The inhibition of oxygen production and the uncoupling of electron transport in photosynthesis in chloroplasts by substituted thiophens.
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Chloroplasts↗
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The accessibility of the chloroplast cytochromes f and b-559 to ferricyanide.
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Chloroplasts↗
Cooperativity between photosystem II centers at the level of primary electron transfer.
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Chlorella↗
Fluorescence yield kinetics in the microsecond-range in Chlorella pyrenoidosa and spinach chloroplasts in the presence of hydroxylamine.
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Chlorella↗
Probing the kinetics of photosystem I and photosystem II fluorescence in pea chloroplasts on a picosecond pulse fluorometer.
Picosecond fluorescence kinetics of pea chloroplasts have been investigated at room temperature using a pulse fluorometer with a resolution time of 10-11 s. Fluorescence has been excited by both a ruby and neodymium-glass mode-locked laser and has been reocrded within the 650 to 800 nm spectral region. We have found three-component kinetics of fluorescence from pea chloroplasts with lifetimes of 80, 300 and 4500 ps, respectively. The observed time dependency of the fluorescence of different components on the functional state of the photosynthetic mechanism as well as their spectra enabled us to conclude that Photosystem I fluoresces with a lifetime of 80 ps (tauI) and Photosystem II fluoresces with a lifetime of 300 ps (tauII). Fluorescence with a lifetime of 4500 ps (tauIII) may be interpreted as originating from chlorophill monomeric forms which are not involved in photosynthesis. It was determined that the rise time of Photosystem I and Photosystem II fluorescence after 530 nm photoexcitation is 200 ps, which corrsponds to the time of energy migration to them from carotenoids.
Chloroplasts↗
Further results on the photoactive chlorophyll aII in photosynthesis.
1. The flash-induced difference spectra of chlorophyll aII in untreated and in heat-treated spinach chloroplasts are presented. 2. Heat-treatment leads to an increase of the half-life of the chlorophyll aII absorbance changes. After the addition of hydroquinone plus sodium ascorbate the half-life of the chlorophyll aII reaction returns nearly to the value in untreated chloroplasts. 3. In digitonin-treated chloroplasts, or in the presence of histone, the chlorophyll aII activity is low when the electron acceptor is benzylviologen and high when ferricyanide is used. 4. The half-life of the chlorophyll aII reaction depends on the temperature of the reaction mixture. The activation energy of the chlorophyll aII reaction is about 7.5 kcal/mol. 5. From the experimental results we conclude that in heat-treated (and in Tris-washed and aged) chloroplasts the linear electron flow in Photosystem II is replaced by a cyclic one, which is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea.
Chlorophyll↗
Light-induced absorbance changes due to photosystems 1 and 2 in spinach chloroplasts at minus 50 degrees C.
Absorbance changes in the region 500-565 nm and at 702 nm, brought about by excitation of Photosystems 1 and 2, respectively, were measured in spinach chloroplasts at minus 50 degrees C. Either dark-adapted chloroplasts were used or chloroplasts preilluminated with a number of short saturating flashes just before cooling. Both photosystems were found to cause a light-induced increase of absorbance at 518 nm (due to "P518"). The System 1-induced change was not affected by pre-illumination. It decayed within 1 s in the dark and showed similar kinetics as P700. Experiments in the presence of external electron acceptors (methylviologen or Fe(CN)6-3-) suggested that P518 was not affected by the redox state of the primary electron acceptor of System 1. The absorbance increase at 518 nm due to System 2 decayed in the dark with a half-time of several min. The kinetics were similar to those of C-550, the presumed indicator of the primary electron acceptor of System 2. After two flashes preillumination the changes due to P518 and C-550 were reduced by about 40%, and a relatively slow, System 2-induced oxidation of cytochrome b559 occurred which proceeded at a similar rate as the increase in yield of chlorophyll a fluorescence. The results indicate that at minus 50 degrees C two different photoreactions of System 2 occur. One consists of a photoreduction of the primary electron acceptor associated with C-550, accompanied by the oxidation of an unknown electron donor; the other is less efficient and results in the photooxidation of cytochrome b559.
