The properties of cytochrome f and P700 in chloroplasts suspended in fluid media at sub-zero temperatures

A thermodynamic study by laser-flash photolysis of plastocyanin and cytochrome c6 oxidation by photosystem I from the green alga Monoraphidium braunii

Plastocyanin and cytochrome c6 from the green alga Monoraphidium braunii reduce the photo-oxidized algal photosystem I (PSI) reaction center chlorophyll (P700) with similar kinetics, as expected from their functional equivalence. The observed P700+ reduction rate constants show a non-linear dependence on metalloprotein concentration, which indicates a (minimal) two-step kinetic mechanism involving complex formation prior to electron transfer. The dependence of the observed rate constants on NaCl concentration suggests that the electrostatic interaction forces between the negatively charged donor proteins and PSI are repulsive at neutral pH and relatively low ionic strength (I), although attractive dipole-dipole interactions may play a role at higher ionic strengths. Activation parameters for P700+ reduction by cytochrome c6 and plastocyanin have been determined by studying the temperature dependence of the respective rate constants at varying ionic strength and pH. Changes in NaCl concentration and pH induce significant changes in the activation free energy of the overall reaction, even though the corresponding values for activation enthalpy and entropy undergo changes in opposite directions. Such a compensation effect between enthalpy and entropy is observed with both cytochrome c6 and plastocyanin. Protein concentration dependencies of the observed rate constants at different temperatures has allowed an estimate of the free energy change during complex association, as well as the activation parameters for electron transfer, according to a two-step kinetic model.

Light-induced proton transport by chloroplasts suspended in fluid media at sub-zero temperatures: kinetics and stoichiometry

1. Chloroplasts suspended in a medium containing ethanediol and water (1 : 1, v/v) at -16 degrees C show light-induced proton uptake and subsequent dark efflux. Proton uptake in continuous light showed biphasic kinetics. 2. A 1 ms flash caused a single turnover of the photochemical centres at -16 degrees C. Under the same conditions 3H+ were taken up from the external medium in the presence of methyl viologen as electron acceptor. 3. The flash-induced proton uptake was exponential and monophasic with t1/2 = 3 s. The flash-induced proton release into the thylakoid interior was biphasic, with half-times of less than 0.1 s and 3 s. The fast phase represented approximately 30% of the total release and may be correlated with the oxidation of water. 4. The half-time of reduction of cytochrome f in the dark following illumination in the presence of 2 mM NH4Cl (2.5 s) is similar to the half-time of the slow phase of proton release, suggesting a correlation between the kinetics of cytochrome f reduction and plastoquinol oxidation.

Redox potentials in hydro-organic media at normal and subzero temperatures. Ferro-ferricyanide and cytochrome c as models

Redox potentials of ferro-ferricyanide and cytochrome c were measured in water/ethylene glycol and water/dimethylsulfoxide (volume ratio from 100/0 to 50/50) between 25 and -25 degrees C. For both systems, the midpoint potential decreases in the presence of organic solvents and increases by cooling. The magnitude of these variations is larger in dimethylsulfoxide than in ethylene glycol; moreover in the same solvent mixture it is larger with ferro-ferricyanide than with cytochrome c, so that the difference between the redox potentials of these two systems can be strongly affected and even reversed. While in pure water (cacodylate buffer pH 7.0, NaCl 0.1 M) they are respectively +388 and +265 mV, in 50% dimethylsulfoxide at 25 degrees C they decrease to +112 and +208 mV. Reduction of cytochrome c by ferro-ferricyanide, in this mixture, is then expected and was indeed observed. On the other hand, as (deltaE/deltaT)T, (E being the redox potential) is higher for ferro-ferricyanide than for cytochrome c, the oxidative power of the former for the latter is expected to increase as temperature decreases. This effect was observed in 50% ethylene glycol at -16 degrees C. Organic solvents and large temperature variations appear then as powerful perturbants of redox reactions. Their effects should be taken into account in studies of redox reactions carried out in cooled hydro-organic media.

