The reaction center P890 is concentrated exclusively in the H-fraction, at a level of 5–7% of the bulk bacteriochlorophyll. The solubilized bacteriochlorophyll absorbing at 780 nm can be totally and irreversibly bleached by 5 mM ferricyanide. The other bacteriochlorophyll forms in the H- and L-fractions are also irreversibly bleached by ferricyanide to variable extents. P890 is the only component that can be re-reduced by ascorbate after ferricyanide oxidation. The P890 content estimated by reversible chemical bleaching agrees well with that obtained by reversible light bleaching. The different bacteriochlorophyll forms, with the exception of the 780-nm absorbing form, are relatively stable toward light bleaching. Again, only P890 is reversibly bleached by light.
Cytochromes-555 and -553 are distributed in both the H-and L-fractions, but not in the solubilized-bacteriochlorophyll G-fraction. However, only cytochromes in the H-fraction which contains all of the P890 can undergo coupled oxidation. Excitation with 20-nsec ruby-laser pulses shows that cytochrome-555 can be oxidized in 2–3 μsec by photooxidized P890, indicating that necessary conformation for rapid electron transport is retained in the subchromatophore particles.
The data on fractionation and redox reactions obtained here, together with direct kinetic measurements recently reported in the literature lend further support to the view that oxidation of these two cytochromes is mediated by the same reaction center, P890. 相似文献
The hypothesis is compatible with the concept of conformational energy conservation. 相似文献
The data on redox titration show that the red and blue shifts of 880-nm bacteriochlorophyll band represent the action of a non-identified component (C340) which has an oxidation-reduction midpoint potential close to 340 mV (n = 1) at pH 6.0–7.6. The Em of this component varies by 60 mV/pH unit between pH 7.6 and 9.2.
The results suggest that the red shift is due to the transmembrane, and the blue shift to the local intramembrane electrical field. The generation of both the transmembrane and local electrical fields is apparently governed by redox transitions of the component C340. 相似文献
The b-type cytochrome in Chromatium has an -band maximum at 560 nm and a midpoint oxidation-reduction potential of −5 mV at pH 8.0. The b-type cytochrome in Chlorobium has an -band maximum at 564 nm and an apparent midpoint oxidation-reduction potential near −90 mV.
Chromatophores isolated from both Chromatium and Chlorobium cells catalyze a photoreduction of cytochrome b that is enhanced in the presence of antimycin A. Antimycin A and 2-n-heptyl-4-hydroxyquinoline-N-oxide inhibit endogenous (but not phenazine methosulfate-mediated) cyclic photophosphorylation in Chromatium chromatophores and non-cyclic electron flow from Na2S to NADP in Chlorobium chromatophores. These observations suggest that b-type cytochromes may function in electron transport reactions in photosynthetic sulfur bacteria. 相似文献
The Em values of P870 and cytochrome c-555 increase strongly with increasing I at low values of I. The Em of cytochrome c-552 also increases with increasing I, though not so strongly. These effects probably cannot be attributed to an influence of I on the activity coefficient of a dissociable ion. We conclude that, when either P870 or cytochrome c-555 loses an electron, no specific ions (including protons) are bound or released in significant amounts, and the absolute value of the charge on the chromatophore decreases.
The Em values of the primary and secondary electron acceptors, X and Y, do not depend on I. Because these Em values have been shown previously to depend on pH, we conclude that the uptake of a proton keeps the charge on the chromatophore constant when either X or Y accepts an electron. This means that the primary and secondary electron transfer reactions in Chromatium result in a net decrease in the charge on the photosynthetic membrane. They do not result in the translocation of protons across the membrane.
