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At least 19 recordsLinked to original sources
PubMed1976

Inhibition of photophosphorylation by ATP and the role of magnesium in photophosphorylation.

ATP and pyrophosphate at high concentration (greater than 1 mM) inhibited photophosphorylation of isolated spinach chloroplasts in the normal salt medium and did not cause stimulation of electron transport. The inhibition of photophosphorylation by ATP or pyrophosphate was shown to be abolished by the addition of excess MgCl2, ADP and phosphate. It has been demonstrated that the rates of photophosphorylation in the absence and presence of ATP or pyrophosphate are determined similarly by the concentrations of magnesium-ADP (Mg - ADP-) and magnesiumphosphate (Mg - Pi) complexes. It is highly probable that Mg - ADP- and Mg - Pi, but not free ADP and free phosphate, are the active form of the substrates of photophosphorylation. This is in support of the view that ATP inhibits photophosphorylation by decreasing the concentration of Mg2+ which is available for the formation of the complex with ADP and phosphate.

Adenosine Diphosphate↗
PubMed1978

Flash-induced photophosphorylation in Rhodospirillum rubrum chromatophores. I. The relationship between cytochrome c-420 content and photophosphorylation.

The content of cytochrome c-420 in Rhodospirillum rubrum chromatophores prepared by grinding with alumina is 5--10% of that in whole cells, and 20--40% in chromatophores by 'French' pressing. Flash-induced phosphorylation of various chromatophores which varied in cytochrome content from 7 to 40% is proportional to the cytochrome content. Extrapolating the cytochrome c-420 content to that observed in whole cells, a ratio ATP/P+X- near 1 is calculated. At low flash intensity the phosphorylation per flash is proportional to flash energy. Photophosphorylation in flashes given after a time of several minutes is only slightly dependent on the number of flashes. If the flashes are spaced from 0.1 to 10 s, relative phosphorylation in the first flash is about 70% and in the second 90+ of that observed in the following flashes. Proton binding is not affected by the cytochrome c-420 content and a ratio of H+/P+x- of 2.3 was found. These results can be explained by a working hypothesis in which charge separation occurring at one reaction centre and the resulting electron transport mediated amongst others by c-420, results in the injection of two protons into an ATPase, this in contrast to a chemiosmotic mechanism, where the protons are released in the chromatophore inner space.

Bacterial Chromatophores↗
PubMed1983

Uncouplers can shuttle between localized energy-coupling sites during photophosphorylation by chromatophores of Rhodopseudomonas capsulata N22.

Two models of the action of uncoupler molecules in inhibiting photophosphorylation in bacterial chromatophores are considered: either uncoupler molecules shuttle rapidly between energy-coupling sites, or uncoupler molecules that are bound to particular sites in the chromatophores for a time that is comparable with the turnover time of the photophosphorylation apparatus may uncouple by a co-operative "substoichiometric' mechanism. It is found that the titre of uncoupler necessary to cause complete uncoupling is lowered if the rate of photophosphorylation is initially decreased by partially restricting electron flow with an appropriate titre of antimycin A. This result indicates that uncoupler molecules shuttle rapidly between energy coupling in which the energized intermediate between electron transport and phosphorylation is delocalized over the entire chromatophore membrane and those in which it is not. If the rate of photophosphorylation is partially restricted with the covalent H+-translocating ATP synthase inhibitor dicyclohexylcarbodi-imide, the titre of uncoupler necessary to effect complete inhibition of photophosphorylation is also decreased relative to that in which the covalent H+-ATP synthase inhibitor is absent. This important result appears to be inconsistent with models of electron-transport phosphorylation in which the "energized state' of the chromatophore membrane that is set up by electron transport and utilized in photophosphorylation is delocalized over the entire chromatophore membrane.

Antimycin A↗
PubMed1977

Bacteriorhodopsin-mediated photophosphorylation in Halobacterium halobium.

