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1.
Nitrogen effects on the regulation of photosynthesis in wheat ( Triticum aestivum L., cv Remia) seedlings were examined. Ribulose 1,5-bisphosphate carboxylase/oxygenase was rapidly extracted and tested for initial activity and for activity after incubation in presence of CO 2 and Mg 2+. Freeze clamped leaf segments were extracted for determinations of foliar steady state levels of ribulose 1,5-bisphosphate, triose phosphate, 3-phosphoglycerate, ATP, and ADP. Nitrogen deficient leaves showed increased ATP/ADP and triose phosphate/3-phosphoglycerate ratios suggesting increased assimilatory power. Ribulose 1,5-bisphosphate levels were decreased due to reduced pentose phosphate reductive cycle activity. Nevertheless, photosynthesis appeared to be limited by ribulose 1,5-bisphosphate carboxylase/oxygenase, independent of nitrogen nutrition. Its degree of activation was increased in nitrogen deficient plants and provided for maximum photosynthesis at decreased enzyme protein levels. It is suggested that ribulose 1,5-bisphosphate carboxylase/oxygenase activity is regulated according to the amount of assimilatory power. 相似文献
2.
1. The carbon isotope discrimination properties of a representative of each of the three types of photosynthetic bacteria Chlorobium thiosulfatophilum, Rhodospirillum rubrum and Chromatium and of the C 3-alga Chlamydomonas reinhardii were determined by measuring the ratio of 13CO 2 to 12CO 2 incorporated during photoautotrophic growth. 2. Chromatium and R. rubrum had isotope selection properties similar to those of C 3-plants, whereas Chlorobium was significantly different. 3. The results suggest that Chromatium and R. rubrum assimilate CO 2 mainly via ribulose 1,5-diphosphate carboxylase and the associated reactions of the reductive pentose phosphate cycle, whereas Chlorobium utilizes other mechanisms. Such mechanisms would include the ferredoxin-linked carboxylation enzymes and associated reactions of the reductive carboxylic acid cycle.Abbreviations RuDP
ribulose 1,5-disphosphate
- PEP
phosphoenolpyruvate 相似文献
3.
The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) FROM SPINACH CHLOROPLASTS IS STRONGLY REGULATED BY THE RATIO OF NADPH/NADP+, with the extent of this regulation controlled by the concentration of ribulose 1,5-diphosphate. Other metabolites of the reductive pentose phosphate cycle are far less effective in mediating the regulation of the enzyme activity by NADPH/NADP+ ratio. With a ratio of NADPH/NADP+ of 2, and a concentration of ribulose 1,5-diphosphate of 0.6 mM, the activity of the enzyme is completely inhibited. This level of ribulose 1,5-diphosphate is well within the concentration range which has been reported for unicellular green algae photosynthesizing in vivo. Ratios of NADPH/NADP+ of 2.0 have been measured for isolated spinach chloroplasts in the light and under physiological conditions. Since ribulose 1,5-diphosphate is a metabolite unique to the reductive pentose phosphate cycle and inhibits glucose-6-phosphate dehydrogenase in the presence of NADPH/NADP+ ratios found in chloroplasts in the light, it is proposed that regulation of the oxidative pentose phosphate cycle is accomplished in vivo by the levels of ribulose 1,5-diphosphate, NADPH, and NADP+. It already has been shown that several key reactions of the reductive pentose phosphate cycle in chloroplasts are regulated by levels of NADPH/NADP+ or other electron-carrying cofactors, and at least one key-regulated step, the carboxylation reaction is strongly affected by 6-phosphogluconate, the metabolic unique to the oxidative pentose phosphate cycle. Thus there is an interesting inverse regulation system in chloroplasts, in which reduced/oxidized coenzymes provide a general regulatory mechanism. The reductive cycle is activated at high NADPH/NADP+ ratios where the oxidative cycle is inhibited, and ribulose 1,5-diphosphate and 6-phosphogluconate provide further control of the cycles, each regulating the cycle in which it is not a metabolite. 相似文献
4.