The Em of the soluble flavocytochrome c-552 from Chromatium depends only weakly on I, but depends strongly on the pH. The uptake of a proton appears to keep the net charge on this cytochrome constant upon reduction. 相似文献
The polarization values of the bacteria studied are similar for the various red bands, indicating a nearly parallel orientation of the adjacent bacteriochlorophylls. 相似文献
The addition of NaCl to the chloroplast suspension produced a 40–80% increase in fluorescence yield measured at 684 nm at room temperature. The fluorescence increase was completed about 5 min after the addition. The effect saturated at 100 mM NaCl. Low-temperature fluorescence spectra showed that NaCl increased the yields of two fluorescence bands of pigment system II at 684 and 695 nm but decreased that of pigment system I at 735 nm. Similar effects on chlorophyll a fluorescence at room and at low temperatures were obtained with NaBr, NaNO3, Na2SO4, LiCl, KCl, RbCl, CsCl, NH4Cl and CH3NH3Cl.
NaCl suppressed the quantum efficiency of NADP+ reduction supported by the ascorbate-2,6-dichlorophenolindophenol (DCIP) couple as an electron donor system in the presence of 3-(3′,4′-chlorophenyl)-1,1-dimethylurea (DCMU). On the other hand, NaCl only slightly enhanced the quantum yield of photoreaction II measured by the Hill reaction with DCIP.
It is concluded that the monovalent cations tested suppressed the excitation transfer from pigment system II to pigment system I; the effects were the same as those of alkaline earth metals and Mn2+ (refs. 1, 2). 相似文献
The state of reduction of Z, which may be a quinone · protein complex near the inner (cytochrome c2) side of the membrane, appears to govern the rate at which the cyclic photosynthetic electron transport system can operate. If Z is oxidized prior to the flash-oxidation of cytochrome c2, the re-reduction of the cytochrome takes hundreds of milliseconds and no third phase of the carotenoid bandshift occurs. In contrast if Z is reduced before flash activation, the cytochrome is rereduced within milliseconds and the third phase of the carotenoid bandshift occurs. The prior reduction of Z also has a dramatic effect on the uncoupler sensitivity of the rate of electron flow; if it is oxidized prior to activation, uncoupler can stimulate the cytochrome re-reduction after several turnovers by less than tenfold, but if it is reduced prior to activation, the stimulation after several turnovers can be as dramatic as a thousandfold. The results suggest that Z plays a central role in controlling electron and proton movements in the ubiquinone cytochrome b-c2 oxido-reductase. 相似文献
The kinetics of the reaction leading to light emission were studied. A biphasic reaction could be induced by incubation of luciferin-luciferase in the absence of O2 or by incubation at pH 4.8 at o°. Injection of reduced flavin mononucleotide resulted in a flash if injected during the reaction but not after. Among metal ions, only Fe2+ showed a strong stimulation. 相似文献
Piericidin A, certain alkyl-substituted quinolinequinones, and a substituted 4,7-dioxobenzothiazole inhibit electron transfer between the primary and secondary acceptors. The sensitivity to these inhibitors, and the participation of Q and non-heme iron suggest that the secondary electron-transfer reaction resembles the reactions catalyzed by respiratory dehydrogenases.
The proton uptake that follows flash excitation does not seem to be tightly linked to the reduction of the secondary electron acceptor. It still occurs (though with decreased amplitude) in extracted chromatophores, and even in the presence of inhibitors of the secondary electron-transfer reaction. 相似文献
The fluorescence of variable yield at 750 nm at −196 °C is due to energy transfer from Photosystem II to Photosystem I. Fluorescence excitation spectra were measured at −196 °C at the minimum, FO, level and the maximum, FM, level of the emission at 750 nm. The difference spectrum, FM–FO, which represents the excitation spectrum for FV is presented as a pure Photosystem II excitation spectrum. This spectrum shows a maximum at 677 nm, attributable to the antenna chlorophyll a of Photosystem II units, with a shoulder at 670 nm and a smaller maximum at 650 nm, presumably due to chlorophyll a and chlorophyll b of the light-harvesting chlorophyll complex.
