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1.
Nearest neighbor analysis of immunocytolocalization experiments indicates that the enzymes glyceraldehyde-3-P dehydrogenase, triose-P isomerase and aldolase are located close to one another in the pea leaf chloroplast stroma, and that aldolase is located close to sedoheptulose bisphosphatase. Direct transfer of the triose phosphates between glyceraldehyde-3-P dehydrogenase and triose-P isomerase, and from glyceraldehyde-3-P dehydrogenase and triose-P isomerase to aldolase, is then a possibility, as is direct transfer of sedoheptulose bisphosphate from aldolase to sedoheptulose bisphosphatase. Spatial organization of these enzymes may be important for efficient CO 2 fixation in photosynthetic organisms. In contrast, there is no indication that fructose bisphosphatase is co-localized with aldolase, and direct transfer of fructose bisphosphate from aldolase to fructose bisphosphatase seems unlikely. 相似文献
2.
A mechanism is proposed for a feed-forward control of photosynthetic sucrose synthesis, which allows withdrawal of carbon from the chloroplast for sucrose synthesis to be coordinated with the rate of carbon fixation. (a) Decreasing the rate of photosynthesis of spinach ( Spinacia oleracea, U.S. hybrid 424) leaf discs by limiting light intensities or CO 2 concentrations leads to a 2-to 4-fold increase in fructose 2,6-bisphosphate. (b) This increase can be accounted for by lower concentrations of metabolites which inhibit the synthesis of fructose 2,6-bisphosphate, such as dihydroxyacetone phosphate and 3-phosphoglycerate. (c) Thus, as photosynthesis decreases, lower levels of dihydroxyacetone phosphate should inhibit the cytosolic fructose bisphosphatase via simultaneously lowering the concentration of the substrate fructose 1,6-bisphosphate, and raising the concentration of the inhibitor fructose 2,6-bisphosphate. 相似文献
3.
The mechanisms regulating transient photosynthesis by soybean ( Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO 2 assimilation attributable to a lightfleck increased. This enhanced lightfleck CO 2 assimilation was accounted for by a greater postillumination CO 2 fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase
fructose 1,6-bisphosphatase (EC 3.1.3.11)
- LUE
lightfleck use efficiency
- P-glycerate
3-phosphoglycerate
- PICF
post-illumination CO 2 fixation
- Ru5P kinase
ribulose 5-phosphate kinase (EC 2.7.1.19)
- RuBP
ribulose 1,5-bisphosphate
- rubisco
ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39)
- SBpase
sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37) 相似文献
4.
Bioengineering of photoautotrophic microalgae into CO 2 scrubbers and producers of value‐added metabolites is an appealing approach in low‐carbon economy. A strategy for microalgal bioengineering is to enhance the photosynthetic carbon assimilation through genetically modifying the photosynthetic pathways. The halotolerant microalgae Dunaliella posses an unique osmoregulatory mechanism, which accumulates intracellular glycerol in response to extracellular hyperosmotic stresses. In our study, the Calvin cycle enzyme sedoheptulose 1,7‐bisphosphatase from Chlamydomonas reinhardtii (CrSBPase) was transformed into Dunaliella bardawil, and the transformant CrSBP showed improved photosynthetic performance along with increased total organic carbon content and the osmoticum glycerol production. The results demonstrate that the potential of photosynthetic microalgae as CO 2 removers could be enhanced through modifying the photosynthetic carbon reduction cycle, with glycerol as the carbon sink. 相似文献
5.
The substrate specificity of purified fructose bisphosphatase form B from Synechococcus leopoliensis (EC 3.1.3.11; cf. K-P Gerbling, M Steup, E Latzko 1985 Eur J Biochem 147: 207-215) has been investigated. Of the phosphate esters tested only fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate were hydrolyzed by the enzyme. Both sugar bisphosphates were cleaved at the carbon 1-ester. Fructose- and sedoheptulose bisphosphate stabilized the activated ( i.e. tetrameric) state of the enzyme and prevented a slow inactivation that is observed in the absence of sugar bisphosphates. With the activated enzyme, kinetic constants (half-saturating substrate concentrations, maximal reaction velocity, and the catalytical constant) were similar for both fructose- and sedoheptulose bisphosphate. The data suggest that fructose bisphosphatase form B from Synechococcus leopoliensis can catalyze both bisphosphatase reactions within the reductive pentose phosphate cycle. 相似文献
6.
