Abbreviations: Rib-5-P, ribose 5-phosphate; Ribul-5-P, ribulose 5-phosphate; Ribul-1,5-P2, ribulose 1,5-diphosphate; HEPES, N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid; MES, 2-(N-morpholino)ethanesulfonic acid 相似文献
The influence of O2 concentrations on the Hill reactions in the presence of p-benzoquinone, ferricyanide, NADP+, NADP+ plus ferredoxin has been studied with isolated spinach chloroplasts.
Because of the partial reoxidation of the hydroquinone, which is depending upon the O2 concentration, it does not seem possible to localize a site of action for O2.
With ferricyanide the influence of O2 is weak. However, the rate of ferricyanide reduction is increased in the presence of O2. The observed stimulation is greater for 21% O2 than for 70% O2. Bicarbonate stimulates the ferricyanide reduction and decreases the stimulating effect of 21% O2.
O2 decreases the rate of NADP+ reduction. Ferredoxin as well as bicarbonate stimulate the NADP+ reduction and reduce the O2 inhibition.
These results seem to indicate that O2 may enter the electron transport chain at a site situated near Photosystem I and before the ferredoxin's site.
The inhibitory effect of O2 on the Hill reactions with p-benzoquinone and NADP+ is depending upon the plants' growth conditions. It is greater with plants grown under weak light. 相似文献
2. Differences in the flurescence yield of chlorophyll a in flowing and stationary suspensions of untreated chloroplasts and of the large fragments are indicative of light-induced photoreduction of the quencher Q of chlorophyll a, associated with pigment System 2 (chlorophyll a2). The relatively low constant fluorescence yield of chlorophyll a in the small fragments indicates the absence of fluorescent chlorophyll a2 from these fragments and suggests that the low fluorescence is due to chlorophyll a, associated with pigmen System 1 (chlorophyll a1). The ratio of the fluorescence yields of chlorophyll a1 and chlorophyll a2 is 0.45:1. In the large particles the concentration ratio of pigment System 1 and System 2 is 1:3.
3. The efficiencies of quanta absorbed at 673, 683 and 705 nm for NADP+ reduction and P 700 oxidation in untreated chloroplasts and chloroplast fragments indicate that digitonin treatment results in a separation of System 2 from System 1 in the small fragments. Sonication does not cause such a separation. Under the conditions used P 700 oxidation and NADP+ reduction in the small fragments separated after digitonin treatment, occurred with maximal efficiency of 0.7 to 1.0 and 0.7, respectively.
4. The constancy of the fluorescence yield of chlorophyll a1 in the small fragments, under conditions at which P 700 is oxidized and NADP+ is reduced, is interpreted as evidence either for the hypothesis that the fluorescence of chlorophyll a1 is controlled by the redox state of the primary photoreductant XH, or alternatively for the hypothesis that energy transfer from fluorescent chlorophyll a1 to P 700 goes via an intrinsically weak fluorescent, still unknown, chlorophyll-like pigment.
5. The low-temperature emission band around 730 nm is argued not to be due to excitation by System 1 only; the relatively large half width of the band, as compared to the emission bands at 683 and 696 nm, suggests that it is possibly due to overlapping emission bands of different pigments. 相似文献
(2) A light dependent increase in the Mg2+ content of the stroma was detected when chloroplasts were subjected to osmotic shock, amounting to 26 nmol/mg chlorophyll. Furthermore, a rapid and reversible light-dependent efflux of Mg2+ has been observed in intact chloroplasts when the divalent cation ionophore A 23 187 was added, indicating a light-dependent transfer of about 60 nmol of Mg2+ per mg chlorophyll from the thylakoid membranes to the stroma.
(3) CO2 fixation, but not phosphoglycerate reduction, could be completely inhibited when A 23 187 was added to intact chloroplasts in the absence of external Mg2+. If Mg2+ was then added to the medium, CO2 fixation was restored. Half of the maximal restoration was achieved with about 0.2 mM Mg2+, which is calculated to reflect a Mg2+ concentration in the stroma of 1.2 mM. The further addition of Ca2+ strongly inhibits CO2 fixation.
