Modulation of proton efflux from chloroplasts in the light by external pH

Simultaneous measurement of deltapH and electron transport in chloroplast thylakoids by 9-aminoacridine fluorescence

Electron transport and the electrochemical proton gradient across the thylakoid membrane are two fundamental parameters of photosynthesis. A combination of the electron acceptor, ferricyanide and the DeltapH indicator, 9-aminoacridine, was used to measure simultaneously electron transport rates and DeltapH solely by changes in the fluorescence of 9-aminoacridine. This method yields values for the rate of electron transport that are comparable with those obtained by established methods. Using this method a relationship between the rate of electron transport and DeltapH at various uncoupler concentrations or light intensities was obtained. In addition, the method was used to study the effect of reducing the disulfide bridge in the gamma-subunit of the chloroplast ATP synthase on the relation of electron transport to DeltapH. When the ATP synthase is reduced and alkylated, the threshold DeltapH at which the ATP synthase becomes leaky to protons is lower compared with the oxidized enzyme. Proton flow through the enzyme at a lower DeltapH may be a key step in initiation of ATP synthesis in the reduced enzyme and may be the way by which reduction of the disulfide bridge in the gamma-subunit enables high rates of ATP synthesis at low DeltapH values.

Proton slip of the chloroplast ATPase: its nucleotide dependence, energetic threshold, and relation to an alternating site mechanism of catalysis

The F-ATPase of chloroplasts couples proton flow to ATP synthesis, but is leaky to protons in the absence of nucleotides. This "proton slip" can be blocked by small concentrations of ADP or by inhibitors of the channel portion, CF0. We studied charge flow through the ATPase by flash spectrophotometry and analyzed the inhibition of proton slip by nucleotides, phosphate/arsenate, and insufficient proton motive force. The following inhibition constants (at given background concentrations) were observed: ADP, 0.2 microM (0.5 mM P(i)); ADP, 13.4 microM (no P(i)); P(i), 43 microM (1 microM ADP); GDP, 2.5 microM (0.5 mM P(i)); ATP, 2 microM. ADP and P(i) mutually lowered their respective inhibition constants. Phosphate could be replaced by arsenate. Proton slip occurred only if the proton motive force exceeded a certain threshold, similar to that for ATP synthesis. The inhibition of proton slip by ADP and GDP qualified the respective nucleotide binding sites as belonging to the subset of two (or three) potentially catalytic sites out of the total of six. We interpreted the ADP-induced transition between different conduction states of the ATPase from "slipping" to "closed" to "coupled" as a consequence of the alternating site mechanism of catalysis. Whereas the proton translocator idles in the absence of nucleotides, the high-affinity binding of the first ADP/P(i) couple to one site clutches proton flow to some (conformational) change that can only be executed after the binding of another ADP/P(i) couple to a second site. From there on these sites alternate in the catalytic cycle. An entropic machine is presented which likewise models proton slip, unisite, and multisite ATP synthesis and hydrolysis.

Effect of low growth temperature on coupling between electron transport and proton flux in Vicia faba thylakoids

Coupling between electron transport and proton flux has been compared in chloroplasts from Vicia faba (cv. Windsor) plants grown at 20 and 5°C. Proton uptake by warm-grown thylakoids was sensitive to external pH and stimulated by micromolar adenine nucleotide above pH 7.0. Electron transport was modulated by pH, adenine nucleotide and energy transfer inhibitors (triphenyltin and Hg(2+) ). By contrast, proton uptake by cold-grown thylakoids was generally lower and was insensitive to micromolar ATP. The rate of non-phosphorylating electron flow in cold-grown thylakoids was relatively insensitive to pH and Hg(2+) and was not modulated by adenine nucleotides or triphenyltin. Stimulation of electron transport by phosphorylating conditions in cold-grown thylakoids was generally lower and insensitive to pH. It is concluded that the control of proton efflux through CF(0) -CF(1) differs in thylakoids of V. faba grown at warm and cold temperatures.