On the estimation of proton gradient and osmotic volume in chloroplast membranes

Distinguishing between luminal and localized proton buffering pools in thylakoid membranes

The dual gradient energy coupling hypothesis posits that chloroplast thylakoid membranes are energized for ATP formation by either a delocalized or a localized proton gradient geometry. Localized energy coupling is characterized by sequestered domains with a buffering capacity of approximately 150 nmol H(+) mg(-1) chlorophyll (Chl). A total of 30 to 40 nmol mg(-1) Chl of the total sequestered domain buffering capacity is contributed by lysines with anomolously low pK(a)s, which can be covalently derivatized with acetic anhydride. We report that in thylakoid membranes treated with acetic anhydride, luminal acidification by a photosystem I (duraquinol [DQH(2)] to methyl viologen [MV]) proton pumping partial reaction was nearly completely inhibited, as measured by three separate assays, yet surprisingly, H(+) accumulation still occurred to the significant level of more than 100 nmol H(+) mg Chl(-1), presumably into the sequestered domains. The treatment did not increase the observed rate constant of dark H(+) efflux, nor was electron transport significantly inhibited. These data provide support for the existence of a sequestered proton translocating pathway linking the redox reaction H(+) ion sources with the CF(0) H(+) channel. The sequestered, low-pK(a) Lys groups appear to have a role in the H(+) diffusion process and chemically modifying them blocks the putative H(+) relay system.

Incorporation of transmembrane hydroxide transport into the chemiosmotic theory

A cornerstone of textbook bioenergetics is that oxidative ATP synthesis in mitochondria requires, in normal conditions of internal and external pH, a potential difference (delta psi) of well over 100 mV between the aqueous compartments that the energy-transducing membrane separates. Measurements of delta psi inferred from diffusion of membrane-permeant ions confirm this, but those using microelectrodes consistently find no such delta psi--a result ostensibly irreconcilable with the chemiosmotic theory. Transmembrane hydroxide transport necessarily accompanies mitochondrial ATP synthesis, due to the action of several carrier proteins; this nullifies some of the proton transport by the respiratory chain. Here, it is proposed that these carriers' structure causes the path of this "lost" proton flow to include a component perpendicular to the membrane but within the aqueous phases, so maintaining a steady-state proton-motive force between the water at each membrane surface and in the adjacent bulk medium. The conflicting measurements of delta psi are shown to be consistent with the response of this system to its chemical environment.

Rates of vectorial proton transport supported by cyclic electron flow during oxygen reduction by illuminated intact chloroplasts

The light-dependent quenching of 9-aminoacridine fluorescence was used to monitor the state of the transthylakoid proton gradient in illuminated intact chloroplasts in the presence or absence of external electron acceptors. The absence of appreciable light-dependent fluorescence quenching under anaerobic conditions indicated inhibition of coupled electron transport in the absence of external electron acceptors. Oxygen relieved this inhibition. However, when DCMU inhibited excessive reduction of the plastoquinone pool in the absence of oxygen, coupled cyclic electron transport supported the formation of a transthylakoid proton gradient even under anaerobiosis. This proton gradient collapsed in the presence of oxygen. Under aerobic conditions, and when KCN inhibited ribulose bisphosphate carboxylase and ascorbate peroxidase, fluorescence quenching indicated the formation of a transthylakoid proton gradient which was larger with oxygen in the Mehler reaction as electron acceptor than with methylviologen at similar rates of linear electron transport. Apparently, cyclic electron transport occured simultaneously with linear electron transport, when oxygen was available as electron acceptor, but not when methylviologen accepted electrons from Photosystem I. The ratio of cyclic to linear electron transport could be increased by low concentrations of DCMU. This shows that even under aerobic conditions cyclic electron transport is limited in isolated intact chloroplasts by excessive reduction of electron carriers. In fact, P700 in the reaction center of Photosystem I remained reduced in illuminated isolated chloroplasts under conditions which resulted in extensive oxidation of P700 in leaves. This shows that regulation of Photosystem II activity is less effective in isolated chloroplasts than in leaves. Assuming that a Q-cycle supports a H(+)/e ratio of 3 during slow linear electron transport, vectorial proton transport coupled to Photosystem I-dependent cyclic electron flow could be calculated. The highest calculated rate of Photosystem I-dependent proton transport, which was not yet light-saturated, was 330 μmol protons (mg chlorophyll h)(-1) in intact chloroplasts. If H(+)/e is not three but two proton transfer is not 330 but 220 μmol (mg Chl H)(-1). Differences in the regulation of cyclic electron transport in isolated chloroplasts and in leaves are discussed.