Electron transport between plastoquinone and chlorophyll Ai in chloroplasts. II. Reaction kinetics and the function of plastocyanin in situ

The light-induced reaction kinetics of electron carriers between the two light reactions were studied in spinach chloroplasts. 1. The difference spectrum of the absorbance changes of plastocyanin in situ was separated from superimposing absorbance changes by flash titration described in the preceding paper (Haehnel, W. (1973) Biochim. Biophys. Acta 305, 618-631). Relative amounts of 2 : 1 electron equivalents were observed for plastocyanin and chlorophyll a1 (P-700). 2. A balance of the electron equivalents released from reduced plastoquinone and simultaneously accepted by oxidized plastocyanin, cytochrome f and chlorophyll a1 indicated a quantitative electron transfer. Additional electron carriers between plastoquinone and light reaction I can be excluded with an accuracy of about +/-0.3 electron equivalents per light reaction II. 3. The time course of the absorbance changes of plastocyanin was measured at 584 nm with negligible interference with other absorbance changes. The reduction kinetics show an initial lag followed by a rise with a half time of about 20 ms. The redox states of plastocyanin and chlorophyll a1 during this reduction via the rate-limiting step between the light reactions and during oxidation by weak far-red light suggest a true equilibrium constant of about 20. 4. The simultaneous oxidation and reduction kinetics of plastoquinone, cytochrome f, plastocyanin and chlorophyll a1 induced by two successive groups of saturating flashes after far-red illumination were measured. The oxidation kinetics of plastocyanin and the simultaneous reduction kinetics of chlorophyll a1 after the single flashes indicate a quantitative electron transfer with a half time of 200 mus. 5. The fast reduction of chlorophyll a1 by plastocyanin showed no effect of the inhibitors 3-(3',4'-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone or of reduced phenazine methosulfate. But it was completed inhibited after KCN incubation. 6. The oxidation kinetics of cytochrome f were reinvestigated with high time resolution from the difference of absorbance changes at 554 minus 540 nm to minimize the disturbing interference with other absorbance changes. Absorbance changes measured 554 nm alone do not reflect kinetics of cytochrome f. The half time of the oxidation was faster than 40 microns. 7. The observed reaction kinetics gave evidence for a function of cytochrome f between plastoquinone and chlorophyll a1 in parallel to plastocyanin. In addition, they indicate that the greater portion of linear electron transport passes plastocyanin. The complex interaction between cytochrome f and chlorophyll a1 is discussed.

Photosynthetic electron transport and electrochromic effects at sub-zero temperatures

Spinach chloroplasts, suspended in a liquid medium containing ethyleneglycol, showed reversible absorbance changes near 700 and 518 nm due to P-700 and "P-518" in the region from -35 to -50 degrees C upon illumination. The kinetics were the same at both wavelengths, provided absorbance changes due to Photosystem II were suppressed. At both wavelengths, the decay was slowed down considerably, not only by the System I electron acceptor methyl viologen, but also by silicomolybdate. The effect of the latter compound is probably not due to the oxidation of the reduced acceptor of Photosystem I by silicomolybdate, but to the enhanced accessibility of the acceptor to some other oxidant. In the presence of both an electron donor and acceptor for System I, a strong stimulation of the extent of the light-induced absorbance increase at 518 nm was observed. The most effective donor tested was reduced N-methylphenazonium methosulphate (PMS). The light-induced difference spectrum was similar to spectra obtained earlier at room temperature, and indicated electrochromic band shifts of chlorophylls a and b and carotenoid, due to a large potential over the thylakoid membrane, caused by sustained electron transport. It was estimated that steady-state potentials of up to nearly 500 mV were obtained in this way; the potentials reversed only slowly in the dark, indicating a low conductance of the membrane. This decay was accelerated by gramicidin D. The absorbance changes were linearly proportional to the membrane potential.