Stimulation of photophosphorylation by low concentrations of uncoupling amines

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Two types of ammonium uncoupling in pea chloroplasts

The effect of ammonium on ATP synthesis, electron transfer, and light-induced uptake of hydrogen ions in pea chloroplasts was studied. It is shown that the dependence of these reactions on ammonium concentration could be due to effects of two different uncoupling processes. The first process is induced by low ammonium concentrations (<0.2 mM); the second one is observed in the NH(4)Cl concentration interval of 0.5-5.0 mM. The first type of uncoupling is stimulated by palmitic acid or by N,N'-dicyclohexylcarbodiimide, while the second is stimulated by chloroplast thylakoid swelling caused by energy-dependent osmotic gradients. In the presence of the fluorescent dye sulforhodamine B, which does not penetrate through the cell membrane, this swelling causes the dye to enter the lumens. It is supposed that ammonium activates two different routes of cation leakage from the lumen. The first route involves channel proteins, while the second is a mechanosensitive pore that opens in response to osmotic gradients.

Synergism of ammonium and palmitic acid in uncoupling of electron transfer and ATP synthesis in chloroplasts

Uncoupling by ammonium of electron transfer and ATP synthesis during linear transfer of electrons from water to photosystem 1 acceptors was studied in pea chloroplasts. It was shown that 40 microM palmitic acid decreased several-fold the ammonium concentrations necessary for 50% inhibition of ATP synthesis. The protonophore carbonyl cyanide m-chlorophenylhydrazone has no such property. The enhancement by palmitate of ammonium-induced uncoupling is accompanied by acceleration of basal electron transfer and decrease in the photoinduced uptake of hydrogen ions (H+). In the absence of ammonium, palmitate has no effect on basal transport and stimulates uptake of hydrogen ions. This means that in the case of combined action of palmitate and ammonium an additional leakage of H+ takes place, resulting in dissipation of the pH gradient. Synergic action of two metabolites, free fatty acid and ammonium, is supposed to provide for functioning of a system of mild regulation of energy coupling processes in native plant cell chloroplasts. Possible mechanisms of synergism are discussed.

Spin-probes designed for measuring the intrathylakoid pH in chloroplasts

Nitroxide radicals are widely used as molecular probes in different fields of chemistry and biology. In this work, we describe pH-sensitive imidazoline- and imidazolidine-based nitroxides with pK values in the range 4.7-7.6 (2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, 4-amino-2,2,5,5-tetramethyl-2,5-dihydro-1H-imidazol-1-oxyl, 4-dimethylamino-2,2-diethyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, and 2,2-diethyl-5,5-dimethyl-4-pyrrolidyline-1-yl-2,5-dihydro-1H-imidazol-1-oxyl), which allow the pH-monitoring inside chloroplasts. We have demonstrated that EPR spectra of these spin-probes localized in the thylakoid lumen markedly change with the light-induced acidification of the thylakoid lumen in chloroplasts. Comparing EPR spectrum parameters of intrathylakoid spin-probes with relevant calibrating curves, we could estimate steady-state values of lumen pHin established during illumination of chloroplasts with continuous light. For isolated bean (Vicia faba) chloroplasts suspended in a medium with pHout=7.8, we found that pHin approximately 5.4-5.7 in the state of photosynthetic control, and pHin approximately 5.7-6.0 under photophosphorylation conditions. Thus, ATP synthesis occurs at a moderate acidification of the thylakoid lumen, corresponding to transthylakoid pH difference DeltapH approximately 1.8-2.1. These values of DeltapH are consistent with a point of view that under steady-state conditions the proton gradient DeltapH is the main contributor to the proton motive force driving the operation of ATP synthesis, provided that stoichiometric ratio H+/ATP is n> or =4-4.7.

Effects of polyamines on the functionality of photosynthetic membrane in vivo and in vitro

The three major polyamines are normally found in chloroplasts of higher plants and are implicated in plant growth and stress response. We have recently shown that putrescine can increase light energy utilization through stimulation of photophosphorylation [Ioannidis et al., (2006) BBA-Bioenergetics, 1757, 821-828]. We are now to compare the role of the three major polyamines in terms of chloroplast bioenergetics. There is a different mode of action between the diamine putrescine and the higher polyamines (spermidine and spermine). Putrescine is an efficient stimulator of ATP synthesis, better than spermidine and spermine in terms of maximal % stimulation. On the other hand, spermidine and spermine are efficient stimulators of non-photochemical quenching. Spermidine and spermine at high concentrations are efficient uncouplers of photophosphorylation. In addition, the higher the polycationic character of the amine being used, the higher was the effectiveness in PSII efficiency restoration, as well as stacking of low salt thylakoids. Spermine with 50 microM increase F(V) as efficiently as 100 microM of spermidine or 1000 microM of putrescine or 1000 microM of Mg(2+). It is also demonstrated that the increase in F(V) derives mainly from the contribution of PSIIalpha centers. These results underline the importance of chloroplastic polyamines in the functionality of the photosynthetic membrane.