Fluorescence at the FO level at 750 nm can be considered in two parts; one part due to the fraction of absorbed quanta, , which excites Photosystem I more-or-less directly and another part due to energy transfer from Photosystem II to Photosystem I. The latter contribution can be estimated from the ratio of FO/FV measured at 692 nm and the extent of FV at 750 nm. According to this procedure the excitation spectrum of Photosystem I at −196 °C was determined by subtracting 1/3 of the excitation spectrum of FV at 750 nm from the excitation spectrum of FO at 750 nm. The spectrum shows a relatively sharp maximum at 681 nm due to the antenna chlorophyll a of Photosystem I units with probably some energy transfer from the light-harvesting chlorophyll complex.
The wavelength dependence of was determined from fluorescence measurements at 692 and 750 nm at −196 °C. is constant to within a few percent from 400 to 680 nm, the maximum deviation being at 515 nm where shows a broad maximum increasing from 0.30 to 0.34. At wavelengths between 680 and 700 nm, increases to unity as Photosystem I becomes the dominant absorber in the photochemical apparatus. 相似文献
The membrane fragments prepared from Anabaena cells grown in the presence of diphenylamine have the activities of both Photosystem 1 (NADP+ reduction with DCIP-ascorbate as electron donor) and Photosystem 2 (DCIP reduction with 1,5-diphenylcarbazide as electron donor).
The fluroescence spectra of these cells at 77°K show peaks at 696 and 731 nm and a shoulder around 687 nm. The fluorescence intensity at 687 and 696 nm is higher in these cells than in normal-Anabaena cells. 相似文献
The mean P50 values of the respiratory pigments were determined at 0 mm Hg CO2 and 28 °C (the average environmental temperature of all the species). Comparison of these data adjusted to the individual mean physiological pH indicate an increase in P50 with terrestrialization, perhaps related to the greater abundance of oxygen in the aerial than in some the aquatic habits, and the progressive elaboration of lung breathing with terrestrialization.
The Bohr shifts (Δ log P50/ΔpH) were determined (using different PCO2 values to vary pH) and were found to decrease with terrestrialization, perhaps in adaptation to an associated rise in internal PCO2 (6–8-fold between the aquatic Callinectes sapidus Rathbun and the terrestrial Cardisoma guanhumi Latreille and probably resulting from progressive gill reduction.
The temperature shifts (ΔH cal/mol) of the haemoeyanins were found and it is suggested that they diminish with increasing evironmental temperature and temperature fluctuation accompanying terrestrialization. 相似文献
Similar measurements made with azurin instead of plastocyanin gave k = 6 · 106 and approx. 2 · 107 M−1 · s−1 for reaction of reduced azurin with cytochrome ƒ and algal cytochrome respectively.
Rate constants of 115 and 80 M−1 · s−1 were found for reduction of plastocyanin by ascorbate and hydroquinone at 298 °K and pH 7.0. The rate constants for the oxidation of plastocyanin, cytochrome ƒ, Pseudomonas cytochrome c-551 and red algal cytochrome c-553 by ferricyanide were found to be between 3 · 104 and 8 · 104 M−1 · s−1.
The results are discussed in relation to photosynthetic electron transport. 相似文献
The NAD+ and NADH concentrations at equilibrium in the light-dependent reaction were determined and the oxidation-reduction potential of this couple calculated. From this value it was calculated that under these experimental conditions the energy requirement to form NADH from the succinate/fumarate couple at Eh = o V was 9.4 kcal.
Particles of R. viridis contained an active transhydrogenase, driven by either light or ATP, that was sensitive to uncouplers of oxidative phosphorylation; the light-driven reaction was insensitive to oligomycin and was inhibited by antimycin A and 2-heptyl-4-hydroxyquinone-N-oxide.
R. viridis did not grow aerobically but particles contained NADH oxidase activity that was cyanide sensitive. There was no spectroscopic evidence for cytochromes of the b-type in reduced-minus-oxidised spectra of particles or in pyridine haemochrome spectra of whole cells. 相似文献