Semiquinone Q in the respiratory chain of electron transport particles: electron spin resonance studies.
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Animals↗
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[ESR study of electron transport in photosynthetic systems. VII. Effects of temperature on the processes of electron transport between two photosystems and the structural state of the chloroplast membrane].
Redox transients of P700 in bean chloroplasts induced by far-red continuous light and brief flashes (t1/2 = 7 musec) of white light were studied under various temperatures. It has been shown that light-induced electron transport between two photosystems occurs at temperatures from -5 degrees to 45 degrees. At temperature interval 5 divided by 30 degrees photosystem 2 donates electrons to P700+ in response to the flash with the maximum efficiency. The number of electrons donated from photosystem 2 does not depend on the temperature in the range 5 divided by 30 degrees. The rate of electron transport between photosystems increases when the temperature is increased from -5 degrees to 20 degrees and does not depend on the temperature above 20 degrees. Structural states of the lipid region of the thylakoid membrane were studied by the spin label method. It was shown that the flexibility and the rate of molecular motion of different spin-labeled fatty acids were increased after temperature variations from -10 degrees to 50 degrees. A correlation between the structural state of the lipid region of thylakoid membrane and the rate of electron transport was found.
Cell Membrane↗
Ionic effects on adrenal steroidogenic electron transport. The role of adrenodoxin as an electron shuttle.
We have shown (Seybert, D., Lambeth, D., and Kamin, H. (1978), J. Biol. Chem. 253, 8355-8358) that, whereas the 1:1 complex between adrenodoxin reductase and adrenodoxin is the active species for cytochrome c reduction, the complex is not sufficient to allow cytochrome P-45011 beta-mediated hydroxylations;adrenodoxin in excess of reductase is required. In the present studies, reduction by NADPH of excess adrenodoxin is shown to occur at a rate sufficient to support both cytochrome P-450 11 beta-mediated hydroxylation of deoxycorticosterone, and cytochrome P-450sec-mediated side chain cleavage of cholesterol. Oxidation-reduction potential and ion effect studies indicate that the mechanism of steroidogenic electron transport involves an adrenodoxin electron "shuttle" rather than a macromolecular complex of reductase, adrenodoxin, and cytochrome. The oxidation-reduction potential of adrenodoxin is shifted about -100 mV when bound to reductase, and reduction of the iron-sulfur protein thus promotes dissociation of the complex. The rate of adrenodoxin reduction is first stimulated, then inhibited by increasing salt; the effect is ion-specific, with Ca2+ approximately Mg2+ greater than Na+ greater than NH/+. Similar ion-specific rate effects are observed for both of the cytochrome P-450-mediated hydroxylations, indicating that the same reduction mechanism is required for these reactions. Increasing salt concentrations caused dissociation of the complex; dissociation of the form of the complex containing reduced adrenodoxin occurred at lower salt concentrations than that containing oxidized adrenodoxin. The order of effectiveness of ions in causing dissociation is the same as the order for stimulation of adrenodoxin reduction, suggesting a dissociation step in the mechanism. This proposed model, together with dissociation constants for the form of the complex containing either oxidized or reduced adrenodoxin, allows accurate prediction of the salt rate effects curve. For all ions, an activity maximum is seen at the ion concentration which produces the largest molar difference between associated-oxidized and dissociated-reduced states, and the model predicts the positions of the maxima for adrenodoxin reduction, 11 beta-hydroxylation, and side chain cleavage. Thus reduction-induced dissociation of adrenodoxin from adrenodoxin reductase appears to be a required step in steroidogenic electron transport by this system, and a role for adrenodoxin as a mobile electron shuttle is proposed.
Adrenal Cortex↗
Characterization of microsomal electron transport components from control, phenobarbital, and 3-methylcholanthrene treated mice: I. Distribution of electron transport components in ammonium-sulfate fractions from mouse liver microsomes.
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Ammonium Sulfate↗
[Kinetics of reactions in polyenzyme systems. III. Electron transport processes].