The chloroplast Tat pathway utilizes the transmembrane electric potential as an energy source

The thylakoid membrane, located inside the chloroplast, requires proteins transported across it for plastid biogenesis and functional photosynthetic electron transport. The chloroplast Tat translocator found on thylakoids transports proteins from the plastid stroma to the thylakoid lumen. Previous studies have shown that the chloroplast Tat pathway is independent of NTP hydrolysis as an energy source and instead depends on the thylakoid transmembrane proton gradient to power protein translocation. Because of its localization on the same membrane as the proton motive force-dependent F(0)F(1) ATPase, we believed that the chloroplast Tat pathway also made use of the thylakoid electric potential for transporting substrates. By adjusting the rate of photosynthetic proton pumping and by utilizing ionophores, we show that the chloroplast Tat pathway can also utilize the transmembrane electric potential for protein transport. Our findings indicate that the chloroplast Tat pathway is likely dependent on the total protonmotive force (PMF) as an energy source. As a protonmotive-dependent device, certain predictions can be made about structural features expected to be found in the Tat translocon, specifically, the presence of a proton well, a device in the membrane that converts electrical potential into chemical potential.

Detecting localized proton currents in photophosphorylation by procaine inhibition of the transthylakoid pH-gradient

The relationship between the transthylakoid pH-gradient, delta pH, and the velocity of photophosphorylation, Vp, in thylakoid membranes from spinach was investigated using the local anesthetic amine procaine as inhibitor of delta pH. When delta pH was driven by Photosystem (PS) II+I-dependent electron flow, passing through the cytochrome b6/f complex, inhibition by procaine was accompanied by an increase of ATP formation. It appeared that procaine allowed for values of Vp similar to those in controls (without procaine) at a significantly lower delta pH than in the controls. In contrast, when delta pH was driven by cyclic electron flow around PS I or by PS-II+I-dependent electron flow via a bypass around the cytochrome b6/f complex, or by PS II alone, procaine simultaneously caused an inhibition of delta pH and a decrease of ATP formation. Inhibition of delta pH by procaine did not induce an electrical membrane potential gradient that otherwise may have energetically compensated for the observed decline of delta pH. The electron flow capacity was unaffected by procaine. However, inhibition of delta pH did not significantly relax pH-dependent control of electron flux. Procaine accelerated ATP hydrolysis by pre-activated thylakoid ATPase to rates which were observed in the presence of uncouplers and had no direct effect on the activation state of the ATPase. The shift in the relationship between delta pH and Vp towards lower delta pH persisted in thermodynamic equilibrium between the phosphorylation potential and delta pH. The data indicated that the unconventional effect of procaine on photophosphorylation may be related to effects on proton translocation at the cytochrome b6/f complex and that a localized protonic coupling may occur between cytochrome b6/f and thylakoid-ATP-synthase complexes.

Photosynthetic control, "energy-dependent" quenching of chlorophyll fluorescence and photophosphorylation under influence of tertiary amines

The effects of the tertiary amines tetracaine, brucine and dibucaine on photophosphorylation and control of photosynthetic electron transport in isolated chloroplasts of Spinacia oleracea were investigated. Tertiary amines inhibited photophosphorylation while the related electron transport decreased to the rates, observed under non-phosphorylating conditions. Light induced quenching of 9-aminoacridine fluorescence and uptake of (14)C-labelled methylamine in the thylakoid lumen declined in parallel with photophosphorylation, indicating a decline of the transthylakoid proton gradient. In the presence of ionophoric uncouplers such as nigericin, no effect of tertiary amines on electron transport was seen in a range of concentration where photophosphorylation was inhibited. Under the influence of the tertiary amines tested, pH-dependent feed-back control of photosystem II, as indicated by energy-dependent quenching of chlorophyll fluorescence, was unaffected or even increased in a range of concentration where 9-aminoacridine fluorescence quenching and photophosphorylation were inhibited. The data are discussed with respect to a possible involvement of localized proton flow pathways in energy coupling and feed-back control of electron transport.