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1.
Isolated, purified mesophyll and guard-cell protoplasts of Vicia faba L. and Allium cepa L. were exposed to 14CO 2 in the light and in the dark. The guard-cell protoplasts of Vicia and Allium did not show any labeling in phosphorylated products of the Calvin cycle, thus appearing to lack the ability to reduce CO 2 photosynthetically. In Vicia, high amounts of radioactivity (35%) appeared in starch after 60-s pulses of 14CO 2 both in the light and in the dark. Presumably, the 14CO 2 is fixed into the malate via PEP carboxylase and then metabolized into starch as the final product of gluconeogenesis. This is supported by the fact that guard-cell protoplasts exposed to malic acid uniformly labeled with 14CO 2 showed high amounts of labeled starch after the incubation, whereas cells labeled with [4- 14C]malate had minimal amounts of labeled starch (1/120).In contrast, the starch-deficient Allium, guard-cell protoplasts did not show any significant 14CO 2 fixation. However, adding PEP to an homogenate stimulated 14CO 2 uptake, thus supporting the interpretation that the presence of starch as a source of PEP is necessary for incorporating CO 2 and delivering malate. With starch-containing Vicia guard-cell protoplasts, the correlation between changes in volume and the interconversion of malate and starch was demonstrated. It was shown that the rapid gluconeogenic conversion of malate into starch prevents an increase of the volume of the protoplasts, whereas the degradation of starch to malate is accompanied by a swelling of the protoplasts.Abbreviations GCPs
guard-cell protoplasts
- MCPs
mesophyll cell protoplasts
- PEP
phosphoenolpyruvate
- DTT
dithiothreitol
- 3-PGA
3-phosphoglyceric acid
- RiBP
ribulose 1,5 bisphosphate
- MDH
malate dehydrogenase
- MES
2-(N-morpholino)ethane sulfonic acid
- CAM
crassulacean acid metabolism 相似文献
2.
Epidermal strips with closed stomata were exposed to malic acid labelled with 14C either uniformly or in 4-C only. During incubation with [U- 14C]malate, radioactivity appeared in products of the tricarboxylic-acid cycle and in transamination products within 10 min, in sugars after 2 h. Hardly any radioactivity was found in sugars if [4- 14C]malate had been offered. This difference in the degree of labelling of sugars indicates that gluconeogenesis can occur in epidermal tissue, involving the decarboxylation of malate. Epidermis incubated with labelled malate was hydrolyzed after extraction with aqueous ethanol. The hydrolysate contained glucose as the only radioactive product, indicating that starch had been formed from malate. Microautoradiograms were black above stomatal complexes, showing that the latter were sites of starch formation. In order to follow the fate of malate during stomatal closure, malate was labelled in guard cells by exposing epidermes with open stomata to 14CO 2 and then initiating stomatal closure. Of the radioactive fixation products of CO 2 only malate was released into the water on which the epidermal samples floated; the epidermal strips retained some of the malate and all of its metabolites. In the case of rapid stomatal closure initiated by abscisic acid and completed within 5 min, 63% of the radioactivity was in the malate released, 22% in the malate retained, the remainder in aspartate, glutamate, and citrate. We conclude that during stomatal closing guard cells can dispose of malate by release, gluconeogenesis, and consumption in the tricarboxylic-acid cycle.Abbreviations ABA
abscisic acid
- NAD
nicotinamide adenine dinucleotide
- NADP
nicotinamide adenine dinucleotide phosphate
- PEP
phosphoenolpyruvate 相似文献
3.
Mesophyll cells were isolated from sunflower leaves by an enzymic procedure. The cell suspensions possessed high photosynthesis rates. The products of cell photosynthesis were similar to the products of leaf disc photosynthesis. The relatively high radioactivity incorporated into malate after 14CO 2 feeding suggests that PEP carboxylase might participate in CO 2 fixation. Sunflower leaf extracts possessed a PEP carboxylase activity slightly higher than that of other C 3 species. Inhibition of PEP carboxylase by maleate decreased cell photosynthesis by only 15% and the first products of cell photosynthesis were phosphorylated compounds. It is concluded that the high photosynthesis rates displayed by sunflower are not due to a parallel C 4 pathway of photosynthesis but are rather dependent, at least in part, on the activity, or the amount, of RuBP carboxylase.Abbreviations PVP
polyvinylpyrrolidone
- PDS
potassium dextran sulfate
- DTT
dithiothreitol
- PEG
polyethyleneglycol
- RuBP
ribulose 1,5-bisphosphate
- PEP
phosphoenolpyruvate
- Mes
2-(N-morpholino) ethanesulfonic acid
- Hepes
N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid 相似文献
4.
