamethicin-induced cytochrome c releases were accompanied by inhibition of ROS generation, which was assessed by measuring mitochondrial H2O2 release with an Amplex Red assay. The mPT inhibitors antagonized suppression of ROS generation caused by BAXoligo but not by alamethicin. Thus, BAXoligo resulted in a complete cytochrome c release from isolated brain mitochondria in the mPT-dependent manner without involvement of oxidative stress by the mechanism requiring mitochondrial remodeling and permeabilization of the OMM.
Keywords: Brain; Mitochondria; Apoptosis; ROS; Calcium; BAX;

The enzyme F1-ATPase is a rotary nanomotor in which the central γ subunit rotates inside the cavity made of α3β3 subunits. The experiments showed that the rotation proceeds in steps of 120° and each 120° step consists of 80° and 40° substeps. Here the Author proposes a stochastic wave mechanics of the F1-ATPase motor and combines it with the structure-based kinetics of the F1-ATPase to form a chemomechanic coupled model. The model can reproduce quantitatively and explain the experimental observations about the F1 motor. Using the model, several rate-limited situations about γ subunit rotation are proposed, the effects of the friction and the load on the substeps are investigated and the chemomechanic coupled time during ATP hydrolysis cycle is determined.
Keywords: Coupled chemomechanics; F1-ATPase; Molecular motor;

Core protein phosphorylation facilitates the repair of photodamaged photosystem II at high light by Mikko Tikkanen; Markus Nurmi; Saijaliisa Kangasjärvi; Eva-Mari Aro (1432-1437).
Phosphorylation of photosystem II (PSII) reaction center protein D1 has been hypothesised to function as a signal for the migration of photodamaged PSII core complex from grana membranes to stroma lamellae for concerted degradation and replacement of the photodamaged D1 protein. Here, by using the mutants with impaired capacity (stn8) or complete lack (stn7 stn8) in phosphorylation of PSII core proteins, the role of phosphorylation in PSII photodamage and repair was investigated. We show that the lack of PSII core protein phosphorylation disturbs the disassembly of PSII supercomplexes at high light, which is a prerequisite for efficient migration of damaged PSII complexes from grana to stroma lamellae for repair. This results in accumulation of photodamaged PSII complexes, which in turn results, upon prolonged exposure to high light (HL), in general oxidative damage of photosynthetic proteins in the thylakoid membrane.
Keywords: Photosystem II; Photoinhibition; Protein phosphorylation; stn mutant; D1 protein; Arabidopsis thaliana;

A maxi-chloride channel in the inner membrane of mammalian mitochondria by Umberto De Marchi; Ildikò Szabò; Grazia M. Cereghetti; Pranvera Hoxha; William J. Craigen; Mario Zoratti (1438-1448).
Patch-clamp experiments on swollen mitochondria of human, mouse and rat origins have revealed activity by an approximately 400 pS (in 150 mM KCl), voltage-dependent and anion-selective channel. This channel is located in the inner membrane, as shown by experiments with mitochondria from cells expressing a fluorescent mitochondrial tag protein and by the co-presence of the 107 pS channel and of the permeability transition pore (PTP). The frequency of appearance was inversely related to the presence of the PTP. This and the comparison of its electrophysiological characteristics with those of the PTP indicate that it is closely related to the latter, possibly corresponding to a monomeric unit whose dimer constitutes the full PTP. The channel is similar but not identical to isolated-and-reconstituted mitochondrial porin, and it is present also in mitochondria from cells lacking porin isoforms. Its identification with porin is therefore to be excluded. It most likely coincides instead with the “maxi-chloride channel” characterized in the plasma membrane of various cell types.
Keywords: Maxi chloride channel; Mitochondria; Permeability transition pore; Porin; Patch-clamp; Translocator protein;