The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) from spinach chloroplasts is strongly regulated by the ratio of NADPH/NADP +, with the extent of this regulation controlled by the concentration of ribulose 1,5-diphosphate. Other metabolites of the reductive pentose phosphate cycle are far less effective in mediating the regulation of the enzyme activity by NADPH/NADP + ratio. With a ratio of NADPH/NADP + of 2, and a concentration of ribulose 1,5-diphosphate of 0.6 mM, the activity of the enzyme is completely inhibited.This level of ribulose 1,5-diphosphate is well within the concentration range which has been reported for unicellular green algae photosynthesizing in vivo. Ratios of NADPH/NADP + of 2.0 have been measured for isolated spinach chloroplasts in the light and under physiological conditions.Since ribulose 1,5-diphosphate is a metabolite unique to the reductive pentose phosphate cycle and inhibits glucose-6-phosphate dehydrogenase in the presence of NADPH/NADP + ratios found in chloroplasts in the light, it is proposed that regulation of the oxidative pentose phosphate cycle is accomplished in vivo by the levels of ribulose 1,5-diphosphate, NADPH, and NADP +.It already has been shown that several key reactions of the reductive pentose phosphate cycle in chloroplasts are regulated by levels of NADPH/NADP + or other electron-carrying cofactors, and at least one key-regulated step, the carboxylation reaction is strongly affected by 6-phosphogluconate, the metabolite unique to the oxidative pentose phosphate cycle. Thus there is an interesting inverse regulation system in chloroplasts, in which reduced/oxidized coenzymes provide a general regulatory mechanism. The reductive cycle is activated at high NADPH/NADP + ratios where the oxidative cycle is inhibited, and ribulose 1,5-diphosphate and 6-phosphogluconate provide further control of the cycles, each regulating the cycle in which it is not a metabolite. 相似文献
5.
Earlier work (SE Taylor, N Terry [1984] Plant Physiol 75: 82-86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO 2 concentrations. Here we monitored leaf metabolites diurnally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet ( Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was little evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diurnal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate. 相似文献
6.
Intact spinach chloroplasts, capable of evolving O 2 in response to CO 2 at rates greater than 70 μmol/h · mg of chlorophyll, synthesize glycolate from added dihydroxyacetone phosphate, ribose 5-phosphate, or xylulose 5-phosphate, when illuminated in the presence of O 2. The synthesis of glycolate from these compounds is dependent upon photophosphorylation and is inhibited by each of the three classes of photophosphorylation inhibitors [Izawa, S., and Good, N. E. (1972) in Methods in Enzymology, Vol. 24, Part B, pp. 355–377)]: an uncoupler, carbonylcyanide-4-trifluoromethoxyphenylhydrazone (FCCP), an energy transfer inhibitor, Dio-9, and a phosphate analog, arsenate. Neither arsenate nor Dio-9 causes the collapse of the light-generated proton gradient between thylakoid and stroma compartments of the chloroplasts, so that the inhibition of glycolate synthesis by these compounds cannot be ascribed to an inactivation of Calvin cycle enzymes thought to be associated with the collapse of such a proton gradient. The dependency of glycolate synthesis upon photophosphorylation indicates that an ATP-consuming reaction(s) is involved in the conversion of the substrates (triose and pentose monophosphates) to glycolate. The formation of dihydroxyethylthiamine pyrophosphate, the “active glycolaldehyde” intermediate of the transketolase reaction, from triose and pentose monophosphates has no known requirements for ATP. On the other hand, the conversion of both triose and pentose monophosphates to ribulose 1,5-bisphosphate, the substrate for the ribulose 1,5-bisphosphate oxygenase reaction, requires ATP. It is concluded that glycolate synthesis by intact isolated chloroplasts is primarily the result of ribulose 1,5-bisphosphate oxygenase activity. No substantial role in glycolate synthesis can be attributed to the oxidation of dihydroxyethylthiamine pyrophosphate, the intermediate of the transketolase reaction. 相似文献
7.