Kinetic relationships of electron transport processes in biological systems have been analysed. Formal-kinetic description and discrimination of the two main models of electron transport processes in systems of homogeneously distributed carriers and in systems of structurally bound carriers is presented including steady-state and non-steady-state reaction kinetics. Methods of determination of limit-stage rate constant and rate constants for all elementary steps of the reaction are given. Dependencies of the degree of electron carriers reduction on donor and acceptor concentrations and the influence of reduction or oxidation of carriers on the kinetic properties of electron transport chain (electron influence factor) in the system of structurally-bound carriers have been analyzed. On the basis of kinetic models a molecular-kinetic interpretation of activation effect of mitochondrial respiratory chain during the process of electron transport is offered.
Cytochromes↗
Characterization of the electron transport system in Brucella abortus.
The electron transport system in Brucella abortus has been characterized. Spectral studies of membrane preparations have indicated the presence of cytochromes a + a3 (maxima at 612 nm), cytochrome b (maxima at 560, 530, and 428 nm), cytochrome c (maxima at 552 and 522 nm), cytochrome o (maxima of carbon monoxide complex at 418 nm), and flavoproteins (minimum at 582 and 450 nm). Cytochromes a + a3 appeared only after cells had reached late log phase, possibly due to lowered oxygen tension in the medium. Dehydrogenases were shown to be present for D-erythritol 1-phosphate, L-lactate, reduced nicotinamide adenine dinucleotide, and succinate. All of the above substrates reduced the electron transport chain and at least some of the flavoproteins, indicating similar pathways of electron transport. N-ethylmaleimide, p-chloromercuribenzoate, and KCN were the only electron transport inhibitors that blocked electron transport by 100%. The system seemed to be uniquely resistant to other electron transport inhibitors.
Ascorbic Acid↗
Some effects of uncouplers and inhibitors on growth and electron transport in rumen bacteria.
Uncouplers and inhibitors of electron transport affected growth and electron transport of rumen bacteria in various ways. Selenomonas ruminantium was not affected by inhibitor and uncoupler concentrations which affected growth and electron transport of Bacteroides ruminicola, B. succinogenes, and Butyrivibrio fibrisolvens. Inhibitors, when active, led to accumulation of reduced electron carriers before the site of action, but differences were found among organisms in the site of action of these inhibitors. Uncouplers reduced the glucose molar growth yields (Ygluc) of B. ruminicola, B. succinogenes, and B. fibrisolvens compared with those obtained without uncouplers. The extent of Ygluc reduction accompanying inhibitor exposure reflected electron transport chain structure. S. ruminantium appeared to obtain its adenosine 5'-triphosphate from substrate-level processes only. The other organisms studied appeared to obtain adenosine 5'-triphosphate both from substrate-level processes and from electron transport but differed in the amount of adenosine 5'-triphosphate obtained from glucose catabolism and in the proportions of adenosine 5'-triphosphate obtained from substrate-level reactions and electron transport.
Adenosine Triphosphate↗
Effects of selected inhibitors on electron transport in Neisseria gonorrhoeae.
The electron transport system of Neisseria gonorrhoeae was partially characterized by using spectrophotometric, spectroscopic, and oxygen consumption measurements. The effects of selected electron transport inhibitors (amytal, rotenone, 2-heptyl-4-hydroxyquinoline, antimycin A1, and potassium cyanide [KCN]) on electron transfer in whole-cell and sonically treated whole-cell preparations of N. gonorrhoeae were examined. The oxidation of reduced nicotinamide adenine dinucleotide, measured as a decrease in absorbance at 340 nm, was inhibited by each of the compounds tested. Oxygen consumption stimulated by reduced nicotinamide adenine dinucleotide was also inhibited, whereas oxygen uptake stimulated by succinate and malate was inhibited by KCN alone, suggesting the presence of a KCN-sensitive terminal oxidase. Room temperature optical difference spectra indicate an operational electron bypass around the amytal-rotenone-binding site. Difference spectra in the presence of 2-heptyl-4-hydroxyquinoline suggest a possible site of interaction of this compound at the substrate side of cytochrome b. Reduced-minus-oxidized spectra of ascorbate-tetramethyl-p-phenylenediamine suggest the participation of b-, a-, and d-type cytochromes in terminal oxidase activity. Hence, N. gonorrhoeae appears to have an electron transport chain containing cytochrome c, two b-type cytochromes (one of which has an oxidase function), and possibly a- and d-type cytochromes. An abbreviated chain exists through which succinate and malate can be oxidized directly by a KCN-sensitive component.