A model of the C
3 photosynthetic system is developed which describes the sensitivity of the steadystate rate of carbon dioxide assimilation to changes in the activity of several enzymes of the system. The model requires measurements of the steady-state rate of carbon dioxide assimilation, the concentrations of several intermediates in the photosynthetic system, and the concentration of the active site of ribulose 1,5-bisphosphate carboxyalse/oxygenase (Rubisco). It is shown that in sunflowers ( Helianthus annuus L.) at photon flux densities that are largely saturating for the rate of photosynthesis, the steady-stete rate of carbon dioxide assimilation is most sensitive to Rubisco activity and, to a lesser degree, to the activities of the stromal fructose, 6-bisphosphatase and the enzymes catalysing sucrose synthesis. The activities of sedoheptulose 1,7-bisphosphatase, ribulose 5-phosphate kinase, ATP synthase and the ADP-glucose pyrophosphorylase are calculated to have a negligible effect on the flux under the high-light conditions. The utility of this analysis in developing simpler models of photosynthesis is also discussed.Abbreviations
c
i
intercellular CO 2 concentration
-
C
infP
supJ
control coefficient for enzyme P with respect to flux J
- DHAP
dihydroxyacetonephosphate
- E4P
erythrose 4-phosphate
- F6P
fructose 6-phosphate
- FBP
fructose 1,6-bisphosphate
- FBPase
fructose 1,6-bisphosphatase
- G3P
glyceraldehyde 3-phosphate
- G1P
glucose 1-phosphate
- G6P
glucose 6-phosphate
- Pi
inorganic phosphate
- PCR
photosynthetic carbon reduction
- PGA
3-phosphoglyceric acid
- PPFD
photosynthetically active photon flux density
-
R
n
J
response coefficient for effector n with respect to flux J
- R5P
ribose 5-phosphate
- Rubisco
ribulose 1,5-bisphosphate carboxylase/oxygenase
- Ru5P
ribulose 5-phosphate
- RuBP
ribulose 1,5-bisphosphate
- S7P
sedoheptulose 7-phosphate
- SBP
sedoheptulose 1,7-bisphosphate
- SBPase
sedoheptulose 1,7-bisphosphatase
- SPS
sucrose-phosphate synthase
- Xu5P
xylulose 5-phosphate
-
n
P
elasticity coefficient for effector n with respect to the catalytic velocity of enzyme P
This research was funded by an Australian Research Council grant to I.E.W. and was undertaken during a visity by K.A.M. to the James Cook University of North Queensland. The expert help of Glenys Hanley and Mick Kelly is greatly appreciated. 相似文献
7.
Isolated mesophyll protoplasts from Valerianella locusta L. were subjected to freeze-thaw cycles. Subsequently, steady-state pool sizes of 14C-labeled intermediates of the photosynthetic carbon reduction cycle were determined by high performance liquid chromatography. Protoplasts in which CO 2 fixation was inhibited by preceding freezing stress, showed a strong increase in the proportion of fructose-1,6-bisphosphate, sedoheptulose-1,7-bisphosphate and triose phosphates. These results indicate an inhibition of the activities of stromal fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase. Furthermore, freezing stress caused a slight increase in the proportion of labeled ribulose-1,5-bisphosphate, which may be based on an inhibition or ribulose bisphosphate carboxylase activity. It was shown earlier (Rumich-Bayer and Krause 1986) that freezing-thawing readily affects photosynthetic CO 2 assimilation independently of thylakoid inactivation. The present results are interpreted in terms of an inhibition of the light-activation system of the photosynthetic carbon reduction cycle, caused by freezing stress.Abbreviations FBP
Fructose-1,6-bisphosphate
- HMP
Hexose Monophosphates
- PGA
3-phosphoglycerate
- PMP
Pentose Monophosphates
- RBP
Ribulose-1,5-bisphosphate
- SBP
Sedoheptulose-1,7-bisphosphate
- TP
Triose Phosphates 相似文献
8.