1. Cell-free extracts of the photosynthetic bacterium Chlorobium thiosulfatophilum, strains 8327 and Tassajara, were assayed for ribulose 1,5-diphosphate (RuDP) carboxylase and phosphoribulokinase-the two enzymes peculiar to the reductive pentose phosphate cycle. 2. RuDP carboxylase was consistently absent in strain 8327. The Tassajara strain showed a low RuDP-dependent CO 2 fixation activity that was somewhat higher in cells following transatlantic air shipment than in freshly grown cells. The stability and behaviour of this activity in sucrose density gradients were similar to those described by other workers. 3. The radioactive carboxylation products formed in the presence of RuDP by enzyme preparations from the Tassajara strain did not include 3-phosphoglycerate-the known product of the RuDP carboxylase reaction, but instead consisted of the unrelated acids glutamate, aspartate and malate. 4. Phosphoribulokinase was absent in all preparations of the two Chlorobium strains tested. By contrast, phosphoribulokinase as well as RuDP carboxylase were readily demonstrated in preparations from pea chloroplasts and the photosynthetic bacterium Rhodospirillum rubrum. 5. It is concluded that C. thiosulfatophilum appears to lack RuDP carboxylase, phosphoribulokinase, and hence, the reductive pentose phosphate cycle.Support of a J. S. Guggenheim Fellowship is gratefully acknowledged 相似文献
8.
Intact chloroplasts isolated from spinach were illuminated in the absence of inorganic phosphate (Pi) or with optimum concentrations of Pi added to the reaction medium. In the absence of Pi photosynthesis declined after the first 1–2 min and was less than 10% of the maximum rate after 5 min. Export from the chloroplast was inhibited, with up to 60% of the 14C fixed being retained in the chloroplast, compared to less than 20% in the presence of Pi. Despite the decreased export, chloroplasts depleted of Pi had lower levels of triose phosphate while the percentage of total phosphate in 3-phosphoglycerate was increased. Chloroplast ATP declined during Pi depletion and reached dark levels after 3–4 min in the light without added Pi. At this point, stromal Pi concentration was 0.2 mM, which would be limiting to ATP synthesis. Addition of Pi resulted in a rapid burst of oxygen evolution which was not initially accompanied by net CO 2 fixation. There was a large decrease in 3-phosphoglycerate and hexose plus pentose monophosphates in the chloroplast stroma and a lesser decrease in fructose-1,6-bisphosphate. Stromal levels of triose phosphate, ribulose-1,5-bisphosphate and ATP increased after resupply of Pi. There was an increased export of 14-labelled compounds into the medium, mostly as triose phosphate. Light activation of both fructose-1,6-bisphosphatase and ribulose-1,5-bisphosphate carboxylase was decreased in the absence of Pi but increased following Pi addition.It is concluded that limitation of Pi supply to isolated chloroplasts reduced stromal Pi to the point where it limits ATP synthesis. The resulting decrease in ATP inhibits reduction of 3-phosphoglycerate to triose phosphate via mass action effects on 3-phosphoglycerate kinase. The lack of Pi in the medium also inhibits export of triose phosphate from the chloroplast via the phosphate transporter. Other sites of inhibition of photosynthesis during Pi limitation may be located in the regeneratige phase of the reductive pentose phosphate pathway.Abbreviations FBP
Fructose-1,6-bisphosphate
- FBPase
Fructose-1,6-bisphosphatase
- MP
Hexose plus pentose monophosphates
- PGA
3-phosphoglycerate
- Pi
inorganic orthophosphate
- RuBP
ribulose-1,5-bisphosphate
- RuBPCase
ribulose-1,5-bisphosphate carboxylase
- TP
Triose Phosphate 相似文献
9.
