Flow-force relationships in lettuce thylakoids. 1. Strict control of electron flow by internal pH

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.

Energetics of protein transport across biological membranes. a study of the thylakoid DeltapH-dependent/cpTat pathway

Among the pathways for protein translocation across biological membranes, the DeltapH-dependent/Tat system is unusual in its sole reliance upon the transmembrane pH gradient to drive protein transport. The free energy cost of protein translocation via the chloro-plast DeltapH-dependent/Tat pathway was measured by conducting in vitro transport assays with isolated thylakoids while concurrently monitoring energetic parameters. These experiments revealed a substrate-specific energetic barrier to cpTat-mediated transport as well as direct utilization of protons from the gradient, consistent with a H+/protein antiporter mechanism. The magnitude of proton flux was assayed by four independent approaches and averaged 7.9 x 10(4) protons released from the gradient per transported protein. This corresponds to a DeltaG transport of 6.9 x 10(5) kJ.mol protein translocated(-1), representing the utilization of an energetic equivalent of 10(4) molecules of ATP. At this cost, we estimate that the DeltapH-dependent/cpTat pathway utilizes approximately 3% of the total energy output of the chloroplast.

Relationship between photosynthetic electron transport and pH gradient across the thylakoid membrane in intact leaves

Under conditions (0.2% CO2; 1% O2) that allow high rates of photosynthesis, chlorophyll fluorescence was measured simultaneously with carbon assimilation at various light intensities in spinach (Spinacia oleracea) leaves. Using a stoichiometry of 3 ATP/CO2 and the known relationship between ATP synthesis rate and driving force (Delta pH), we calculated the light-dependent pH gradient (Delta pH) across the thylakoid membrane in intact leaves. These Delta pH values were correlated with the photochemical (qP) and nonphotochemical (qN) quenching of chlorophyll fluorescence and with the quantum yield of photosystem II (phiPSII). At Delta pH > 2.1 all three parameters (qP, qN, and phiPSII) changed very steeply with increasing DeltapH (decreasing pH in the thylakoid). The observed pH dependences followed hexacooperative titration curves with slightly different pKa values. The significance of the steep pH dependences with slightly different pKa values is discussed in relation to the regulation of photosynthetic electron transport in intact leaves.