Chloroplasts↗
A rapid, light-induced transient in electron paramagnetic resonance signal II activated upon inhibition of photosynthetic oxygen evolution.
A rapid, light-induced reversible component in Signal II is observed upon inhibition of oxygen evolution in broken spinach chloroplasts. The inhibitory treatments used include Tris washing, heat, treatment with chaotropic agents, and aging. This new Signal II component is in a 1 : 1 ratio with Signal I (P700). Its formation corresponds to a light-induced oxidation which occurs in less than 500 mus. The subsequent decay of the radical results from a reduction which occurs more rapidly as this free radical component is complete following a single 10-mus flash, and it occurs with a quantum efficiency similar to that observed for Signal I formation. Red light is more effective than far-red light in the generation of this species, and, in preilluminated chloroplasts, 3-(3,4-dichlorophenyl)-1,1-dimethylurea blocks its formation. Inhibition studies show that the decline in oxygen evolution parallels the activation of this Signal II component. These results are interpreted in terms of a model in which two pathways, one involving water, the other involving the rapid Signal II component, compete for oxidizing equivalents generated by Photosystem II. In broken chloroplasts this Signal II pathway is deactivated and water is the principal electron donor. However, upon inhibition of oxygen evolution, the Signal II pathway is activated.
Chloroplasts↗
The rapid component of electron paramagnetic resonance signal II: a candidate for the physiological donor to photosystem II in spinach chloroplasts.
Rapid light-induced transients in EPR Signal IIf (F-+) are observed in 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated, Tris-washed chloroplasts until the state F P680 Q minus is reached. In the absence of exogenous redox mediators several flashes are required to saturate this photoinactive state. However, the Signal IIf transient is observed on only the first flash following DCMU addition if an efficient donor to Signal IIf, phenylenediamine or hydroquinone, is present. Complementary polarographic measurements show that under these conditions oxidized phenylenediamine is produced only on the first flash of a series. The DCMU inhibition of Signal IIf can be completely relieved by oxidative titration of a one-electron reductant with E'Os.o equals to + 480 mV. At high reduction potentials the decay time of Signal IIf is constant at about 300 ms, whereas in the absence of DCMU the decay time is longer and increases with increasing reduction potential. A model is proposed in which Q minus, the reduced Photosystem II primary acceptor, and D, a one-electron 480 mV donor endogenous to the chloroplast suspension, compete in the reduction of Signal IIf (F-+). At high potentials D is oxidized in the dark, and the (Q-+F-+) back reaction regenerates the photoactive F P680 Q state. The electrochemical and kinetic evidence is consistent with the hypothesis that the Signal IIf species, F, is identical with Z, the physiological donor to P680.
Chloroplasts↗
Induction kinetics of delayed light emission in spinach chloroplasts.
Induction curves of the delayed light emission in spinach chloroplasts were studied by measuring the decay kinetics after each flash of light. This study differs from previous measurements of the induction curves where only the intensities at one set time after each flash of light were recorded. From the decay kinetics after each flash of light, the induction curves of the delayed light emission measured 2 ms after a flash of light were separated into two components: one component due to the last flash only and one component due to all previous flashes before the last one. On comparing the delayed light induction curves of the two components with the fluorescence induction curves in chloroplasts treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and in chloroplasts treated with hydroxylamine and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the component due to the last flash only is found to be dependent on the concentration of open reaction centers and the component due to all previous flashes except the last is dependent on the concentration of closed reaction centers. This implies that the yield of the fast decaying component of the delayed light emission is dependent on the concentration of open reaction centers and the yield of the slow decaying component is dependent on the concentration of closed reaction centers.