The capacity of ribulose-1,5-bisphosphate carboxylase to bind reversibly chloroplast metabolites which are the substrates for both thylakoid and stromal enzymes was assessed using spinach chloroplasts and chloroplast extracts and with pure wheat ribulose-1,5-bisphosphate carboxylase. Measurements of the rate of coupled electron flow to methyl viologen in ‘leaky’ chloroplasts (which retained the chloroplast envelope and stromal enzymes but which were permeable to metabolites) and also with broken chloroplasts and washed thylakoids were used to study the effects of binding ADP and inorganic phopshate to ribulose-1,5-bisphosphate carboxylase. The presence of ribulose-1,5-bisphosphate carboxylase significantly altered the values obtained for apparent Km for inorganic phosphate and ADP of coupled electron transport. The Km (P i) in washed thylakoids was 60–80 μM, in ‘leaky’ chloroplasts it was increased to 180–200 μM, while in ‘leaky’ chloroplasts preincubated with KCN and ribulose 1,5-bisphosphate the value was decreased to 40–50 μM. Similarly, the Km (ADP) of coupled electron transport in washed thylakoids was 60–70 μM, in ‘leaky’ chloroplasts it was 130–150 μM and with ‘leaky’ chloroplasts incubated in the presence of KCN and ribulose 1,5-bisphosphate a value of 45–50 μM was obtained. The ability of ribulose 1,5-bisphosphate carboxylase to reduce the levels of free glycerate 3-phosphate in the absence of ribulose 1,5-bisphosphate was examined using a chloroplast extract system by varying the concentrations of stromal protein or purified ribulose 1,5-bisphosphate carboxylase. The effect of binding glycerate 3-phosphate to ribulose-1,5-bisphosphate carboxylase on glycerate 3-phosphate reduction was to reduce both the rate an the amount of NADPH oxidation for a given amount of glycerate 3-phosphate added. The addition of ribulose 1,5-bisphosphate reinitiated NADPH oxidation but ATP or NADPH did not. Incubation of purified ribulose-1,5-bisphosphate carboxylase with carboxyarabinitolbisphosphate completely inhibited the catalytic activity of the enzyme and decreased inhibition of glycerate-3-phosphate reduction. Two binding sites with different affinities for glycerate 3-phosphate were observed with pure ribulose-1,5-bisphosphate carboxylase. 相似文献
10.
Hydrogen peroxide (6x10 -4 M) causes a 90% inhibition of CO 2-fixation in isolated intact chloroplasts. The inhibition is reversed by adding catalase (2500 U/ml) or DTT (10 mM). If hydrogen peroxide is added to a suspension of intact chloroplasts in the light, the incorporation of carbon into hexose- and heptulose bisphosphates and into pentose monophosphates is significantly increased, whereas; carbon incorporation into hexose monophosphates and ribulose 1,5-bisphosphate is decreased. At the same time formation of 6-phosphogluconate is dramatically stimulated, and the level of ATP is increased. All these changes induced by hydrogen peroxide are reversed by addition of catalase or DTT. Additionally, the conversion of [ 14C]glucose-6-phosphate into different metabolites by lysed chloroplasts in the dark has been studied. In presence of hydrogen peroxide, formation of ribulose-1,5-bisphosphate is inhibited, whereas formation of other bisphosphates,of triose phosphates, and pentose monophosphates is stimulated. Again, DTT has the opposite effect. The release of 14CO 2 from added [ 14C]glucose-6-phosphate by the soluble fraction of lysed chloroplasts via the reactions of oxidative pentose phosphate cycle is completely inhibited by DTT (0.5 mM) and re-activated by comparable concentrations of hydrogen peroxide. These results indicate that hydrogen peroxide interacts with reduced sulfhydryl groups which are involved in the light activation of enzymes of the Calvin cycle at the site of fructose- and sedoheptulose bisphophatase, of phosphoribulokinase, as well as in light-inactivation of oxidative pentose phosphate cycle at the site of glucose-6-phosphate dehydrogenase.Abbreviations ADPG
ADP-glucose
- DHAP
dihydroxyacetone phosphate
- DTT
dithiothreitol
- FBP
fructose-1,6-bisphosphate
- HEPES
N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid
- HMP
hexose monophosphates (fructose-6-phosphate, glucose-6-phosphate, glucose-1-phosphate)
- 6-PGI
6-phosphogluconate
- PMP
pentose monophosphates (xylulose-5-phosphate, ribose-5-phosphate, ribulose-5-phosphate)
- RuBP
ribulose-1,5-bisphosphate
- S7P
sedoheptulose-7-phosphate
- SBP
sedoheptulose-1,7-bisphosphate
Dedicated to Prof. Dr. W. Simonis on the occasion of his 70th birthday 相似文献
11.