It is reported that O2 is required for the activation of photosynthesis in dark adapted Chlamydomonas reinhardtii in State 1, under low light intensity. The concentration of dissolved O2 of ca. 9 µM is sufficient to saturate the requirement. When the concentration of O2 is 3 μM or below, the activation of photosynthesis is strongly inhibited by myxothiazol, a specific inhibitor of the mitochondrial cytochrome bc 1. The effect of this inhibitor decreases as the O2 concentration is raised, to disappear completely above 50 μM. Low concentrations of uncouplers delay the activation of photosynthesis, but do not inhibit it when steady state is reached. It is concluded that in State 1 C. reinhardtii mitochondrial respiration is required for the activation of photosynthesis upon illumination of dark adapted cells only when the concentration of O2 is too low (less than 5 μM) to allow an appreciable activity of the Mehler reaction. The role of respiration does not seem to be due to the synthesis of ATP by oxidative phosphorylation, because photosynthesis activation is not sensitive to oligomycin.
Keywords: Mtochondrial respiration; Photosynthesis activation; Dark adapted C. reinhardtii; Myxothiazol; Oligomycin; Uncoupler;

The Toxoplasma gondii type-II NADH dehydrogenase TgNDH2-I is inhibited by 1-hydroxy-2-alkyl-4(1H)quinolones by San San Lin; Stefan Kerscher; Ahmad Saleh; Ulrich Brandt; Uwe Groß; Wolfgang Bohne (1455-1462).
The apicomplexan parasite Toxoplasma gondii does not possess complex I of the mitochondrial respiratory chain, but has two genes encoding rotenone-insensitive, non-proton pumping type-II NADH dehydrogenases (NDH2s). The absence of such “alternative” NADH dehydrogenases in the human host defines these enzymes as potential drug targets. TgNDH2-I and TgNDH2-II are constitutively expressed in tachyzoites and bradyzoites and are localized to the mitochondrion as shown by epitope tagging. Functional expression of TgNDH2-I in the yeast Yarrowia lipolytica as an internal enzyme, with the active site facing the mitochondrial matrix, permitted growth in the presence of the complex I inhibitor DQA. Bisubstrate kinetics of TgNDH2-I measured within Y. lipolytica mitochondrial membrane preparations were in accordance with a ping-pong mechanism. Using inhibition kinetics we demonstrate here that 1-hydroxy-2-alkyl-4(1)quinolones with long alkyl chains of C12 (HDQ) and C14 are high affinity inhibitors for TgNDH2-I, while compounds with shorter side chains (C5 and C6) displayed significantly higher IC50 values. The efficiency of the various quinolone derivatives to inhibit TgNDH2-I enzyme activity mirrors their inhibitory potency in vivo, suggesting that a long acyl site chain is critical for the inhibitory potential of these compounds.
Keywords: Alternative (type-II) NADH dehydrogenase (NDH2); Toxoplasma gondii; Inhibition kinetics; Ping-pong mechanism; 1-Hydroxy-2-alkyl-4(1)quinolone; HDQ;

The negatively charged amino acids in the lumenal loop influence the pigment binding and conformation of the major light-harvesting chlorophyll a/b complex of photosystem II by Chunhong Yang; Petar Lambrev; Zhi Chen; Tamás Jávorfi; Anett Z. Kiss; Harald Paulsen; Győző Garab (1463-1470).
The major chlorophyll (Chl) a/b complexes of photosystem II (LHCIIb), in addition to their primary light-harvesting function, play key roles in the organization of the granal ultrastructure of the thylakoid membranes and in various regulatory processes. These functions depend on the structural stability and flexibility of the complexes. The lumenal side of LHCIIb is exposed to broadly variable pH environments, due to the build-up and decay of the pH gradient during photosynthesis. Therefore, the negatively charged amino acids in the lumenal loop might be of paramount importance for adjusting the structure and functions of LHCIIb. In order to clarify the structural roles of these residues, we investigated the pigment stoichiometries, absorption, linear and circular dichroism spectra of the reconstituted LHCIIb complexes, in which the negatively charged amino acids in the lumenal loop were exchanged to neutral ones (E94G, E107V and D111V). The mutations influenced the pigment binding and the molecular architecture of the complexes. Exchanging E94 to G destabilized the 310 helix in the lumenal loop structure and led to an acquired pH sensitivity of the LHCIIb structure. We conclude that these amino acids are important not only for pigment binding in the complexes, but also in stabilizing the conformation of LHCIIb at different pHs.
Keywords: Major light-harvesting a/b complex of photosystem II; Spectroscopy; Mutagenesis; Low pH;