The rate of halobacterial photophosphorylation was found to be a linear function of light intensity over a wide range (between 1 and 20 mW/cm2). At higher light intensities (above 25 mW/cm2) the ATP-synthesizing system itself limits the maximal rate of photophosphorylation. The optimal external pH range for this type of photophosphorylation is between pH 6.2 and 7.2 external. The photophosphorylation rate is directly proportional to the bacteriorhodopsin content of the cells. The quantum requirement for photophosphorylation was found to be 22 +/- 5 photons per ATP molecule synthesized. According to Mitchell's chemiosmotic hypothesis of energy coupling phosphorylation can be driven by a membrane potential or a pH gradient or a combination of both. From the results of experiments with drugs which abolish or reduce either one of the two components we conclude that the major driving force for photophosphorylation above an external pH value of 6.5 is the membrane potential, while at more acidic pH value the pH gradient becomes dominating. We did not observe a correlation between a transient alkalinization of the medium and ATP-synthesis upon illumination under certain conditions.

Adenosine Triphosphate↗
PubMed1967

Inhibitory effect of peroxyacetyl nitrate on cyclic photophosphorylation by chloroplasts from black valentine bean leaves.

The inhibitory effect of peroxyacetyl nitrate on the cyclic photophosphorylation of chloroplasts isolated from Black Valentine variety bean leaves (Phaseolis vulgaris L.) has been studied. Peroxyacetyl nitrate caused inhibition to photophosphorylation, in either the dark or the light, by affecting the chloroplast. Evidence is presented which suggests that peroxyacetyl nitrate could oxidize sulfhydryl groups on enzymes necessary for photophosphorylation. The inhibition to photophosphorylation caused by peroxyacetyl nitrate cannot be reversed by glutathione, even when added in large amounts, whereas the inhibition to photophosphorylation caused by para-chloromercuriphenylsulfonic acid is easily reversed by small quantities of glutathione. This suggests that if peroxyacetyl nitrate is oxidizing sulfhydryl groups necessary for photophosphorylation, this oxidation is proceeding beyond the disulfide state.

Air Pollution↗
PubMed1985

Protonmotive force and photophosphorylation in single swollen thylakoid vesicles.

Swollen vesicles generally 40 micron in diameter were prepared from spinach chloroplasts. These vesicles appear to originate from thylakoids. The present study reports results obtained with individual vesicles using micromanipulative procedures. The electric potential across the membrane was measured with microelectrodes and the pH of the internal space was calculated from the fluorescence of the pH indicator pyranine. The individual vesicles photophosphorylate as measured with luciferin-luciferase. Impalement with microelectrodes did not affect the ability of individual vesicles to photophosphorylate. However, there was no significant membrane potential either with continuous illumination or light flashes. In contrast, we found a delta pH of 3.7 under photophosphorylative conditions and the incubation with the appropriate buffers blocked photophosphorylation presumably by preventing formation of a pH gradient. We propose that, in these vesicles, the membrane potential plays no role in photophosphorylation, whereas a pH gradient is obligatory.

Adenosine Triphosphate↗
PubMed1982

Effect of pyridine homologues on proton flux through the CF0 . CF1 complex and photophosphorylation in chloroplasts.

At concentrations below 1 mM, hydrophobic pyridine homologues decrease the rate of photophosphorylation and light-stimulated hydrolysis of ATP and light-activated exchange of the tightly bound nucleotides in chloroplasts, but increase the rate of the Hill reaction. Unlike uncoupling agents, the presence of the organic base at such low concentrations decreases the rate of light-dependent leakage and has no effect on the efficiency of two-stage photophosphorylation in broken chloroplasts. By assuming that the organic base is bound to independent equivalent sites in the thylakoid membrane, a simple expression can be derived which relates the observed rates of photophosphorylation and light-stimulated hydrolysis of ATP quantitatively to the concentration of the organic base in solution and gives dissociation equilibrium constants which are on the order of the relative hydrophobicities of the pyridine homologues. A possible mechanistic model for the CF0 . CF1 complex is proposed which could serve as the basis for a unified interpretation of the kinetics of proton translocation in illuminated chloroplasts, the steady-state rate of photophosphorylation, the light-stimulated ATPase activity, and the light-activated exchange of tightly bound adenine nucleotides.

Adenine Nucleotides↗
PubMed1976

Phosphorylation of nucleotides bound to chloroplast membranes and their role in photophosphorylation.