The potential of control analysis to aid our understanding of regulation and control of photosynthetic carbon metabolism is investigated. Methods of metabolic control analysis are used to determine flux control coefficients of photosynthetic reactions from enzyme elasticities. Equations expressing control coefficients symbolically by enzyme elasticities are derived, and general properties of these expressions are analysed. Suggestions for experimental determination of flux control coefficients from enzyme elasticities are given. A simplified model of the Calvin-Benson cycle is used to illustrate interrelations between patterns of photosynthetic metabolites and that of control coefficients.Abbreviations GAPDH
glyceraldehyde phosphate dehydrogenase
- PGA
3-phosphoglycerate
- PGK
3-phosphoglycerate kinase
- Pi
inorganic phosphate
- PRK
phosphoribulokinase
- RuBP
ribulose-1,5-bisphosphate( total, free)
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- Ru5P
ribulose-5-phosphate 相似文献
12.
(i) We have studied the influence of reduced phosphoglucose-isomerase (PGI) activity on photosynthetic carbon metabolism in mutants of Clarkia xantiana Gray (Onagraceae). The mutants had reduced plastid (75% or 50% of wildtype) or reduced cytosolic (64%, 36% or 18% of wildtype) PGI activity. (ii) Reduced plastid PGI had no significant effect on metabolism in low light. In high light, starch synthesis decreased by 50%. There was no corresponding increase of sucrose synthesis. Instead glycerate-3-phosphate, ribulose-1,5-bisphosphate, reduction of Q A (the acceptor for photosystem II) and energy-dependent chlorophyll-fluorescence quenching increased, and O 2 evolution was inhibited by 25%. (iii) Decreased cytosolic PGI led to lower rates of sucrose synthesis, increased fructose-2,6-bisphosphate, glycerate-3-phosphate and ribulose-1,5-bisphosphate, and a stimulation of starch synthesis, but without a significant inhibition of O 2 evolution. Partitioning was most affected in low light, while the metabolite levels changed more at saturating irradiances. (iv) These results provide decisive evidence that fructose-2,6-bisphosphate can mediate a feedback inhibition of sucrose synthesis in response to accumulating hexose phosphates. They also provide evidence that the ensuing stimulation of starch synthesis is due to activation of ADP-glucose pyrophosphorylase by a rising glycerate-3-phosphate: inorganic phosphate ratio, and that this can occur without any loss of photosynthetic rate. However the effectiveness of these mechanisms varies, depending on the conditions. (v) These results are analysed using the approach of Kacser and Burns (1973, Trends Biochem. Sci. 7, 1149–1161) to provide estimates for the elasticities and flux-control coefficient of the cytosolic fructose-1,6-bisphosphatase, and to estimate the gain in the fructose-2,6-bisphosphate regulator cycle during feedback inhibition of sucrose synthesis.Abbreviations and symbols Chl
chlorophyll
- Fru6P
fructose-6-phosphate
- Frul,6bisP
fructose-1,6-bisphosphate
- Fru-1,6Pase
fructose-1,6-bisphosphatase
- Fru2,6bisP
fructose-2,6-bisphosphate
- Fru2,6Pase
fructose-2,6-bisphosphatase
- Glc6P
glucose-6-phosphate
- PGI
phosphoglucose isomerase
- Pi
inorganic phosphate
- Q A
acceptor for photosystem II
- Ru1,5bisP
ributose-1,5-bisphosphate
- SPS
sucrose-phosphate synthase 相似文献
13.