(4) The results suggest that illumination of intact chloroplasts causes an increase in the Mg2+ concentration of 1–3 mM in the stroma. Compared to the total Mg2+ content of chloroplasts, this increase is very low, but it appears to be high enough to have a possible function in the light regulation of CO2 fixation. 相似文献
The influence of very low O2 concentration on the NADP+ reduction by isolated spinach chloroplasts has been studied.
The results show that in the presence of very low O2 concentration (< 0.3%) NADP+ reduction is partially inhibited. This inhibition may be partially reversed under some conditions, especially when, in spite of the presence of an O2 trap (glucose plus glucose oxidase (EC 1.1.3.4)) an O2 evolution is observed. 相似文献
The rate of CO2 fixation is strongly increased by addition of Calvin cycle intermediates if the catalase activity of the preparation is low. However, at high catalase activity addition of Calvin cycle intermediates remains without effect. Obviously the hydrogen peroxide formed at low catalase activity leads to a loss of Calvin cycle substrates which reduces the rate of CO2 fixation.
3-Phosphoglycerate-dependent O2-evolution is not influenced by hydrogen peroxide at a concentration (5 · 10−4 M) which inhibits CO2 fixation almost completely. Therefore the inhibition site of hydrogen peroxide cannot be at the step of 3-phosphoglycerate reduction. Dark CO2 fixation of lysed chloroplasts in a hypotonic medium is not or only slightly inhibited by hydrogen peroxide (2.5 · 10−4 M), if ribulose-1,5-diphosphate, ribose 5-phosphate or xylulose 5-phosphate were added as substrates. However, there is a strong inhibition of CO2 fixation by hydrogen peroxide, if fructose 6-phosphate together with triose phosphate are used as substrates. This indicates that hydrogen peroxide interrupts the Calvin cycle at the transketolase step, leading to a reduced supply of the CO2-acceptor ribulose 1,5-diphosphate. 相似文献
H2O → IIbhv → C550 → cyt. b559 → PC → IIahv → Fd → NADP+
Photoreaction IIb involves an electron transfer from water to C550 that does not require plastocyanin and is the first known System II photoreaction resistant to inhibition by 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) and o-phenanthroline. Cytochrome b559 is reduced by C550 in a reaction that is readily inhibited by DCMU or o-phenanthroline. Thus, the site of DCMU (and o-phenanthroline) inhibition of System II appears to lie between C550 and cytochrome b559. Photoreaction IIa involves an electron transfer from cytochrome b559 and plastocyanin to ferredoxin-NADP+. 相似文献
Optimum reaction conditions were developed and the properties of the isolated particles investigated. Maximal activities are obtained when the sucrose concentration is maintained below 0.4 M; the pH optimum with Tricine buffer is between 7.8–8.1; and at least 30 mM Cl− is required. Red actinic light (wavelength >620 nm) with an intensity of 106 ergs/cm2 per s is required for saturation.
Ferrodoxin and ferredoxin-NADP+ oxidoreductase are lost from the particles during the preparatory procedures and maximum photochemical activity is attained only when they are added back in balanced amounts. Stimulatory effects of added plastocyanin and cytochrome c-553 are noted only with particles having an initially low photochemical activity. 相似文献
1. 1. The effect of the Mg2+ concentration on the CO2 fixation activity in situ in isolated and intact spinach chloroplasts upon suspension in hypotonic medium was examined. CO2 fixation in the dark was activated 25–100 fold by 20 mM Mg2+ in the presence of added ATP plus either ribulose 5-phosphate or ribose 5-phosphate. 20 mM Mg2+-stimulated fixation only 2–3 fold in the presence of the substrate of fixation, ribulose 1,5-diphosphate. The highest Mg2+-stimulated rate of fixation in the dark observed with chloroplasts was 480 μmoles CO2 fixed per mg chlorophyll per h.
2. 2. The concentration of bicarbonate at half of the maximal velocity (apparent Km) during the Mg2+-stimulated fixation of CO2 was 0.4 mM in the presence of ATP plus ribose 5-phosphate and 0.6 mM with ribulose 1,5-diphosphate.
3. 3. Dithioerythritol or light enhanced Mg2+-stimulated CO2 fixation 1–3 fold in the presence of ATP plus ribose 5-phosphate but not ribulose 1,5-diphosphate.