1. The initial rapid phosphorylation of membrane-bound ADP yields [gamma- 32P]-ATP. Long term illumination of chloroplasts resulted in the introduction of label in the beta position of ADP and ATP. 2. It is concluded that the initial acceptor in photophosphorylation is ADP and not AMP. The appearance of 32Pi in the beta position of the nucleotide fractions is probably the result of side reactions not directly involved in the photophosphorylation mechanism. 3. The inhibitor phlorizin affects similarly the phosphorylation of bound ADP and the net photophosphorylation reaction. 4. The uncoupler nigericin has different effects on the phosphorylation of membrane-bound ADP and the net photophosphorylation reaction. Dissipation of energy by low concentrations of the uncoupler affects primarily the step(s) of ATP release to the medium, while steps leading to the interconversion of Pi and ADP to form bound ATP are less sensitive and are inhibited by relatively higher uncoupler concentrations.

Adenosine Diphosphate↗
PubMed1979

The influence of energy-transfer inhibitors on proton permeability and photophosphorylation in normal and preilluminated Rhodospirillum rubrum chromatophores.

(1) Chromatophores were preilluminated in the presence of phenazine methosulphate or diaminodurene, and without phosphorylation substrates; next they were transferred to fresh medium and assayed for light-induced proton uptake, light-induced 9-aminoacridin fluorescence quenching, and photophosphorylation. (2) Preillumination in the presence of phenazine methosulphate or diaminodurene causes an inhibition of the photophosphorylation rate. The presence of ADP + MgCl2 + phosphate, or ADP + MgCl2 + arsenate during preillumination provides full protection against this effect. (3) Preilluminated chromatophores are leaky for protons. The leak is expressed as an accelerated dark decay, and a diminished extent of succinate-supported, light-induced proton uptake. The extent of light-induced 9-aminoacridin fluorescence quenching is also diminished. (4) The proton leak can be closed by oligomycin and by dicyclohexyl carbodiimide (at concentrations similar to those used to inhibit photophosphorylation), but not by aurovertin. Closure of the proton leak results in partial restoration of the photophosphorylation rate. (5) The inhibition of phosphorylation by oligomycin or dicyclohexyl carbodiimide is time-dependent. In untreated chromatophores, the time-dependence is determined by the extent of membrane energization. In preilluminated chromatophores, the time-dependence is determined in addition by the extent to which the proton leaks have been closed. The reasons for this are briefly discussed.

Antimycin A↗
PubMed1981

Models for protocellular photophosphorylation.

Several photoreactions for transducing light energy have been analyzed for their relevance as models for protocellular photophosphorylation. Inorganic ions and compounds could have played a role in protocellular photophosphorylation. Organic catalysts may have been the next significant agents used by protocells for photophosphorylation. Membranous photophosphorylation probably became the most recent type of photoenergy transduction to be acquired by protocells; it is still used by modern cells although components of the other types of phosphorylation are found in present day cells. Recorded yields of energy-rich phosphates from the model reactions discussed are small. Arguments are advanced that such yields could have been sufficient to have fueled protocellular metabolism which was probably very slow compared to modern cellular metabolism. Future prospects for research in this area are discussed.

Adenosine Triphosphatases↗
PubMed1974

Inhibition and uncoupling of photophosphorylation in isolated chloroplasts by organotin, organomercury and diphenyleneiodonium compounds.