A procedure for the facile measurement, in intact cells, of two enzymes unique to the Calvin cycle, ribulose 1,5-bisphosphate carboxylase and phosphoribulokinase, is described. The procedure involved a simple toluene treatment to render phototrophic cells permeable to the necessary substrates, effectors, and cofactors. Whole-cell ribulose 1,5-bisphosphate carboxylase activity quantitatively approximates the activity obtained in cell-free extracts. In addition, the activity measured with toluene-treated whole cells results in a stoichiometric carboxylation of ribulose 1,5-bisphosphate to phosphoglyceric acid. The assay procedures described are most convenient for determining enzyme levels as a function of growth. Moreover, such an assay should open the way to further studies on the regulation of CO 2 assimilation by direct measurement of the enzymes concerned within the cell. 相似文献
14.
Temperature, activating metal ions, and amino-acid substitutions are known to influence the CO 2/O 2 specificity of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. However, an understanding of the physical basis for enzyme specificity has been elusive. We have shown that the temperature dependence of CO 2/O 2 specificity can be attributed to a difference between the free energies of activation for the carboxylation and oxygenation partial reactions. The reaction between the 2,3-enediolate of ribulose 1,5-bisphosphate and O 2 has a higher free energy of activation than the corresponding reaction of this substrate with CO 2. Thus, oxygenation is more responsive to temperature than carboxylation. We have proposed possible transition-state structures for the carboxylation and oxygenation partial reactions based upon the chemical natures of these two reactions within the active site. Electrostatic forces that stabilize the transition state of the carboxylation reaction will also inevitably stabilize the transition state of the oxygenation reaction, indicating that oxygenase activity may be unavoidable. Furthermore, the reduction in CO 2/O 2 specificity that is observed when activator Mg 2+ is replaced by Mn 2+ may be due to Mg 2+ being more effective in neutralizing the negative charge of the carboxylation transition state, whereas Mn 2+ is a transition-metal ion that can overcome the triplet character of O 2 to promote the oxygenation reaction.Abbreviations CABP
2-carboxyarabinitol 1,5-bisphosphate
- enol-RuBP
2,3-enediolate of ribulose 1,5-bisphosphate
- K c
K mfor CO 2
- K o
K mfor O 2
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate
- V c
V
max for carboxylation
- V o
V
max for oxygenation 相似文献
15.
An assay was developed for simultaneous kinetic analysis of the activities of the bifunctional plant enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase [EC 4.1.1.39]. [1- 14C,5- 3H]Ribulose 1,5-bisphosphate (RuBP) was used as the labeled substrate. Tritium enrichment of the doubly labeled 3-phosphoglycerate (3-PGA) product, common to both enzyme activities, may be used to calculate V c/V o ratios from the expression A/(B-A) where A and B represent the 3H/ 14C isotope ratios of doubly labeled RuBP and 3-PGA, and V c and V o represent the activities of carboxylase and oxygenase, respectively. Doubly labeled substrate was synthesized from [2- 14C]glucose and [6- 3H]glucose using the enzymes of the pentose phosphate pathway coupled with phosphoribulokinase. 相似文献
16.
