Light-dependent Modification of Spinach Chloroplast Coupling Factor 1 by Permanganate Ion

Anacardic Acid Derivatives Affect the in Vitro Reactions of Photosynthesis

The dichloromethane extract of the cashew nuts from Anacardium occidentale was fractionated by rotation locular countercurrent chromatography aimed at discovering metabolites that could be useful as new models for photosynthesis inhibitors. The chemical fractionation afforded a complex mixture of anacardic acids, which upon catalytic hydrogenation yielded anacardic acid (1). Methylation of 1 via reaction with diazomethane afforded an ester 2. Both compounds were evaluated using polarographic approaches and fluorescence studies of chlorophyll a (ChL a). The in vitro assays informed the decision for the classification of 1 and 2 as Hill reaction inhibitors. Besides that, 1 inhibited the donor side of the PSII, while 2 acted as an energy transfer inhibitor. Therefore, this study is important for the development of herbicides.

Inhibition of photophosphorylation and electron transport chain in thylakoids by lasiodiplodin, a natural product from Botryosphaeria rhodina

Four natural products were isolated from the fungus Botryosphaeria rhodina, and their effects on photosynthesis were tested. Only lasiodiplodin (1) inhibited ATP synthesis and electron flow from water to methylviologen; therefore, it acts as a Hill reaction inhibitor in freshly lysed spinach thylakoids. Photosystem I and II and partial reactions as well as ATPase were measured in the presence of 1. Three new different sites of 1 interaction and inhibition were found: one at CF1, the second in the water-splitting enzyme, and the third at the electron-transfer path between P680 and QA; these targets are different from that of the synthetic herbicides present. Electron transport chain inhibition by 1 was corroborated by fluorescence induction kinetics studies.

Mitochondrial ATPase

Considerable progress has been made in recent years in our understanding of the phosphorylating apparatus in mitochondria, chloroplasts, and bacteria. It has become clear that the structure and the function of the ATP synthesizing apparatus in these widely divergent organisms is similar if not virtually identical. The subunit composition of F1, its molecular architecture, the location and function of substrate binding sites, as well as putative control sites, understanding of the component parts of the oligomycin-sensitive ATPase complex, and the role of these components in the function of the complex all are under active investigation in many laboratories. The developing information and the new insights provided have begun to permit experimental approaches, at the molecular level, to the mode of action of the ATPase in electron-transport-coupled ATP synthesis.

5-O-β-D-galactopyranosyl-7-methoxy-3',4'-dihydroxy-4-phenylcoumarin, an inhibitor of photophosphorylation in spinach chloroplasts

5-O-β-D-galactopyranosyl-7-methoxy-3',4'-dihydroxy-4-phenylcoumarin isolated from Exostema caribaeum (Rubiaceae) has been found to act as an energy-transfer inhibitor in spinach chloroplasts. ATP synthesis and phosphorylating (coupled) electron flow were inhibited by 89 and 72%, respectively, at a concentration of 400 μM. H(+)-uptake, basal and uncoupled electron transport were not affected by the coumarin. The light-activated Mg(+2)-ATPase activity from bound membrane thylakoid chloroplasts was slightly inhibited by the coumarin. Also, the heat-activated Ca(+2)-ATPase activity of the isolated coupling factor protein was insensitive to this compound. In chloroplasts partially stripped of coupling factor 1 by an EDTA treatment, the coumarin showed a restoration of the proton uptake process. These results suggest that the 4-phenylcoumarin under investigation inhibited phosphorylation in chloroplasts by specifically blocking the transport of protons through a membrane-bound component or a carrier channel (CFO) located in a hydrophobic region at or near the functional binding site for the coupling factor 1.