4. 4. These results indicate that Mg2+ fluxes in the stroma of the chloroplast could control the activity of the phosphoribulokinase with a lesser effect on the ribulosediphosphate carboxylase. An increase in Mg2+ of 6–10 mM in the stroma region of the chloroplast would be enough to activate CO2 fixation during photosynthesis.
A sulfite reductase was purified from spinach leaves. Broken chloroplasts and sulfite reductase reduced sulfite to sulfide in the light when ferredoxin was added. NADP+ was not required for this reduction.
The results suggest that in chloroplasts a sulfate activated by ATP (phosphoadenosine phosphosulfate) is reduced to sulfite by a sulfhydryl compound and that sulfite is reduced to sulfide by a ferredoxin-dependent sulfite reductase. 相似文献
At moderate light intensities the steady state rate of the [ascorbate + DCIPH2 → NADP+] reaction (A) in the presence of DCMU and added ferredoxin can be increased more than 3 times when saturating amounts of plastocyanin and ferredoxin-NADP reductase are added to the chloroplasts. Similarly, the steady-state rate of the [H2O → DCIP] Hill reaction (B) is increased about 3-fold by added MgCl2 and plastocyanin, but added ferredoxin or ferredoxin-NADP reductase have no effect on this reaction. Plastocyanin appears to be the electron transport component which couples to DCIP, either in the oxidized or in the reduced form, in the reaction media. The steady-state rate of the [H2O → NADP+] reaction (C) with saturating amounts of ferredoxin can be further increased more than 3-fold when MgCl2, plastocyanin and ferredoxin-NADP reductase are added. 相似文献
Between 525 and 690 mμ, the phosphorylation yield for the normal system is constant ( = 0.15 ATP/hv), maintaining a constant P/2e ratio of unity. The P/2e ratios indicate a tight coupling between phosphorylation and electron transport encompassing a single phosphorylation site for the transfer of two electrons.
Between 525 and 680 mμ, the phosphorylation yield for the donor system is constant ( approx. 0.04 ATP/hv), maintaining a P/2e ratio of approx. 0.5. At longer wavelengths (>690 mμ) the phosphorylation yield of the donor system rises ( approx. 0.07–0.08 ATP/hv) concomitant with the rise in the yield of electron flow.
These experiments suggest the possibility that two types of phosphorylation processes operate in chloroplasts, (1) a short-wavelength process coupled to the normal O2-evolving activity, and (2) a long-wavelength process coupled to the electron-donor activity of reagents such as DCIP. 相似文献
2. The chloroplasts isolated in MES-sorbitol media exhibited induction phenomena which were similar to those shown by chloroplasts isolated in orthophosphate-sugar mixtures. Added ribose 5-phosphate shortened the lags which preceded the attainment of maximal rates of CO2 fixation and O2 evolution. O2 evolution reached its maximum rate almost immediately in the presence of 3-phosphoglycerate. Induction periods were shortened by pre-illumination of the parent tissue prior to separation of the chloroplasts.
3. In the absence of added substrate (other than CO2) lags exhibited by chloroplasts isolated in MES-sorbitol were shorter than those observed with chloroplasts prepared in orthophosphate-sorbitol. These shorter lags could be extended by briefly exposing the chloroplasts to sugar media containing orthophosphate, malate or acetate or to Tris-NaCl.
4. The results are discussed in relation to photosynthetic induction phenomena and current methods of chloroplast isolation. 相似文献
In the absence of an artificial, low-potential electron acceptor, addition of ascorbate stimulates photophosphorylation in isolated chloroplasts. This effect of ascorbate is abolished by superoxide dismutase, indicating that both the superoxide free radical ion and ascorbate are responsible for the stimulation of photophosphorylation. In this case, the superoxide free radical ion seems to be formed during the autooxidation of an endogenous electron acceptor.
In the presence of ferredoxin and NADP+, photophosphorylation in isolated chloroplasts stops as soon as the available NADP+ is fully reduced. If ascorbate is present in this system, however, a linear rate of photophosphorylation is maintained in spite of the fact, that NADP+ is fully reduced. This ascorbate-mediated photophosphorylation again is abolished by superoxide dismutase.
During the catalysis of this oxygen-dependent photophosphorylation, ascorbate consumption is not observed. These findings support the idea, that in chloroplasts ascorbate together with the superoxide free radical ion may function in providing additional ATP by an oxygen-dependent photophosphorylation. 相似文献