Location and dynamics of alamethicin in unilamellar vesicles and thylakoids as model systems. A spin label study

Location and dynamics of the voltage-dependent pore-forming icosapeptide alamethicin have been studied using spin labels which were linked directly and via spacers to the C-terminus of the amphiphilic alpha-helix. Ion-transport activities of these derivatives were found to be very similar to those of natural alamethicin in green plant thylakoids chosen as a model system. The shape of the electron spin resonance spectra indicates segmental motion of the nitroxide rather than rotation of the whole peptide. A population of spins showing narrow lines in the presence of thylakoids or lipid vesicles is attributed to alamethicin in the aqueous solution. A second population shows rotational correlation times greater than 10(-9) s and is bound to the membranes, the C-termini residing in an environment with a polarity close to that of water. This population is inaccessible to the hydrophilic, charged line broadening agent chromium oxalate. Since spectral shapes and amplitudes of spectra are unchanged by additions of unlabelled peptide, it is concluded that the ESR detectable spins are bound to peptides essentially in the monomeric state. Alamethicin induced pore formation under flash illumination is demonstrated by measurement of kinetics of proton deposition in the thylakoid interior. When pores are opened by illuminating thylakoids and thus applying a membrane potential, mainly the bound population is affected by a process reversibly suppressing the signal, whereas only limited disappearance of label from the external medium is detected. Apparently, the potential causes a change in the conformation of the peptide which leads to a further immobilisation of the label, possibly due to a deeper insertion of the alpha-helices into the lipid membrane. However, evidence has been presented experimentally that there is no detectable change of potential prior to the opening of the pore.

Intrinsic and artifactual pH buffering in chloroplast thylakoids

After an increase in pH of the suspension medium, a pH gradient across the membrane of chloroplast thylakoids stored at pH greater than or equal to 6.5 is often maintained for several minutes. The intrinsic hydrogen ion buffering capacity of the thylakoid membranes between pH 6.5 and 8.5 is about 40 neq/mg chlorophyll, but can be artificially inflated by penetration of the external buffer into the thylakoid vesicle. A delta pH imposed across the thylakoid membrane by an acid/base transition cannot be estimated accurately by the fluorescent probe 9-aminoacridine, especially with osmotically shrunken thylakoids in which 9-aminoacridine appears to become bound or adsorbed to the membrane. This interaction may be related to the existence of the previously demonstrated special pool of slowly equilibrating, "sequestered" protons.

Calibration of the response of 9-amino acridine fluorescence to transmembrane pH differences in bacterial chromatophores

The spectral characteristics of absorption and fluorescence emission of 9-amino acridine are not altered by the interaction with bacterial chromatophores, except for the attenuation of both the absorption and emission following the formation of a protonic gradient. The lifetime of fluorescence of the dye is significantly affected in the presence of membranes, and even more following illumination. The shortening of the lifetime induced by light is reversible and prevented by nigericin and K+. The onset kinetics of the fluorescence quenching following the generation of an artificial transmembrane pH difference is temperature dependent, with an activation energy of 17 +/- 3 kcal/mol. The effect of pH on the rate constants is consistent with a model assuming that the diffusion of the unprotonated species is the limiting step in the quenching phenomenon. The response of 9-amino acridine to artificially imposed delta pH's has been utilized as a calibration method for the measurements of the light-induced protonic gradient. The apparent inner volume of chromatophores, evaluated from the extraplation of the response at delta pH = 0, was found to be much larger (15- to 40-fold) than the true osmotic volume, indicating that most of the dye is bound to the membrane when accumulated into the inner lumen.

Permeability of alkylamines across phosphatidylcholine vesicles as studied by 1H-NMR

The exchange rate and enthalpy and entropy of activation of the diffusion of the first five n-alkylamines across egg phosphatidylcholine vesicles has been measured by 1H-NMR spectroscopy employing the 1:2 Gd3+-EDTA complex as a relaxation reagent. The permeability determined from the exchange rate of the ethyl through the pentyl derivatives increased sequentially with increasing chain length from 7.10(-7) to 4.10(-4) cm/s, respectively, at 25 degrees C. The permeability of methylamine was similar to that of ethylamine (1.10(-6) cm/s at 25 degrees C) and exhibited a relatively smaller entropy increase. The enthalpy of activation for the transfer reaction was high for all amine derivatives (20 kcal/mol). The entropy of activation increased with increasing chain length. The results indicate that the rate of diffusion is dominated by the partition into the membrane. Methylamine, being the smallest molecule in this series, can probably diffuse also through vacancies formed by the internal motions of the lipid chains.