Chloroplasts↗
Laser-flash-activated electron paramagnetic resonance studies of primary photochemical reactions in chloroplasts.
Electron paramagnetic resonance studies of the primary reactants of Photosystems I and II have been conducted at cryogenic temperatures after laser-flash activation with monochromatic light.P-700 photooxidation occurs irreversibly in chloroplasts and in Photosystem I fragments after activation with a 730 nm laser flash at a temperature of 35 degrees K. Flash activation of chloroplasts or Photosystem II chloroplast fragments with 660 nm light results in the production of a free-radical signal (g = 2.002, linewidth approximately 8 gauss) which decays with a half-time of 5.0 ms at 35 degrees K. The half-time of decay is independent of temperature in the range of 10-77 degrees K. This reversible signal can be eliminated by preillumination of the sample at 35 degrees K with 660 nm light (but not by 730 nm light), by preillumination with 660 nm light at room temperature in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) plus hydroxylamine, or by adjustment of the oxidation-reduction potential of the chloroplasts to - 150 mV prior to freezing. In the presence of ferricyanide (20-50 mM), two free-radical signals are photoinduced during a 660 nm flash at 35 degrees K. One signal decays with a half-time of 5 ms, whereas the second signal is formed irreversibly. These results are discussed in terms of a current model for the Photosystem II primary reaction at low temperature which postulates a back-reaction between P-680+ and the primary electron acceptor.
Ascorbic Acid↗
Light-induced turnover of chloroplast cytochrome b-559 in the presence of N-methylphenazonium methosulphate.
In the presence of 0.1-5 muM N-methylphenazonium methosulphate approx. 50-70% oxidation of cytochrome b-559 can be induced by far-red light. The oxidation is best observed with long wavelength far-red light (732 nm) of moderate intensities (approx. 10(4) ergs/cm2 per s) and is reversed by subsequent illumination with red light. Concentrations of N-methylphenazonium methosulphate above 5 muM are inhibitory probably due to cyclic electron flow. The far-red oxidation is inhibited by low concentrations of the plastoquinone antagonist 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, while 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits red light reduction and increases the amplitude of far-red oxidation. The effect of N-methylphenazonium methosulphate is mimicked by N-methyl-phenazonium ethosulphate, but not by pyocyanine or diaminodurene. Low concentrations (2-3 muM) of N-methylphenazonium methosulphate also stimulate a 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone-inhibitable red light reduction of cytochrome f.
Chlorophyll↗
Analysis of anaerobic fluorescence decay in Scenedesmus obliquus.
With reduction of System II acceptors during dark anaerobic adaptation in Scenedesmus obliquus fluorescence yield rises to a maximum value in two distinct transitions. Subsequent illumination results in a decay of fluorescence yield with the following characteristics: 1. In low intensity light it is independent of temperature and is an expression of light reaction I. 2. In high intestity light it reflects the dark limiting step in the reoxidation mechanism of System II primary acceptors. 3. There is strong inhibition by agents known to block electron transport between the two systems. 4. At light limiting conditions decay kinetics include an initial delay phase and thereafter close to second order behaviour. 5. Following a single brief saturating flash a maximum of 80% quenching is restored and a second flash yields approx. 95% restoration. Comparison with the fluorescence rise in the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea suggests that the decay reflects transfer of one positive charge from System I to the primary System II acceptor with the intermediary carrier pool remaining reduced.
Anaerobiosis↗
The phosphorylation site associated with the oxidation of exogenous donors of electrons to photosystem I.
