Light-dependent proton efflux from chloroplast thylakoids

This work clarifies certain aspects of proton conductivity estimates in the light compared to dark conditions for spinach chloroplast thylakoid membranes. A method is presented, with kinetic analysis to justify it, that permits the separation of the proton influx and efflux rate constants, the sum of which contributes to the measured apparent first-order rate constant for proton efflux linked to basal electron transport. Proton fluxes linked to ATP formation were not dealt with. Using this technique it was shown that dicyclohexylcarbodiimide, an inhibitor of proton channel function, completely blocks a component of proton efflux in the light, as well as partially blocking the proton efflux in the dark. Antibody against purified chloroplast coupling factor (CF1) inhibits the light-dependent proton efflux, but has no effect on the dark proton efflux. Those data are consistent with there being a proton efflux pathway through the coupling factor complex, both in the light and the dark. The H+ efflux through the coupling factor was closely correlated with adenine nucleotide exchange activity. As suggested by others, such exchange activity may be an indication of conformational changes linked to the activation of the coupling factor. A plausible model is that the positive proton electrochemical potential gradient leads to an interaction between protons and the coupling factor, causing a conformational change, which leads to adenine nucleotide exchange linked to the passage of protons through the coupling complex. The nucleotide exchange activity reflects a transition from a higher to a lower binding affinity. Some of the Gibbs free energy lost in the dissipation of the proton gradient must be conserved in the transition to the lower affinity adenine nucleotide binding form of the coupling factor protein complex.

pH effects on light dependence of the stoichiometry of proton influx and efflux to electron transport in spinach chloroplasts

Initial and steady state rates of proton transport at low light intensity have been measured and compared with steady state rates of electron transport in the pH range of 6.0-7.6 in envelope-free spinach chloroplasts. At pH 6-7, the H(+)/e(-) values computed using the initial rate of proton transport varied with light intensity, from a value of 2 at low light to almost 5 at high light. In contrast, the H(+)/e(-) values computed using the steady state rate of proton transport did not show a dependence on light intensity, having a constant value of 1.7±0.2. Likewise, at pH 7.6, the H(+)/e(-) ratio, computed using either the initial or steady state rates of proton transport did not vary with light intensity but was constant at H(+)/e(-)=1.7±0.1. Analysis of the light dependence of electron and proton transport allowed determination of (a) the quantam requirements of transport, (b) the rates of transport at light saturation, and (c) H(+)/e(-) ratios for initial and steady state proton transport. Extrapolating the initial proton transport to zero light, we found that both H(+)/photon and H(+)/e(-) values were not strongly dependent on pH, approaching a near constant value of 2.0. Using the initial rate of proton transport extrapolated to saturating light intensity we found the H(+)/e(-) ratio to be strongly pH-dependent. We suggest that internal pH controls electron transport at high light intensities. The true stoichiometry is reflected only in measurements taken at low light using the initial proton transport data. Our findings and interpretation reconcile some conflicting data in the literature regarding the pH-dependence of the H(+)/e(-) ratio and support the concept that internal pH controls noncyclic electron transport.

Factors controlling the binding of two protons per electron transferred through the ubiquinone and cytochrome b/c2 segment of Rhodopseudomonas sphaeroides chromatophores

1. On every turnover, 2.0 protons can be bound by the membrane for each single electron moving through the Q-b/c2 oxidoreductase. 2. One proton (H+II) binding reaction is, and one (H+I) is not, sensitive to antimycin. 3. The redox states of electron transfer components other than the proton binding agents can affect both the rate of proton uptake and the apparent pK values on the agents binding the protons. 4. The presence of valinomycin under certain well-defined conditions can strongly influence the value of the measured pK on the H+II binding agent.

Light-driven proton translocations in Halobacterium halobium

The purple membrane of Halobacterium halobium acts as a light-driven proton pump, ejecting protons from the cell interior into the medium and generating electrochemical proton gradient across the cell membrane. However, the type response of cells to light as measured with a pH electrode in the medium consists of an initial net inflow of protons which subsides and is then replaced by a net outflow which exponentially approaches a new lower steady state pH level. When the light turned off a small transient acidification occurs before the pH returns to the original dark level. We present experiments suggesting that the initial inflow of protons is triggered by the beginning ejection of protons through the purple membrane and that the initial inflow rate is larger than the continuing light-driven outflow. When the initial inflow has decreased exponentially to a small value, the outflow dominates and causes the net acidification of the medium. The initial inflow is apparently driven by a pre-existing electrochemical gradient across the membrane, which the cells can maintain for extended times in the absence of light and oxygen. Treatments which collapse this gradient such as addition of small concentrations of uncouplers abolish the initial inflow. The triggered inflow occurs through the ATPase and is accompanied by ATP synthesis. Inhibitors of the ATPase such as N,N'-dicyclohexylcarbodiimide (DCCD) inhibit ATP synthesis and abolish the inflow. They also abolish the transient light-off acidification, which is apparently caused by a short burst of ATP hydrolysis before the enzyme is blocked by its endogenous inhibitor. Similar transient inflows and outflows of protons are also observed when anaerobic cells are exposed to short oxygen pulses.

A partial reaction in photosystem II: reduction of silicomolybdate prior to the site of dichlorophenyldimethylurea inhibition

Silicomolybdate functions as an electron acceptor in a Photosystem II water oxidation (measured as O2 evolution) partial reaction that is 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) insensitive, that is, reduction os silicomolybdate occurs at or before the level of Q, the primary electron acceptor for Photosystem II. This report characterizes the partial reaction with the principal findings being as follows: 1. Electron transport to silicomolybdate significantly decreased room temperature Photosystem I side of the DCMU had no effect on the fluorescence level, consistent with silicomolybdate accepting electrons at or before Q. In the absence of DCMU, silicomolybdate is also reduced at a site on the Photosystem I side of the DCMU block, prior to or at plastoquinone, since the plastoquinone antagonist dibromothymoquinone (DBMIB) did not affect the electron transport rate. 3. Electron transport from water to silicomolybdate (+ DCMU) is not coupled to ATP formation, nor is there a measurable accumulation of protons within the membrane (measured by amine uptake). Silicomolybdate is not inhibitory to phosphorylation per se since neither cyclic nor post-illumination (XE) phosphorylation were inhibited. 4. Uncouplers stimulated electron transport from water to silicomolybdate in the pH range of 6 to 7, but inhibited at pH values near 8. These data are consistent with the view that when electron flow is through the abbreviated sequence of water to Photosystem II to silicomolybdate (+ DCMU), conditions are not established for the water protons to be deposited within the membrane. Experiments reported elsewhere (Fiaquinta, R.T., Dilley, R.A. and Horton, P.(19741 J. Bioenerg. 6, 167-177) and these data, are consistent with the hypothesis that electron transport between Q and plastoquinone energizes a membrane conformational change that is required to interact with the water oxication system so as to result in the deposition of water protons either within the membrane itself or within the inner oxmotic space.