Amobarbital↗
[Effect of dibromothymoquinone on photosynthetic electron transport].
The effect of dibromothymoquinone on photosynthetic electron transport in pea dependent on concentration was studied. Dibromothymoquinone inhibited general electron transport from water to NADP+ in isolated chloroplasts and ethiochloroplasts and the electron transfer via plastoquinone and cytochrome f in the leaves and isolated plastids. At all concentrations studied dibromothymoquinone significantly affected the absorption changes at 590 nm in the ethiochloroplasts associated with plastocyanine photoreactions. Possible location of electron carriers in the photosynthetic electron transport chain is discussed.
Chloroplasts↗
Metabolite transport in mutants of Escherichia coli K12 defective in electron transport and coupled phosphorylation.
1. The uptakes of Pi and serine by whole cells of mutant strains of Escherichia coli K12, grown under both aerobic and anaerobic conditions, were studied. 2. Uptake by aerobic cells was low in a ubiquinone-less mutant but normal in two mutant strains unable to couple phosphorylation to electron transport. 3. One of these uncoupled strains, carrying the unc-405 allele, does not form a membrane-bound Mg2+-stimulated adenosine triphosphatase aggregate, and it is concluded that the Mg2+-stimulated adenosine triphosphatase does not serve a structural role in the aerobic active transport of Pi or serine. 4. The other uncoupled strain, in which aerobic uptake is unaffected, carries a mutation in the uncB gene, thus distinguishing this gene from the etc gene, previously shown to be concerned with the coupling of electron transport to active transport. 5. The uptakes of Pi and serine by anaerobic cells were normal in the ubiquinone-less mutant, but defective in both the uncoupled strains. 6. The uptake of Pi and serine by anaerobic cells of the uncB mutant could be increased by the addition of fumarate to the uptake medium. The unc-405 mutant, however, required the addition of fumarate for growth and for uptake. 7. The uncB mutant, unlike the unc-405 mutant, is able to grow anaerobically in a minimal medium with glucose as sole source of carbon. Similarly a strain carrying a mutation in the frd gene, which is the structural gene for the enzyme fumarate reductase, is able to grow anaerobically in a glucose-minimal medium. However, a mutant strain carrying mutations in both the uncB and frd genes resembles the unc-405 mutant in not being able to grow under these conditions.
Adenosine Triphosphatases↗
Electron paramagentic resonance studies of photosynthetic electron transport: photoreduction of ferredoxinand membrane-bound iron-sulfur centers.
Electron paramagnetic resonance spectrometry was used to investigate, at physiological temperatures, light-induced electron transport from membrane-bound iron-sulfur components (bound ferredoxin) to soluble ferredoxin and NADP(+) in membrane fragments (from the blue-green alga, Nostoc muscorum) that had high rates of electron transport from water to NADP(+) and from an artificial electron donor, reduced dichlorophenolindophenol (DCIPH(2)) to NADP(+). Illumination at 20 degrees resulted in the photoreduction of membrane-bound iron-sulfur centers A and B. Photoreduction by water gave electron paramagnetic resonance signals of both centers A and B; photoreduction by DCIPH(2) was found to generate a strong electron paramagnetic signal of only center B. When water was the reductant, the addition and photoreduction of soluble ferredoxin generated additional signals characteristics of soluble ferredoxin without causing a decrease in the amplitude of the signals due to centers A and B. The further addition of NADP(+) (and its photoreduction) greatly diminished signals due to the bound iron-sulfur centers and to soluble ferredoxin. An outflow of electrons from center B to soluble ferredoxin and NADP(+) was particularly pronounced when DCIPH(2) was the reductant. These observations provide the first evidence for a light-induced electron transport between membrane-bound iron-sulfur centers and ferredoxin-NADP(+). The relationship of these observations to current concepts of photosynthetic electron transport is discussed.