Freezing injury in cold-acclimated and unhardened spinach leaves : I. Photosynthetic reactions of thylakoids isolated from frost-damaged leaves

Spinach plants (Spinacia oleracea L.) were frost-hardened by cold-acclimation to 1° C or kept in an unhardy state at 20°/14° C in phytotrons. Detached leaves were exposed to temperatures below 0°C. Rates of photosynthetic CO2 uptake by the leaves, recorded after frost treatment, served as a measure of freezing injury. Thylakoid membranes were isolated from frost-injured leaves and their photosynthetic activities tested. Ice formation occurred at about-4° to-5° C, both in unhardened and cold-acclimated leaves. After thawing, unhardened leaves appeared severely damaged when they had been exposed to-5° to-8° C. Acclimated leaves were damaged by freezing at temperatures between-10° to-14° C. The pattern of freezing damage was complex and appeared to be identical in hardened and unhardened leaves: 1. Inactivation of photosynthesis and respiration of the leaves occurred almost simultaneously. 2. When the leaves were partly damaged, the rates of photosynthetic electron transport and noncyclic photophosphorylation and the extent of light-induced H(+) uptake by the isolated thylakoids were lowered at about the same degree. The dark decay of the proton gradient was, however, not stimulated, indicating that the permeability of the membrane to-ward protons and metal cations had not increased. 3. As shown by partial reactions of the electron transport system, freezing of leaves predominantly inhibited the oxygen evolution, but photosystem II and photosystem I-dependent electron transport were also impaired. 4. Damage of the chloroplast envelope was indicated by a decline in the percentage of intact chloroplasts found in preparations from injured leaves. The results are discussed in relation to earlier studies on freezing damage of thylakoid membranes occurring in vitro.

Metabolic regulation by pH gradients. Inhibition of photosynthesis by indirect proton transfer across the chloroplast envelope

Anions of several weak acids inhibited photosynthesis in isolated spinach chloroplasts. Inhibition was drastic at low pH and weak or absent at high pH. Glyoxylate was particularly effective and inhibition decreased in the order: glyoxylate, nitrite, glycerate, formate, hydroxypyruvate, glycolate, propionate, acetate, pyruvate. These anions operated as indirect proton shuttles across the chloroplast envelope. They compensated active proton fluxes into the medium, minimized gradients in proton activity across the chloroplast envelope, and so prevented light-dependent stroma alkalization. This caused inhibition of sugar bisphosphatases which are known to be pH-regulated. At concentrations that caused potosynthesis inhibition, the proton shuttles were not effective in decreasing the proton gradient across the thylakoids. Some anions also inhibited fructose-bisphosphatase directly, when present at concentratins higher than needed for photosynthesis inhibition.

Measurement of chloroplast internal protons with 9-aminoacridine. Probe binding, dark proton gradient, and salt effects

A defined ratio, gamma, of the total proton uptake to the concentration change of free internal H+ is observed for illuminated envelope-free chloroplasts (Haraux, F. and de Kouchkovsky, Y. (1979) Biochim. Biophys. Acta, 546, 455-471). Proton uptake is measured by the external pH shift, free internal H+ by 9-aminoacridine fluorescence quenching. Extension of this work leads to the following conclusions, which, in the case of 9-aminoacridine behaviour, should apply to any kind of diffusible protonizable delta pH probe: 1. The gamma constancy is preserved when the internal volume (Vi) is modulated by chlorophyll and osmolarity changes: thus, 9-aminoacridine behaves as expected from the delta pH distribution of an amine of high pK; previous doubts on this point are attributed to the lack of control of the external proton uptake. 2. With variable 9-aminoacridine concentration, however, some variation of gamma confirms the existence of slight light-induced probe-membrane interactions. 3. According to the diffuse layer theory, salts decrease the negative potential at the 'plane of closest approach' of the thylakoids, thereby releasing the excess 9-aminoacridine in this diffuse layer, which increases its fluorescence. Although of equal valency, NH4+ is more potent than K+, suggesting competition between amines for specific anionic binding sites. 4. Two categories of membrane modifications are induced by salts: in addition to the above-mentioned electrical effect, mono- and divalent cations at high concentration increase the chloroplast proton binding capacity. La3+ is only able to release the excess dye in the diffuse layer and leaves gamma unchanged. Therefore the probe-membrane interactions should have limited importance for steady-state delta pH measurement. 5. A Donnan-type dark pH difference, which could seriously bias these delta pH estimates, is found experimentally to be less than 2 (no significant gamma change when Vi varies) and even theoretically less than 1 (on the basis of the concentration of the non-diffusible internal protonizable groups). Similarly, the predictable errors of Vi and its possible light-induced variations must have a small effect on delta pH under present experimental conditions.