Spinach chloroplast fructose bisphosphatase (EC 3.1.3.11.) exists in both oxidised and reduced forms. Only the latter has the kinetic properties that allow it to function at physiological concentrations of fructose 1,6-bisphosphate and Mg 2+. Illumination of freshly prepared type A chloroplasts causes a conversion of oxidised to reduced enzyme. The rate of this conversion does not limit the rate of CO 2 fixation. In the dark the reduced enzyme partially reverts back to the oxidised form. If catalase is omitted from the reaction medium the rate of CO 2 fixation by chloroplasts is decreased and seems to be limited by the rate of conversion of the enzyme to the reduced form. The physiological significance of the light dependent generation of dithiol compounds (such as thioredoxin) within chloroplasts is discussed. 相似文献
9.
When envelope-free spinach chloroplasts are incubated with stromal protein, catalytic NADP, catalytic ADP, radioactive bicarbonate and fructose 1,6-bisphosphate, 14CO 2 fixation starts immediately upon illumination but oxygen evolution is delayed. The delay is increased by the addition of fructose 6-phosphate and by a variety of factors known (or believed) to increase fructose bisphosphatase activity (such as dithiothreitol, more alkaline pH, higher [Mg] and antimycin A). Conversely, the lag can be decreased or eliminated by the addition of an ATP-generating system. Bearing in mind the known inhibition, by ADP, of sn-phospho-3-glycerate (3-phosphoglycerate) reduction it is concluded that the lag in O 2 evolution results from the production of ribulose 5-phosphate from fructose bisphosphate and that this in turn inhibits the reoxidation of NADPH by adversely affecting the ADP/ATP ratio. The results are discussed in their relation to the mode of action of antimycin A and to regulation of the reductive pentose phosphate pathway. 相似文献
10.
The effects of reduced osmotic potential on the photosynthetic carbon reduction cycle were investigated by monitoring photosynthetic processes of spinach ( Spinacia oleracea L. var. Long Standing Bloomsdale) chloroplasts exposed to increased assay medium sorbitol concentrations. CO 2 assimilation was found to be inhibited at 0.67 molar sorbitol by about 60% from control rates at 0.33 molar sorbitol. This level of stress inhibition was greater than that affecting the reductive phase of the cycle; glycerate 3-phosphate reduction was inhibited at 0.67 molar by 27 to 40%. Sorbitol (0.67 molar) inhibited the rate of O 2 evolution at saturating and limiting concentrations of NaHCO 3, and extended the lag phase of O 2 evolution. This indicated that factors which are rate-limiting to the photosynthetic process are adversely affected by reduced osmotic potential. Analysis of photosynthetic products following CO2 fixation in 0.33 molar sorbitol and 0.67 molar sorbitol indicated that reduced osmotic potential facilitated increases in the levels of fructose 1,6-bisphosphate and triose phosphates with reductions in glucose 6-phosphate and fructose 6-phosphate, implicating fructose 1,6-bisphosphatase as a site of osmotic stress. Osmotic inhibition of the reductive portion (glycerate 3-phosphate to triose phosphate) of the photosynthetic carbon reduction cycle was partially attributed to feedback inhibition by the product, triose phosphate, on glycerate 3-phosphate reduction. A saturating concentration of ribose 5-phosphate partially overcame osmotic inhibition of CO2-supported O2 evolution, indicating another but apparently less severe site of stress inhibition in the sequence of ribose 5-phosphate to glycerate 3-phosphate. 相似文献
11.
Thiol-treated spinach (Spinacia oleracea) chloroplast fructose bisphosphatase is powerfully inhibited by Ca2+ non-competitively with respect to its substrate, fructose 1,6-bisphosphate. 500 microM-Ca2+ causes virtually complete inhibition and the Ki is 40 microM. Severe inhibition of sedoheptulose bisphosphatase is also caused by Ca2+. A role for Ca2+ in regulation of the Calvin cycle in spinach chloroplasts is proposed. 相似文献
12.