Photoassimilation of 14CO 2 by intact chloroplasts from the Crassulacean acid metabolism plant Sedum praealtum was investigated. The main water-soluble, photosynthetic products were dihydroxyacetone phosphate (DHAP), glycerate 3-phosphate (PGA), and a neutral saccharide fraction. Only a minor amount of glycolate was produced. A portion of neutral saccharide synthesis was shown to result from extrachloroplastic contamination, and the nature of this contamination was investigated with light and electron microscopy. The amount of photoassimilated carbon partitioned into starch increased at both very low and high concentrations of orthophosphate. High concentrations of exogenous PGA also stimulated starch synthesis. DHAP and PGA were the preferred forms of carbon exported to the medium, although indirect evidence suported hexose monophosphate export. The export of PGA and DHAP to the medium was stimulated by high exogenous orthophosphate, but depletion of chloroplastic reductive pentose phosphate intermediates did not occur. As a result only a relatively small inhibition in the rate of CO2 assimilation occurred. The rate of photoassimilation was stimulated by exogenous PGA, ribose 5-phosphate, fructose 1,6-bisphosphate, fructose 6-phosphate, and glucose 6-phosphate. Inhibition occurred with phosphoenolpyruvate and high concentrations of PGA and ribose 5-phosphate. PGA inhibition did not result from depletion of chloroplastic orthophosphate or from inhibition of ribulose 1,5-bisphosphate carboxylase. Exogenous PGA and phosphoenolpyruvate were shown to interact with the orthophosphate translocator. 相似文献
17.
A simple approach to determine CO 2/O 2 specificity factor () of ribulose 1,5-bisphosphate carboxylase/oxygenase is described. The assay measures the amount of CO 2 fixation at varying [CO 2]/[O 2] ratios after complete consumption of ribulose 1,5-bisphosphate (RuBP). Carbon dioxide fixation catalyzed by the carboxylase was monitored by directly measuring the moles of 14CO 2 incorporated into 3-phosphoglycerate (PGA). This measurement at different [CO 2]/[O 2] ratios is used to determine graphically by several different linear plots the total RuBP consumed by the two activities and the CO 2/O 2 specificity factor. The assay can be used to measure the amounts of products of the carboxylase and oxygenase reactions and to determine the concentration of the substrate RuBP converted to an endpoint amount of PGA and phosphoglycolate. The assay was found to be suitable for all [CO 2]/[O 2] ratios examined, ranging from 14 to 215 micromolar CO 2 (provided as 1–16 mM NaHCO 3) and 614 micromolar O 2 provided as 50% O 2. The procedure described is extremely rapid and sensitive. Specificity factors for enzymes of highly divergent values are in good agreement with previously published data.Abbreviations HEPPS
N-(2-hydroxyethyl)piperazine-N-(3-propanesulfonic acid)
- L
large subunit of rubisco
- PGA
3-phosphoglyceric acid
- rubisco
ribulose 1,5-bisphosphate carboxylase/oxygenase
- RuBP
d-ribulose 1,5-bisphosphate
- S
small subunit of rubisco
- XuBP
d-xylulose 1,5-bisphosphate 相似文献
18.
The rate of CO 2 assimilation and levels of metabolites of the C 4 cycle and reductive pentose phosphate pathway in an attached leaf of maize ( Zea mays L) were measured over a range of intercellular CO 2 concentration (Ci) of 10 to 190 microliters per liter. The CO 2 assimilation rate was saturated at a Ci of around 175 microliters per liter. The levels of ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate decreased substantially with increasing Ci. The levels of 3-phosphoglycerate, phosphoenolpyruvate (PEP), and pyruvate increased with increasing Ci. The level of dihydroxyacetone phosphate increased moderately from Ci of 10 microliters per liter to 20 to 50 microliters per liter and stayed almost constant over the rest of the range of Ci investigated. The levels of fructose 6-phosphate did not show any significant changes over the range of Ci. The levels of glucose 6-phosphate decreased slightly with increasing Ci. Although photosynthetically inactive pools of malate, asparate, and alanine could mask real changes in levels of the photosynthetically active pools of these compounds, the apparent levels of these compounds and the total amount of intermediates in the C 4 cycle (malate, aspartate, pyruvate, PEP, and alanine) increased with increasing Ci. The results suggest that there is carbon input into the C 4 cycle from the reductive pentose phosphate pathway which increases the level of total intermediates of the C 4 cycle with increasing Ci. 相似文献
19.