Cyanobacteria↗
Morphogenesis of the membrane-bound electron-transport system in sporulating Bacillus megaterium KM.
The properties of electron transport systems present in soluble and particulate fractions of spores of Bacillus megaterium KM?HAVE BEEN COMPARED WIth those of similar fractions prepared from exponential-phase vegetative cells of this organism. The timing and localization of modifications of the electron transport system occurring during sporulation have been investigated by using a system for separating forespores from mother cells at all stages during development [8]. Spore membranes contained cytochromes a + a3, and o at lower concentrations than in vegetative membranes, and in addition cytochrome c, which was not found in exponential-phase vegetative membranes. An NADH oxidase activity of similar specific activity was found in both spore and vegetative membranes but DL-glycerol 3-phosphate and L-malate oxidase activities were found only in vegetative membranes. A soluble NADH oxidase of low specific activity was found in spores and vegetative cells which probably involves a flavoprotein reaction with oxygen because the activity was stimulated by FAD or FMN and difference spectra of concentrated soluble fractions showed spectra typical of a flavoprotein. Particulate NADH oxidase was sensitive to all classical inhibitors of electron transport tested whereas soluble NADH oxidase was insensitive to many of these inhibitors. Cytochrome c was formed between stage I and II of sporulation and this coincided with a five-fold increase in NADH-cytochrome c reductase activity. Forespore membranes had lower contents of cytochromes than sporangial cell membranes but similar levels of NADH and L-malate oxidases; DL-glycerol 3-phosphate oxidase activity could not be detected in either membranes by stage III of sporulation. This characterization of spore electron transport systems provides a basis for suggestions concerning initial metabolic events during spore germination and the effect of a number of germination inhibitors.
Adenosine Triphosphatases↗
Photosystem I electron transport and phosphorylation supported by electron donation to the plastoquinone region.
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Chloroplasts↗
Proton translocation and ATP formation coupled to electron transport from H2O to the primary acceptor of photosystem 2.
1. The rate of electron transport from H2O to silicomolybdate in the presence of 3-(3-4-dichlorophenyl)-1,1-dimethylurea (diuron) (which involves the oxygen-evolving enzyme, the photochemistry of photosystem 2 and the primary electron acceptor of photosystem 2) is controlled by internal pH. This is based on the shift of the pH profile of the rate of electron transport upon addition of uncouplers, or by using EDTA-treated chloroplasts. Both stimulation and inhibition of electron transport by addition of uncouplers (depending on external pH) could be observed. These effects are obtained in the diuron-insensitive photoreductions of either silicomolybdate or ferricyanide. These experiments provide strong evidence that a proton translocating site exists in the sequence of the electron transport H2O leads to Q (the primary acceptor of photosystem 2). 2. The photoreduction of silicomolybdate in the presence of diuron causes the formation of delta pH. The value of delta pH depends on the external pH and its maximal value was shown to be 2.4. The calculated internal pH at different external pH values was found to be rather constant, namely between 5.1 -- 5.2. 3. Electron transport from H2O to silicomolybdate (in the presence of diuron) does not support ATP formation. It is suggested that this is due to the fact that the delta pH formed is below the "threshold" delta pH required for the synthesis of ATP. By adding an additional source of energy in the form of a dark diffusion potential created in the presence of K+ and valinomycin, significant amounts of ATP are formed in this system.
Adenosine Triphosphate↗
The electron transport system of the anaerobic Propionibacterium shermanii: cytochrome and inhibitor studies.