1. Trialkyltin, triphenyltin and diphenyleneiodonium compounds inhibited ADP-stimulated O(2) evolution by isolated pea chloroplasts in the presence of phosphate or arsenate. Tributyltin and triphenyltin were the most effective inhibitors, which suggests a highly hydrophobic site of action. Phenylmercuric acetate was a poor inhibitor of photophosphorylation, which suggests that thiol groups are not involved. 2. Triethyltin was a potent uncoupler of photophosphorylation by isolated chloroplasts in media containing Cl(-), but had little uncoupling activity when Cl(-) was replaced by NO(3) (-) or SO(4) (2-), which are inactive in the anion-hydroxide exchange. It is suggested that uncoupling by triethyltin is a result of the Cl(-)-OH(-) exchange together with a natural uniport of Cl(-). Tributyltin, triphenyltin and phenylmercuric acetate had low uncoupling activity, probably because in these compounds the uncoupling activity is partially masked by inhibitory effects. 3. At high concentrations the organotin compounds caused inhibition of electron transport uncoupled by carbonyl cyanide m-chlorophenylhydrazone or NH(4)Cl. At these high concentrations the organotin compounds may be producing a detergent-like disorganization of the membrane structure. In contrast, diphenyleneiodonium sulphate inhibited uncoupled electron transport at low concentrations; however, this inhibition is less than the inhibition of photophosphorylation, which suggests that the compound also inhibits the phosphorylation reactions as well as electron transport. 4. The effects of these compounds on basal electron transport were complex and depended on the pH of the reaction media. However, they can be explained on the basis of three actions: inhibition of the phosphorylation reactions, uncoupling and direct inhibition of electron transport. 5. The inhibition of cyclic photophosphorylation in the presence of phenazine methosulphate by diphenyleneiodonium sulphate shows that it inhibits in the region of photosystem 1.

Adenosine Diphosphate↗
PubMed1989

Chloroplast ATP synthase contains one single copy of subunit delta that is indispensable for photophosphorylation.

F0F1 ATP synthases synthesize ATP in their F1 portion at the expense of free energy supplied by proton flow which enters the enzyme through their channel portion F0. The smaller subunits of F1, especially subunit delta, may act as energy transducers between these rather distant functional units. We have previously shown that chloroplast delta, when added to thylakoids partially depleted of the coupling factor CF1, can reconstitute photophosphorylation by inhibiting proton leakage through exposed coupling factor CF0. In view of controversies in the literature, we reinvestigated two further aspects related to subunit delta, namely (a) its stoichiometry in CF0CF1 and (b) whether or not delta is required for photophosphorylation. By rocket immunoelectrophoresis of thylakoid membranes and calibration against purified delta, we confirmed a stoichiometry of one delta per CF0CF1. In CF1-depleted thylakoids photophosphorylation could be reconstituted not only by adding CF1 and subunit delta but, surprisingly, also by CF1 (-delta). We found that the latter was attributable to a contamination of CF1 (-delta) preparations with integral CF1. To lesser extent CF1 (-delta) acted by complementary rebinding to CF0 channels that were closed because they contained delta [CF0(+delta)]. This added catalytic capacity to proton-tight thylakoid vesicles. The ability of subunit delta to control proton flow through CF0 and the absolute requirement for delta in restoration of photophosphorylation suggest an essential role of this small subunit at the interface between the large portions of ATP synthase: delta may be part of the coupling site between electrochemical, conformational and chemical events in this enzyme.

Amino Acids↗
PubMedDate not supplied

[Effect of IAA on the photophosphorylation of pea isolated chloroplasts].

Effect of IAA (10(-10)-10(-3) M) on photophosphorylation, NADP reduction and the oxygen exchange is investigated. It is shown that low concentrations of IAA (10(-10)-10(-7) M) increase the photophosphorylation reaction and the flow of electrones to NADP under the phosphorylation conditions in the chloroplasts, and their effect on the O2 exchange is not the same in different types of photophosphorylation. It is supposed that the effect of IAA on the photophosphorylation is connected with H292 metabolism in chloroplasts and with catalase and peroxidase functions.

Chloroplasts↗
PubMed1982

[The special reaction of photophosphorylation using epsilon ADP--a fluorescent analog of ADP].

Photophosphorylation of epsilon ADP in a chloroplast synthetase system reconstituted with CF1 or with CF1 modified by covalently bound epsilon ADP has been studied. The reconstitution of EDTA-treated chloroplasts with CF1 restores the photophosphorylating activity to about 90%. When the CF1 modified by covalently bound epsilon ADP is used for reconstitution the photophosphorylating activity of EDTA-treated chloroplasts is restored to 37%. Based on the results of a photochemical study of the chloroplast ATP-synthetase system reconstituted with CF1 with covalently bound epsilon ADP it may be assumed that the substrate, adenine, participates in proton translocation to inorganic phosphate in the active center of the coupling enzyme during photophosphorylation.

Adenosine Diphosphate↗