Photosynthetic ATPases: purification, properties, subunit isolation and function

Photosynthetic coupling factor ATPases (F1-ATPases) generally censist of five subunits named α, β, γ, δ and ε in order of decreasing apparent molecular weight. The isolated enzyme has a molecular weight of between 390,000 to 400,000, with the five subunits probably occurring in a 3:3:1:1:1 ratio. Some photosynthetic F1 ATPases are inactive as isolated and require treatment with protease, heat or detergent in order to elicit ATPase activity. This activity is sensitive to inhibition by free divalent cations and appears to be more specific for Ca(2+) vs. Mg(2+) as the metal ion substrate chelate. This preference for Ca(2+) can be explained by the higher inhibition constant for inhibition of ATPase activity by free Ca(2+). Methods for the assay of a Mg-dependent ATPase activity have recently been described. These depend on the presence of organic solvents or detergents in the reaction mixture for assay. The molecular mechanism behind the expression of either the Ca- or Mg-ATPase activities is unknown. F1-ATPases function to couple proton efflux from thylakoid membranes or chromatophores to ATP synthesis. The isolated enzyme may thus also be assayed for the reconstitution of 'coupling activity' to membranes depleted of coupling factor 1.The functions of the five subunits in the complex have been deduced from the results of chemical modification and reconstitution studies. The δ subunit is required for the functional binding of the F1 to the F0. The active site is probably contained in the β (and α) subunit(s). The proposed functions for the γ and ε subunits are, however, still matters of controversy. Coupling factors from a wide variety of species including bacteria, algae, C3 and C4 plants, appear to be immunologically related. The β subunits are the most strongly related, although the α and γ subunits also show significant immunological cross-reactivity. DNA sequence analyses of the genes for the β subunit of CF1 have indicated that the primary sequence of this polypeptide is highly conserved. The genes for the polypeptides of CF1 appear to be located in two cellular compartments. The α, β and ε subunits are coded for on chloroplast DNA, whereas the γ and δ subunits are probably nuclear encoded. Experiments involving protein synthesis by isolated chloroplasts or protein synthesis in the presence of inhibitors specific for one or the other set of ribosomes in the cell suggest the existence of pools of unassembled CF1 subunits. These pools, if they do exist in vivo, probably make up no greater than 1% of the total CF1 content of the cell.

Kinetics of nucleotide binding to chloroplast coupling factor (CF1)

Studies of the kinetics of association and dissociation of the formycin nucleotides FTP and FDP with CF1 were carried out using the enhancement of formycin fluorescence. The protein used, derived from lettuce chloroplasts by chloroform induced release, contains only 4 types of subunit and has a molecular weight of 280 000. In the presence of 1.25 mM MgCl2, 1 mol of ATP or FTP is bound to the latent enzyme, with Kd = 10(-7) or 2 . 10(-7), respectively. The fluorescence emission (lambdamax 340 nm) of FTP is enhanced 3-fold upon binding, and polarization of fluorescence is markedly increased. The fluorescence changes have been used to follow FTP binding, which behaves as a bimolecular process with k1 = 2.4 . 10(4) M-1 . s-1. FTP is displaced by ATP in a process apparently involving unimolecular dissociation of FTP with K-1 = 3 . 10(-3) S-1. The ratio of rates is comparable to the equilibrium constant and no additional steps have been observed. The protein has 3 sites for ADP binding. Rates of ADP binding are similar in magnitude to those for FTP. ADP and ATP sites are at least partly competitive with one another. The kinetics of nucleotide binding are strikingly altered upon activation of the protein as an ATPase. The rate of FTP binding increases to at least 10(6) M-1 . s-1. This suggests that activation involves lowering of the kinetic barriers to substrate and product binding-dissociation and has implications for the mechanism of energy transduction in photophosphorylation.

Gated proton conduction via the coupling factor of photophosphorylation modified by N,-N-orthophenyldimaleimide

The membrane bound coupling factor of photophosphorylation is studied after pretreatment of broken chloroplasts with the bifunctional N,N-orthophenyldimaleimide under energization of the thylakoid membrane by mild flashing light. The proton conduction of the membrane is monitored both via the electrochromic absorption changes and via selective pH-indicating dyes. It is found that the coupling factor, after interaction with N,N-orthophenyldimaleimide during the preillumination period, shortcircuits one of the two protons pumped inside after excitation of chloroplasts with one short flash of light. In contrast to the low proton conductivity of the unperturbed thylakoid membrane (relaxation time for a proton gradient greater than 5s), this extra proton channel leads to a partial relaxation of a proton gradient within a few ms. Although limited to only one proton per electron, this extra proton conducting pathway is not otherwise specific. It operates with protons resulting from both Photosystem I and Photosystem II activity. In addition it operates with protons already present in the internal phase before firing of the exciting light flash. These effects are prevented by the presence of ATP (but not GTP) during the preillumination period. It is suggested that the modified coupling factor is gated open by the light induced electric field across the thylakoid membrane while self closing after passage of one proton per activated coupling factor.

Arginine modifiers as energy transfer inhibitors in photophosphorylation

Photophosphorylation by spinach chloroplasts is inhibited after they have been incubated in the dark with either phenylglyoxal or butanedione. Inhibition by phenylglyoxal is strongest when N-ethylmorpholine is the buffer used during the incubation; that by butanedione requires the presence of borate as buffer. The inhibitions are not reversed by simply washing out the inhibitor, suggesting that a covalent modification of one or more arginine residues is responsible. This is supported by the reversibility of the butanedione inhibition if both the inhibitor and borate buffer are removed. ATPase of the chloroplasts, and of extracted protein, is inhibited, whether activated by trypsin or by heating. This indicates that arginine residues of the coupling factor are the probable major site(s) for attack by these modifiers, leading to the observed inhibitions.