Bundle sheath strands capable of assimilating up to 68 μmoles CO 2 per mg chlorophyll per hr in the dark have been isolated from fully expanded leaves of Zea mays L. This dark CO 2-fixing system is dependent on exogenous ribose-5-phosphate, ADP or ATP, and Mg 2+ for maximum activity. The principal product of dark fixation in this system is 3-phosphoglycerate, indicating that the CO 2-fixing reaction is mediated by ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). The rate of dark CO 2 uptake in the strands in the presence of saturating levels of ribose-5-phosphate plus ADP is inhibited by oxygen. The inhibitory effect of oxygen is rapidly and completely reversible, and is relieved by increased levels of CO 2. Glycolate is synthesized in this dark system in the presence of [U- 14C]ribose-5-phosphate, ADP, oxygen, and an inhibitor of glycolate oxidase (EC 1.1.3.1). Glycolate formation is completely abolished by heating the strands, and the rate of glycolate synthesis is markedly reduced by either lowering the oxygen tension or increasing the level of CO 2.These results, obtained with intact cells in the absence of light, indicate that the direct inhibitory effect of oxygen on photosynthesis is associated with photosynthetic carbon metabolism, probably at the level of ribulose-1,5-bisphosphate carboxylase, and not with photophosphorylation or photosynthetic electron transport. Furthermore, the findings indicate that the synthesis of glycolate from exogenous substrate can readily occur in the absence of photosynthetic electron transport, an observation consistent with the ribulose-1, 5-bisphosphate “oxygenase” scheme for glycolate formation during photosynthesis. 相似文献
13.
A range of studies of C 3 plants have shown that there is a change in both the carbon flux and the pattern of nitrogen allocation when plants are grown under enhanced CO 2. This paper examines evidence that allocation of nitrogen both to and within the photosynthetic system is optimised with respect to the carbon flux. A model is developed which predicts the optimal relative allocation of nitrogen to key enzymes of the photosynthetic system as a function of CO 2 concentration. It is shown that evidence from flux control analysis is broadly consistent with this model, although at high nitrogen and under certain conditions at low nitrogen experimental data are not consistent with the model. Acclimation to enhanced CO 2 is also assessed in terms of resource allocation between photosynthate sources and sinks. A means of assessing the optimisation of this source-sink allocation is proposed, and several studies are examined within this framework. It is concluded that C 3 plants probably possess the genetic feedback mechanisms required to efficiently smooth out any imbalance within the photosynthetic system caused by a rise in atmospheric CO 2.Abbreviations A
net rate of CO 2 assimilation
-
c
i
intercellular CO 2 concentration
- C R
A
flux control coefficient for Rubisco with respect to flux A
- FBPase
fructose 1,6-bisphosphatase
- k app
apparent catalytic rate constant
- PCO
photorespiratory carbon oxidation
- PCR
photosynthetic carbon reduction
- PPFD
photosynthetically active photon flux density
- Rubisco
ribulose 1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate
- Ru5P
ribulose 5-phosphate
- SBPase
sedoheptulose 1,7-bisphosphatase 相似文献
14.
Phosphorus-deficient spinach plants were grown by transferring them to nutrient solutions without PO 4. Photosynthetic rates were measured at a range of intercellular CO 2 partial pressures from 50–500 bar and then the leaves were freeze-clamped in situ to measure ribulose bisphosphate carboxylase (Rubisco) activity and metabolite concentrations. Compared with control leaves, deficient leaves had significantly lower photosynthetic rates, percentage activation of Rubisco, and amounts of ribulose bisphosphate and 3-phosphoglycerate at all CO 2 partial pressures. After feeding 10 mM PO 4 to the petioles of detached deficient leaves, all these measurements increased within 2 hours. At atmospheric CO 2 partial pressure the photosynthetic rate was stimulated in 19 mbar O 2 compared with 200 mbar. At higher CO 2 partial pressures this stimulation was less but the percentage stimulation in deficient leaves was no different from controls in either CO 2 partial pressure. It was concluded that phosphorus deficiency affects both Rubisco activity and the capacity for ribulose bisphosphate regeneration, and possible causes are discussed.Abbreviations A
CO 2 assimilation rate
- C i
intercellular CO 2 partial pressure
- PGA
3-phosphoglycerate
- RuP 2
ribulose 1,5-bisphosphate
- Rubisco
RuP 2 carboxylase/oxygenase 相似文献
15.