1. The Photosystem I-mediated transfer of electrons from diaminodurene, diaminotolune and reduced 2,6-dichlorophenolindophenol to methylviologen is optimal at pH 8-8.5, where phosphorylation is also maximal. In the presence of superoxide dismutase, the efficiency of phosphorylation rises from smaller than or equal to 0.1 at pH 6.5 to 0.6-0.7 at pH 8-8.5, regardless of the exogenous electron donor used. 2. The apparent Km (at pH 8.1) for diaminodurene is 6-10-minus 4 M and for diaminotoluene is 1.2- 10- minus 3 M. The concentrations of diaminodurene and diaminotoluene required to saturate the electron transport processes are greater than 2 mM and greater than 5 mM, respectively. At these higher electron donor concentrations the rates of electron transport are markedly increased by phosphorylation (1.5-fold) or by uncoupling conditions (2-fold). 3. Kinetic analysis of the transfer of electrons from reduced 2,6-dichlorophenolindophenol (DCIPH2) to methylviogen indicates that two reactions with very different apparent Km values for DCIPH2 are involved. The rates of electron flux through both pathways are increased by phosphorylation or uncoupling conditions although only one of the pathways is coupled to ATP formation. No similar complications are observed when diaminodurene or diaminotoluene serves as the electron donor. 4. In the diaminodurene yields methylviologen reaction, ATP formation and that part of the electron transport dependent upon ATP formation are partially inhibited by the energy transfer inhibitor HgC12. This partial inhibition of ATP formation rises to about 50 percent at less than 1 atom of mercury per 20 molecules of chlorophyll, then does not further increase until very much higher levels of mercury are added. 5. It is suggested that exogenous electron donors such as diaminodurene, diaminotoluene and DCIPH2 can substitute for an endogenous electron carrier in donating electrons to cytochrome f via the mercury-sensitive coupling site (Site I) located on the main electron-transporting chain. If this is so, there would seem to be no reason for postulating yet another coupling site on a side branch of the electron transport chain in order to account for cyclic photophosphorylation.
Animals↗
Light-induced de-epoxidation of violaxanthin in lettuce chloroplasts. IV. The effects of electron-transport conditions on violaxanthin availability.
1. In isolated chloroplasts of Lactuca sativa var. Manoa, the size of the violaxanthin fraction which is available for de-epoxidation is not directly dependent on electron transport but rather related to the reduced level of some electron carrier between the photosystems. This is concluded from the effects of various electron-transport conditions on violaxanthin availability: Under conditions of electron transport through both photosystems, availability was saturated at a lower electron-transport rate with actinic light at 670 than at 700 nm. Under conditions of electron transport through Photosystem I, availability was smaller for linear electron flow from reduced N-methylphenazonium methosulfate via methylviologen to oxygen than from cyclic electron flow mediated by either N-methylphenazonium methosulfate or 2,6-dichlorophenolindophenol; in addition for linear flow from reduced N-methyphenazonium methosulfate via methylviologen to oxygen, availability increased with decreasing light intensity. 2. The postulated carrier whose reduced level is related to availability seems to be some carrier between plastoquinone and the primary acceptor of Photosystem II or plastoquinone itself. This conclusion follows from the fact that availability increased with increasing light intensity under conditions of electron flow through both photosystems and that 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (greater than or equal to 1 mu M) had no effect on availability, whereas low levels of 3, 3-(3',4'-dichlorophenyl)-1,1-dimethylurea resulted in decreased availability (50 percent decrease at 1 mu M). Furthermore, availability in 3,3-(3',4'-dichlorophenyl)-1,1-dimethylurea-poisoned chloroplasts was fully restored by 2-methyl-1,4-naphtoquinone (menadione) which mediates cyclic electron flow through plastoquinone. 3. Violaxanthin availability was zero in the dark and increased in the light to maximum of 67 percent of the total violazanthin in chloroplasts. It is proposed that this variable violaxanthin availability reflects conformational changes on the internal surface of the thylakoid membrane which result in variable exposure of violaxanthin to the de-epoxidase. The fact that not all of the violaxanthin was available for de-epoxidation may indicate a heterogenous distribution of violaxanthin in the membrane.
Aerobiosis↗
On the steady-state electrical potential difference across the thylakoid membranes of chloroplasts in illuminated plant cells.