Sulphydryl groups in photosynthetic energy conservation. IV. Inhibition of the ATPase of chloroplast coupling factor 1 by sulphydryl reagents

1. O-Iodosobenzoate and 2,2'-dithio bis-(5-nitropyridine) inhibited by about fifty per cent the ATPase activity of heat-activated chloroplast coupling factor 1 only when present during the heating but were without effect when added before or after the activation. Reversion of this inhibition was only obtained by a second heat treatment with 10 mM dithioerythritol. 2. The inhibition of the Ca2+-ATPase of coupling factor 1 by o-iodosobenzoate or 2,2'-dithio bis-(5-nitropyridine) was not additive with similar inhibitions obtained with the alkylating reagents iodoacetamide and N-ethylmaleimide. 3. The heat-activated ATPase of o-iodosobenzoate-treated coupling factor 1 had a higher Km for ATP, without modification of V. The modified enzyme was desensitized against the allosteric inhibitor ADP.

Photoinactivation of photophosphorylation and dark ATPase in Rhodospirillum rubrum chromatophores

Preillumination of Rhodospirillum rubrum chromatophores with strong, far-red light in the presence of phenazine methosulfate under non-phosphorylation conditions results in a selective, irreversible inactivation (typically about 70%) of photophosphorylation and of uncoupler-stimulated dark ATPase. The time course of the photoinactivation is similar to the light-on kinetics of the light-induced proton uptake in the absence of ADP. Only little photoinactivation occurs when the uncoupler carbonyl cyanide m-chlorophenyl hydrazone is present or when phenazine methosulfate is absent during the preillumination, indicating that the reaction occurs only when the membrane is energized. Phosphorylation conditions offer a practically complete protection against the photoinactivation. Inorganic phosphate, Mg2+ or ADP do not provide a significant protection against the photoinactivation, nor does ATP. The pH-dependence of the reaction(s) leading to photoinactivation may indicate that a partial reaction of the photophosphorylation process (perhaps only a conformational change of the coupling factor) precedes the photoinactivation.

Salt inactivation as a mechanistic probe of membrane-bound chloroplast coupling factor 1

Conditions are reported under which ATP protects membrane-bound coupling factor 1 against sodium bromide inactivation. The presence of Mg2+ was found to be obligatory for this protection. ADP and GTP also protected the enzyme against salt inactivation but to a much smaller extent. Other nucleotides tested were ineffective. At low ATP concentrations ADP prevented the effect of ATP and modified the saturation curve for ATP from hyperbolic to sigmoidal. Treatment of chloroplasts with 0.4 M MgCl2 or 2 M LiCl resulted in inactivation of photophosphorylation. In contrast to NaBr-depleted particles the MgCl2 or LiCl-depleted chloroplasts can be reconstituted by purified coupling factor 1. A binding site for Mg2+ and two different sites for ATP upon the coupling factor 1 are suggested to explain the mechanism of their protection against salt inactivation.

Sulphydryl groups in photosynthetic energy conservation. I. Light-dependent inhibition of photophosphorylation by the sulphydryl reagent 2-2'dithio bis-(5-nitropyridine)

1. The sulphydryl reagent 2-2'dithio bis-(5-nitropyridine) (DTNP) inhibited photophosphorylation when the chloroplasts were preincubated with the reagent in the light. A maximum inhibition of about 50% was obtained in the presence of pyocyanine and MgCl 2 at 0.3 mumol DTNP per mg chlorophyll and was completed in about 40 s of preillumination. 2. Dithioerythritol, ADP plus Pi (or arsenate) and uncouplers prevented the inhibition when present during the preillumination while phloridzin, Dio-9 and discarine B were ineffective. Low concentrations of ADP or ATP afforded partial protection but other nucleotides had no effect. 3. DTNP inhibited the coupled electron transport rate to the basal level and had no effect on the uncoupled electron transport. The stimulation of proton uptake and inhibition of electron transport by ATP was prevented by DTNP. 4. The trypsin-activated but not the light- and dithioerythritol-triggered ATPase was inhibited by light preincubation of chloroplasts with DTNP. 5. Reversal of DTNP inhibition of photophosphorylation was obtained by a second preillumination in the presence of thiol groups. 6. More DTNP reacted with chloroplasts in the light than in the dark. Two mol of thione were formed in the light per mol of DTNP disappeared. 7. The results suggested that DTNP inhibition is related to the oxidation by DTNP of chloroplast vicinal dithiols probably exposed by a light-induced conformational change.