Oxygen requirement of photosynthetic CO2 assimilation

In the absence of electron acceptors and of oxygen a proton gradient was supported across thylakoid membranes of intact spinach chloroplasts by far-red illumination. It was decreased by red light. Inhibition by red light indicates effective control of cyclic electron flow by Photosystem II. Inhibition was released by oxygen which supported a large proton gradient. Oxygen appeared to act as electron acceptor simultaneously preventing over-reduction of electron carriers of the cyclic electron transport pathway. It thus has an important regulatory function in electron transport. Under anaerobic conditions, the inhibition of electron transport caused by red illumination could also be released and a large proton gradient could be established by oxaloacetate, nitrite and 3-phosphoglycerate, but not by bicarbonate. In the absence of oxygen, ATP levels remained low in chloroplasts illuminated with red light even when bicarbonate was present. They increased when electron acceptors were added which could release the over-reduction of the electron transport chain. Inhibition of electron transport in the presence of bicarbonate was relieved and CO2-fixation was initiated by oxygen concentrations as low as about 10 microM. Once CO2 fixation was initiated, very low oxygen levels were sufficient to sustain it. The results support the assumption that pseudocyclic electron transport is necessary to poise the electron transport chain so that a proper balance of linear and cyclic electron transport is established to supply ATP for CO2 reduction.

A quantitative study of the slow decline of chlorophyll a fluorescence in isolated chloroplasts

A detailed study of the photo-induced decline in chlorophyll a fluorescence intensity (Kautsky phenomenon) in coupled isolated chloroplasts from a high level (P) to a low stationary level (S) is presented. 1. A linear relationship between P leads to S quenching and intrathylakoid H+ concentration was found. When the light-induced proton gradient was abolished by uncoupling, the fluorescence emission at room temperature was lowered proportionally to increased H+ concentration in the medium. 2. Fluorescence spectra at -196 degrees C of samples frozen at the P and S states showed no significant differences in the Photosystem I/Photosystem II ratio of fluorescence emission. Furthermore, freezing to -196 degrees C reversed the P leads to S quenching. This indicates that the P leads to S quenching is not related to an increase of spillover of excitation energy from Photosystem II to Photosystem I. 3. When Mg2+ was added to thylakoids suspended in a medium free of divalent cations, the inhibition of spillover required lower Mg2+ concentrations (half saturation at 0.6 mM). Increased proton concentration in the medium also inhibited spillover. 4. The results are interpreted in terms of two sites of Mg2+ and H+ effects on excitation deactivation in Photosystem II. One site is located on the outer face of the thylakoid membrane; action of both Mg2+ and H+ at this side diminishes spillover. The second site is located on the inner face of the membrane; as Mg2+ is displaced there by protons, a non-photochemical quenching of Photosystem II fluorescence is induced, which is manifested by the P leads to S decline.

Quantitative estimation of the photosynthetic proton binding inside the thylakoids by correlating internal acidification to external alkalinisation and to oxygen evolution in chloroplasts

The external alkalinisation delta pHe, or the rate of oxygen evolution vO2, of a suspension of envelope-free chlorplasts was correlated with their internal acidification, estimated from the transmembrane delta pHei. Knowing the external buffer value, the concentration of the total protons moved Hi was calculated from the delta pHe, measured with a glass electrode ([Hi] was also obtained from vO2), and the free proton concentration [Hi+] was determined from delta pHei, measured with 9-aminoacridine. This gives a ratio gamma i = theta [Hi]/theta [Hi+], which is independent of the thylakoids internal volume. Within a large pHi range, scanned by varying the light intensity, gamma i was kept reasonably constant; it was hardly sensitive to pHi. This apparent invariability implies a continuous change of the internal buffer value beta i with pHi, since beta i/gamma i = -2.3.....10pHi, a relationship which inlcudes neither the total concentration of protonizable groups [Ai] nor pKi. As gamma i approximately Ki[Ai]/(Ki + [Hi+i]2, to keep gamma i constant when pHi drops, pKi and [Ai] must increase. This may be achieved by a progressive unmasking of anionic functions, initially inaccessible in the membrane. The relative slowness of this process may explain why gamma i calculated from the initial kinetics was sometimes smaller in high than in low light, where it always equalled that measured from the steady-state amplitude at all intensities. A small deficit of [Hi+] deduced from what could have been expected from delta pHe may reflect a limited binding of protons in the membrane itself, about 1 H+ for 30--130 chlorophylls (gamma i could be between 70 and 240, more frequently around 100); these numbers varied depending on the samples, but were constant for a given preparation.