The potential defference across the thyladoid membranes under steady-state saturating light conditions, measured with microcapillary glass electrodes, was found to be small as compared to the potential initially generated at the onset of illunimation. This result is discussed to be in agreement with quantitative estimates on the approximate magnitudes of the potential generating electron flux through the photo-synthetic electron transport chain and of the potential dissipating ion fluxes across the thylakoid membrane under steady-state conditions. It is concluded that a pH gradient of approx. 3-3.4 units is built up in the light across the membrane. The negative diffusion potential associated with this gradient is suggested to cause the transient negative potential observed in the dark after illumination.
2,6-Dichloroindophenol↗
The reduction of artificial electron acceptors at sub-zero temperatures by chloroplasts suspended in fluid media.
1. Chloroplasts can be suspended in aqueous/organic mixtures which are liquid at sub-zero temperatures with a good retention of the ability to reduce artificial electron acceptors. The reduction of ferricyanide and 2,6-dichlorophenolindophenol at temperatures above 0 degrees C is about 50% inhibited by 50% (v/v) ethylene glycol. Higher concentrations cause more extensive inhibition. 2. Different solvents were compared on the basis of their ability to cause a given depression of the freezing point of an aqueous solution. Ethylene glycol caused less inhibition of electron transport than glycerol, which in turn was found to be superior to methanol. 3. The reduction of oxidised 2,3,5,6-tetramethyl-p-phenylenediamine could be measured at -25 degrees C in 40% (v/v) ethylene glycol. Using an acceptor with a high extinction coefficient, methyl purple (a derivative of 2,6-dichlorophenolindophenol) it was possible to observe electron flow at temperatures as low as -40 degrees C in 50% (v/v) ethylene glycol. 4. From studies of the effects of the inhibitors 3(3,4-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone it is suggested that electron flow from the donor side of Photosystem II to the acceptor side of Photosystem I can occur at temperatures at least as low as -25 degrees C. The ultimate electron donor is presumably water but it was not possible to demonstrate this directly.
2,6-Dichloroindophenol↗
A partial reaction in photosystem II: reduction of silicomolybdate prior to the site of dichlorophenyldimethylurea inhibition.
Silicomolybdate functions as an electron acceptor in a Photosystem II water oxidation (measured as O2 evolution) partial reaction that is 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) insensitive, that is, reduction os silicomolybdate occurs at or before the level of Q, the primary electron acceptor for Photosystem II. This report characterizes the partial reaction with the principal findings being as follows: 1. Electron transport to silicomolybdate significantly decreased room temperature Photosystem I side of the DCMU had no effect on the fluorescence level, consistent with silicomolybdate accepting electrons at or before Q. In the absence of DCMU, silicomolybdate is also reduced at a site on the Photosystem I side of the DCMU block, prior to or at plastoquinone, since the plastoquinone antagonist dibromothymoquinone (DBMIB) did not affect the electron transport rate. 3. Electron transport from water to silicomolybdate (+ DCMU) is not coupled to ATP formation, nor is there a measurable accumulation of protons within the membrane (measured by amine uptake). Silicomolybdate is not inhibitory to phosphorylation per se since neither cyclic nor post-illumination (XE) phosphorylation were inhibited. 4. Uncouplers stimulated electron transport from water to silicomolybdate in the pH range of 6 to 7, but inhibited at pH values near 8. These data are consistent with the view that when electron flow is through the abbreviated sequence of water to Photosystem II to silicomolybdate (+ DCMU), conditions are not established for the water protons to be deposited within the membrane. Experiments reported elsewhere (Fiaquinta, R.T., Dilley, R.A. and Horton, P.(19741 J. Bioenerg. 6, 167-177) and these data, are consistent with the hypothesis that electron transport between Q and plastoquinone energizes a membrane conformational change that is required to interact with the water oxication system so as to result in the deposition of water protons either within the membrane itself or within the inner oxmotic space.
Benzenesulfonates↗