Addition of dihydroxyacetone phosphate (2.5 mM) or 3-phosphoglycerate (2.5 mM) to a suspension of isolated intact chloroplasts, which contains P i only in low concentrations (0.2 mM) leads to a competitive inhibition of P i uptake in the light. In consequence, the ATP/ADP ratio is strongly decreased. The rate of O 2 evolution is also reduced under these conditions, but the degree of inhibition is much higher after addition of dihydroxyacetone phosphate than after addition of 3-phosphoglycerate. Therefore, besides the competitive inhibition of P i uptake, additional effects of dihydroxyacetone phosphate and 3-phosphoglycerate on O 2 evolution and CO 2 fixation of isolated intact chloroplasts must occur, which are discussed. 相似文献
16.
Nitrite reduction in either whole, isolated spinach chloroplasts ( Spinacia oleracea L.) or in reconstituted spinach chloroplasts is stimulated by a short period of photosynthetic CO 2 fixation in the light prior to nitrite addition. With reconstituted chloroplasts, a similar stimulation can be obtained in nitrite reduction without CO 2 fixation by the addition of dihydroxyacetone phosphate or fructose 6-phosphate. Specific intermediate metabolites of the photosynthetic carbon reduction cycle may have a regulatory role in nitrite reduction in chloroplasts in the light. 相似文献
17.
Photosynthetic CO 2 fixation rates in leaves and intact chloroplasts of spinach measured at 18°–20° C are substantially decreased by pretreatment at temperatures exceeding 20° C. Mild heating which causes 80% inhibition of CO 2 fixation does not affect phosphoglyceroacid reduction and causes increases in the ATP/ADP ratio and the light-induced transthylakoid proton gradient. The inactivation of the CO 2 fixation is completely reversible with half-times of recovery in the order of 15–20 min. Comparison of steady-state patterns of 14C labeled Calvin cycle intermediates of heat-treated and control samples reveals a large increase in the ribulose-1,5-bisphosphate/phosphoglyceroacid ratio and a large decrease in the phosphoglyceroacid/triosephosphate ratio. It is concluded that inactivation of CO 2 fixation occurring at elevated temperatures is caused by inhibition of the ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). Measurements of light-induced light scattering changes of thylakoids and of the light-induced electrochromic absorption shift show that these signals are affected by mild heating in a way which is strictly correlated with the inactivation of the CO 2 fixation. It is proposed that the function of the ribulose-1,5-bisphosphate carboxylase in vivo requires a form of activation that involves properties of the thylakoid membrane which are affected by the heat treatment. The fact that these changes in thylakoid membrane properties and of ribulose-1,5-bisphosphate carboxylase activity are already affected at elevated temperatures which can still be considered physiological, and the reversible nature of these changes, suggest that they may play a role in temperature regulation of the overall photosynthetic process.Abbreviations 9-AA
9-aminoacridine
- DMO
5,5-dimethyloxazolidine-2,4-dione
- FBP
fructose-1,6-bisphosphate
- HEPES
N-2-hydroxyethylpiperazine N-2-ethane sulfonic acid
- HMP
hexose monophosphates
- PGA
3-phosphoglycerate
- PMP
pentose monophosphates
- RuBP
ribulose-1,5-bisphosphate
- SBP
seduheptulose-1,7-bisphosphate
- TP
triose monophosphates 相似文献
18.
Profile analyses of the enzymes comprising the photosynthetic carbon reduction cycle have been performed in extracts of dark grown and greening Euglena gracilis var. bacillaris. Chlorella pyrenoidosa grown photoautotrophically, in the light with glucose or in the dark with glucose, Tolypothrix tenuis, Chromatium and leaves of spinach. Amounts of activity are compared with the level of photosynthetic CO 2 fixation. Only in Chromatium were all enzyme activities sufficient to support the in vivo rate of CO 2 fixation. In organisms other than Chromatium, some enzymes and particularly fructose 1,6-phosphatase and ribulose 1.5-diphosphate carboxylase appeared to be present in insufficient amounts to support the photosynthetic rate of the intact cell. Developmental studies with Euglena and growth studies with Chlorella led to the conclusion that these enzymes were associated with the cycle. Suppression of CO 2 fixation in heterotrophically grown Chlorella was accompanied by a striking decrease in the same enzymes whose activities increased in greening Euglena. 相似文献
19.