1. Electron transport particles obtained from cell-free extracts of Propionibacterium shermanii by centrifugation at 105000 times g for 3 hrs oxidized NADH, D,L-lactate, L-glycerol-3-phosphate and succinate with oxygen and, except for succinate, with fumarate, too. 2. Spectral investigation of the electron transport particles revealed the presence of cytochromes b, d and o, and traces of cytochrome alpha1 and a c-type cytochrome. Cytochrome b was reduced by succinate to about 50%, and by NADH, lactate or glycerol-3-phosphate to 80--90%. 3. The inhibitory effects of amytal and rotenone on NADH oxidation, but not on the oxidation of the other substrates, indicated the presence of the NADH dehydrogenase complex, or "site I region", in the electron transport system of P. shermanii. 4. NQNO inhibited substrate oxidations by oxygen and fumarate, as well as equilibration of the flavoproteins of the substrate dehydrogenases by way of menaquinone. The inhibition occurred at low concentrations of the inhibitor and reached 80--100%, depending on the substrate tested. The site of inhibition of the respiratory activity was located between menaquinone and cytochrome b. In addition, inhibition of flavoprotein equilibration suggested that NQNO acted upon the electron transfer directed from menaquinol towards the acceptor to be reduced, either cytochrome b or the flavoproteins, which would include fumarate reductase. 5. In NQNO-inhibited particles, cytochrome b was not oxidized by oxygen-free fumarate, but readily oxidized by oxygen. It was concluded from this and the above evidence that the branching-point of the electron transport chain towards fumarate reductase was located at the menaquinone in P. shermanii. It was further concluded that all cytochromes were situated in the oxygen-linked branch of the chain, which formed a dead end of the system under anaerobic conditions. 6. Antimycin A inhibited only oxygen-linked reactions of the particles to about 50% at high concentrations of the inhibitor. Inhibitors of terminal oxidases were inactive, except for carbon monoxide.
Aerobiosis↗
The effect of dimethyl sulphoxide on electron transport in chloroplasts.
The effect of DMSO (dimethyl sulphoxide) on electron transport in chloroplast membranes has been studied. It has been found that concentrations of DMSO up to 20% (v/v) do not inhibit electron transport in freshly isolated chloroplasts, but that higher concentrations start to cause inhibition. However, in chloroplasts that have been aged for 8 to 24 hours by storage at 4 degrees C, the addition of DMSO at concentrations up to 20% causes stimulation of electron transport. Possible mechanisms for this effect are discussed.
Chloroplasts↗
[Inhibition of electron transport and photophosphorylation in chloroplasts by quercetin].
The influence of quercetin on electron transport and photophosphorylation of pea isolated chloroplasts with methylviologen and NADP+ has been studied. Quercetin inhibits ATP synthesis and phosphorylating electron transport but does not affect the basal electron transport in the presence of methylviologen. In view of these data and because of the increase of the proton uptake by chloroplasts in the presence of quercetin we consider it as an inhibitor of energy transfer. Under conditions of NADP+ photoreduction quercetin acts also as an inhibitor of electron transfer, interacting with ferredoxin, though a complete inhibition of electron transfer has not been observed. This last phenomenon may be of importance for the understanding of the detailed mechanism of NADP+ reduction by chloroplasts.
Chloroplasts↗
[Functional characteristics of the electron transport chain in Candida mycoderma yeasts exposed to benzhydroxamic acid].
The effect of antimycin A and benzhydroxamic acid (BHA) on functioning of the electron transport system was studied with the resting cells of Candida mycoderma grown in a medium containing glucose and collected at the beginning of the deceleration phase. In the original ("control") cells, the processes of oxygen consumption were shown to be mediated mainly by the phosphorylating electron transport chain. When the cells were incubated withe glucose, the cyanide resistant electron transport chain (CRETC) started to operate in these processes. Antimycin A stimulated the operation of CRETC. BHA blocked CRETC and, at the same time, prevented the participation of pyridine nucleotides (PN) in the processes of oxygen utilization. Apparently, electron donors (or reduced equivalents) in the electron transport chain were flavin enzymes operating in the cytochrome b region of the chain instead of PN. Similar changes in the electron transport system (when PN were not involved in the processes of oxygen utilization) were observed if resting cells were incubated in the presence of ethanol.
Antimycin A↗