Characterization of nucleotide binding sites on chloroplast coupling factor 1

A study of the equilibrium binding of ADP, 1,N6-ethenoadenosine diphosphate, adenylyl imidodiphosphate, and 1,N6-ethenoadenylyl imidodiphosphate to solubilized spinach chloroplast coupling factor 1 (CF1) has been carried out. All four nucleotides were found to bind to two apparently identical "tight" sites, with characteristic dissociation contants generally less than 10 muM. The binding to these "tight" sites is similar in the presence of Mg2+ and Ca2+, is stronger in 0.1 M NaC1 than in 20 mM Tris-C1, and is only slightly altered by heat activation. The slow rate of association of ADP and 1,N6-ethenoadenosine diphosphate at these sites rules out the possibility that they are catalytic sites for ATPase activity on the solubilized enzyme. A third tight site for adenylyl imidodiphosphate was found on the heat-activated enzyme. The dissociation constant for this interaction (7.6 muM) is similar to the adenylyl imidodiphosphate competitive inhibition constant for ATPase activity (4 muM). ADP, which inhibits ATPase activity but is not a strong competitive inhibitor, binds only weakly at a third site (dissociation constant greater than 70 muM). One mole of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole reacted per mole of CF1 prevents ADP and adenylyl imidodiphosphate binding at the "catalytic" site and abolishes the ATPase activity. A model is proposed in which the "tight" nucleotide binding sites act as allosteric conformational switches for the ATPase activity of solubilizedCF1.

Delayed light studies on photosynthetic energy conversion. VIII. Evidence from millisecond emission of chloroplasts for two adenylate binding sites on membrane-bound coupling factor, CF1

Effects of adenylates on chloroplast delayed light emission, at millisecond dark times, are inverse to the previously characterized effects of adenylates on electron transport rates. Either ADP alone or ATP alone increase intensity of delayed light, while ADP plus Pi decrease it. ADP alone requires the presence of an electron acceptor to have this effect on delayed light, but ATP does not. All three adenylate effects are abolished by uncoupling with gramicidin, by partial removal of photophosphorylation coupling factor (CF1) with EDTA, and by antibody to CF1. Readdition of CF1 re-established the adenylate effects in EDTA-stripped membranes. The three adenylate effects are differentially sensitive to pH, and pH differentially affected their abolition by antibody to CF1. The two adenylate effects shown in the absence of Pi are exhibited at lower adenylate concentrations than the ADP plus Pi effect, and are also less sensitive to phloridzin. These results are discussed in terms of probable adenylate effects on membrane-bound chloroplast coupling factor, CF1. At least two ADP binding sites would differ with respect to adenylate concentration for half maximal binding; pH of optimal binding capacity; phloridzin sensitivity; and functional regulation of electron transport, proton uptake, and energy storage within the membrane as measured by delayed light emission. It remains unclear whether the high affinity ADP binding site is identical to a high affinity ATP binding site on CF1.

Possible role of firmly bound ATP in the energy transduction of photosynthetic membranes

Chromatophores of Rhodospirillum rubrum and spinach chloroplasts contain firmly bound ATP that is rapidly labeled along with ADP in the presence of 32Pi and endogenous nucleotides. The labeling is not entirely dependent on light. In chloroplasts three types of bound ATP can be defined methodologically by their extraction properties: buffer-soluble; acid-soluble; and SDS-soluble or firmly bound ATP. Extensive washing of the chloroplasts does reduce buffer-soluble but not acid-soluble and firmly bound ATP. Buffer-soluble [32P] ATP is almost exclusively gamma labeled while acid-soluble and firmly bound ATP are labeled in the beta and gamma position equally. CCCP, desaspidin, and phlorizin do not inhibit the labeling of firmly bound ATP, whereas the phosphorylation is almost abolished. However, EDTA and NEM pretreatments of the chloroplasts affect both reactions similarly. The postillumination [32P] ATP synthesis with chromatophores can be inhibited by adding ATP to the incubation mixture after illumination if 32Pi is included only during the dark incubation, but is without effect if 32Pi is present only during illumination. On the other hand, ADP added after illumination inhibits post-illumination [32P] ATP formation in both chromatophores and chloroplasts only if 32Pi is present during illumination. The data can be explained by a coupling factor having two sites, as proposed previously on the basis that firmly bound ATP does not transfer its phosphoryl group but seems to drive a synthesis of acid-soluble ATP which incorporates free phosphate.