Chloroplasts isolated from pine needles were found to be inactive with respect to CO 2 fixation. Since it was suspected that pine needles may contain substances inhibitory to photosynthesis, studies were carried out using photosynthetically active isolated spinach chloroplasts and chloroplasts isolated from pine needles. When isolated pine chloroplasts were suspended in buffer and were added to isolated spinach chloroplasts they inhibited photosynthetic CO 2 fixation. When the pine chloroplasts were separated from the medium by centrifugation, the separated pine chloroplasts severely inhibited CO 2 fixation by isolated spinach chloroplasts, but the supernatant solution from the pine chloroplasts was not inhibitory. As little as 5% pine chloroplasts (based on chlorophyll content) produced 50% inhibition of CO 2 fixation by the spinach chloroplasts. Studies of fixation of 14C-labelled CO 2 by spinach chloroplasts were carried out in which after 5 min photosynthesis the pine chloroplasts were added. It was found that the subsequent inhibition of spinach CO 2 fixation was neither due to any effect on the rate of export of photosynthetic metabolites from the chloroplasts to the medium, nor to a direct effect on the RUBP carboxylase reaction. The principal effect was found to be an inhibition of the conversion of fructose-1,6-bisphosphate and sedoheptulose-1,7-bisphosphate to the respective monophosphates and inorganic phosphate. From this finding it was concluded that a principal effect of the inhibition by pine chloroplasts is probably an inhibition either directly or indirectly of the bisphosphatase enzymes in the spinach chloroplasts. Based on its distribution between organic and aqueous acidic or neutral solutions, the inhibitory factor of the pine chloroplasts must be lipophilic. Most of the factor could be transferred to an aqueous phase in a strongly alkaline solution. Following subsequent acidification of the aqueous phase the activity could be completely transferred back into the organic phase. This procedure allowed for separation of the inhibitory factor from most of the pigments and other lipophilic substances present in the pine chloroplasts and yielded a preparation which could be subsequently fractionated by thin layer chromatography. UV absorption was found in two fast moving spots and at the origin. The fastest running spot from the thin layer chromatography plate was found to be the one containing most of the inhibitory activity. 相似文献
20.
Rates of photosynthesis of spinach leaves were varied by varying light intensity and CO 2 concentration. Metabolism of the leaves was then arrested by freezing them in liquid nitrogen. Chloroplasts were isolated by a nonaqueous procedure. In the chloroplast fractions, levels of intermediates of the carbon reduction cycle were determined and considered in relation to the photosynthetic flux situation of the leaves at the time before freezing. During induction of photosynthesis, ribulose 1,5-bisphosphate levels increased in parallel with CO 2 fixation. In the steady state, a similar relation between ribulose 1,5-bisphosphate levels and CO 2 uptake was observed at light intensities between 0 and 50 W·m −2. A further increase in light intensity increased CO 2 fixation rates but not ribulose 1,5-bisphosphate levels. Increasing the CO 2 concentration resulted in increased CO 2 uptake, whereas ribulose 1,5-bisphosphate levels decreased. Even under CO 2 saturation, ribulose 1,5-bisphosphate levels were about 100 nmol/mg chlorophyll corresponding to about 3.5 mM ribulose 1,5-bisphosphate in the chloroplast stroma. This suggests that even under CO 2 saturation, ribulose-1,5-bisphosphate carboxylase limits photosynhetic CO 2 uptake. Mass action ratios calculated from measured metabolite levels demonstrated that the thermodynamic gradient required for the regeneration of ribulose 1,5-bisphosphate from hexosephosphate and triosephosphate increased considerably as photosynthetic flux increased. Similar calculations revealed that the enzymatic apparatus responsible for the reduction of 3-phosphoglycerate to dihydroxyacetone phosphate is not displaced much from equilibrium even under maximum rates of photosynthesis at saturating CO 2. The same is true for aldolase. Fructose-1,6-bisphosphatase also did not limit Calvin cycle turnover. Only at very low light intensities and during the first minutes of the induction period was the ratio of fructose 1,6-bisphosphate to fructose 6-phosphate high. This observation was more readily explained in terms of fructose 1,6-bisphosphate binding to ribulose-1,5-bisphosphate carboxylase than by a rate limitation imposed by insufficient activation of fructose-1,6-bisphosphatase. 相似文献
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