Several representatives of the euryarchaeal class Archaeoglobi are able to grow facultative autotrophically using the reductive acetyl-CoA pathway, with 'Archaeoglobus lithotrophicus' being an obligate autotroph. However, genome sequencing revealed that some species harbor genes for key enzymes of other autotrophic pathways, i.e. 4-hydroxybutyryl-CoA dehydratase of the dicarboxylate/hydroxybutyrate cycle and the hydroxypropionate/hydroxybutyrate cycle and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) of the Calvin-Benson cycle. This raised the question of whether only one or multiple autotrophic pathways are operating in these species. We searched for the presence of enzyme activities specific for the dicarboxylate/hydroxybutyrate or the hydroxypropionate/hydroxybutyrate cycles in 'A. lithotrophicus', but such enzymes could not be detected. Low Rubisco activity was detected that could not account for the carbon dioxide (CO(2)) fixation rate; in addition, phosphoribulokinase activity was not found. The generation of ribulose 1,5-bisphosphate from 5-phospho-D-ribose 1-pyrophosphate was observed, but not from AMP; these sources for ribulose 1,5-bisphosphate have been proposed before. Our data indicate that the reductive acetyl-CoA pathway is the only functioning CO(2) fixation pathway in 'A. lithotrophicus'. 相似文献
20.
Photosynthetic rates and photosynthate partitioning were studied in three-week-old soybean [ Glycine max (L.) Merr. cv. Williams] plants exposed to either ambient (35 Pa) or elevated (70 Pa) CO 2 in controlled environment chambers. Ambient CO 2-grown plants also were given a single 24 h treatment with 70 Pa CO 2 1 d prior to sampling. Photosynthetic rates of ambient CO 2-grown plants initially increased 36% when the measurement CO 2 was doubled from 35 to 70 Pa. Photosynthetic rates of the third trifoliolate leaf, both after 1 and 21 d of elevated CO 2 treatment, were 30 to 45% below those of ambient CO 2-grown plants when measured at 35 Pa CO 2. These reduced photosynthetic rates were not due to increased stomatal resistance and were observed for 2 to 8 h after plants given 1 d of CO 2 enrichment were returned to ambient CO 2. Initial and total ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities, percent activation, Rubisco protein, soluble protein and leaf chlorophyll content were similar in all CO 2 treatments. Quantum yields of photosynthesis, determined at limiting irradiances and at 35 Pa CO 2, were 0.049±0.003 and 0.038±0.005 mol CO 2 fixed per mol quanta for ambient and elevated CO 2-grown plants, respectively (p<0.05). Leaf starch and sucrose levels were greater in plants grown at 70 than at 35 Pa CO 2. Starch accumulation rates during the day were greater in ambient CO 2-grown plants than in plants exposed to elevated CO 2 for either 1 or 21 d. However, the percentage of C partitioned to starch relative to total C fixed was unaffected by 1 d of CO 2 enrichment. The above results showed that both photosynthetic and starch accumulation rates of soybean leaflets measured at 35 Pa CO 2 were temporarily reduced after 1 and 21 d of CO 2 enrichment. The biochemical mechanism affecting these responses was not identified.Abbreviations SLW-
specific leaf weight (g m –2)
- Rubisco-
ribulose 1,5-bisphosphate carboxylase/oxygenase
- Rul-
5bisP, ribulose 1,5 bisphosphate
- DAP-
days after planting
- SAR-
starch accumulation rate
- C i-
intercellular CO 2 concentration 相似文献
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