The leaf anatomy and certain photosynthetic properties of nitrate- and ammonia-grown plants of Moricandia arvensis (L.) DC., a species previously reported to be a C 3-C 4 intermediate, were investigated. Nitrate-grown plants had a high level of malate in the leaves while ammonia-grown plants had low levels of malate. In young leaves of nitrate-grown plants, there was a diurnal fluctuation of malate content, increasing during the day and decreasing during the night. Titratable acidity remained low in leaves of both nitrate- and ammonia-grown plants. In nitrate-grown plants, the activity of phosphoenolpyruvate (PEP) carboxylase was about 2-fold higher than in ammonia-grown plants, the latter having activity typical of C3 species. Also, in nitrate-grown plants, the ratio of activities of ribulose 1,5-bisphosphate (RuBP) carboxylase/PEP carboxylase was lower than in ammonia-grown plants. Nitrate reductase activities were higher in nitrate- than in ammonia-grown plants and the greatest activity was found in younger leaves. With nitrate-grown plants, during a pulse-chase experiment the label in malate, as a percentage of the total labeled products, increased from about 7% after a 10-second pulse with 14CO2 up to 17% during a 5-minute chase with 12CO2. The pattern of 14C labeling in various metabolites suggests the primary carboxylation is through RuBP carboxylase with a secondary carboxylation through PEP carboxylase. In similar experiments, with ammonia-grown plants, the percentage label in malate was only 0% to 4% with no increase in malate labeling during the chase period. The CO2 compensation point was lower in nitrate-grown than ammonia-grown plants. There was no evidence of Kranz-like anatomy in either the nitrate or ammonia-grown plants. Mitochondria of bundle-sheath cells were strikingly positioned along the inner tangential wall. This might allow the chloroplasts of these cells to fix the mitochondrial photorespired CO2 more effectively and contribute to the low CO2 compensation point in the species. Chloroplasts of bundle-sheath cells and contiguous mesophyll cells were similar in size and structure in plants grown on different media, although chloroplast thylakoids and stromata of the ammonia-grown plants stained more intensely than those of nitrate-grown plants. In addition, irregular clusters of phytoferritin particles occurred in the chloroplasts of the ammonia-grown plants. The results indicate that the substantial activity of PEP carboxylase, incorporation of CO2 into malate, the high malate content, and in part the relatively low CO2 compensation point in Moricandia arvensis may be accounted for by metabolism of nitrate rather than by a state of C3-C4 intermediacy. 相似文献
5.
The 24 h O 2 uptake and release together with the CO 2 balance have been measured in two CAM plants, one a non-succulent Sempervivum grandifolium, the other a succulent Prenia sladeniana. The O 2 uptake was estimated by the use of 18O 2. It was found that the mean hourly O 2 uptake in the light was 7 times that in the dark for Sempervivum and 5 times that for Prenia, after correction for the lightdark temperature difference. It was estimated that oxygen uptake in the light was 2.4 times greater than oxygen release (=net photosynthesis) in Sempervivum and 1.4 times greater in Prenia. In both plants there was a positive carbon balance over the 24 h period under the experimental conditions. It was estimated that malate formed during the night could, if completely oxidized to CO 2 and water, account for 74% of the light phase O 2 uptake in Sempervivum. In Prenia the O 2 uptake was more than sufficient to account for a full oxidation of malate.Abbreviations CAM
Crassulacean acid metabolism
- PAR
photosynthetically active radiation
- PEP
phosphoenolpyruvate
- RrBP
ribulose-1,5-bisphosphate
- TCA
tricarboxylic acid cycle 相似文献
6.
Abstract Photosynthetic properties of cell suspension cultures derived from the callus proliferation of cladophyll explants of Chamaecereus sylvestrii Spegazzini were studied. High content of chlorophyll (105–120 μg/g fresh weight), cyanide sensitive O 2 uptake and maximal rates of O 2 evolution (100–115 μmol/mg Chl x h) and CO 2 fixation (130–150 μmol/mg Chl x h) were some of the properties of the exponential phase cells. Determination of the component reactions, viz. photosystems I and II and photophosphorylation of the chloroplasts isolated from the cells, indicated normal development and functioning of the photosynthetic machinery. Studies on the enzymatic reactions as well as the determination of the early products of 14CO 2 fixation in light in these cells implicated the operation of both autotrophic and non-auto-trophic pathways, the latter being less pronounced. The diurnal oscillation of titratable acidity and malate content found in the intact cladophyll tissues was absent in the cultured cells. Evidences for a rapid and continuous drain of carbon from malate into the citrate and isocitrate components of the TCA cycle via pyruvate after decarboxylation, and then into the amino acid pool are presented. The absence of large vacuoles and the rapid turnover of malate are considered to account for the lack of diurnal fluctuation of organic acide in the cell cultures. 相似文献
7.
Phyllodia of the Crassulacean acid metabolism (CAM) plant Kalanchoë tubiflora were allowed to fix 13CO 2 in light and darkness during phase IV of the diurnal CAM cycle, and during prolongation of the regular light period. After 13CO 2 fixation in darkness, only singly labelled [ 13C]malate molecules were found. Fixation of 13CO 2 under illumination, however, produced singly labelled malate as well as malate molecules which carried label in two, three or four carbon atoms. When the irradiance during 13CO 2 fixation was increased, the proportion of singly labelled malate decreased in favour of plurally labelled malate. The irradiance, however, did not change either the ratio of labelled to unlabelled malate molecules found in the tissue after the 13CO 2 application, or the magnitude of malate accumulation during the treatment with label. The ability of the tissue to store malate and the labelling pattern changed throughout the duration of the prolonged light period. The results indicate that malate synthesis by CAM plants in light can proceed via a pathway containing two carboxylation steps, namely ribulose-1,5-bisphosphate-carboxylase/oxygenase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) which operate in series and share common intermediates. It can be concluded that, in light, phosphoenolpyruvate carboxylase can also synthesize malate independently of the proceeding carboxylation step by ribulose-1,5-bisphosphate carboxylase/oxygenase.Abbreviations CAM
Crassulacean acid metabolism
- PEP
phosphoenolpyruvate
- PEPCase
phosphoenolpyruvate carboxylase (EC 4.1.1.31)
- RuBPCase
ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39)
- TMS
trimethylsilyl 相似文献
8.
Phosphoenolpyruvate (PEP) carboxylation was measured as dark 14CO 2 fixation in leaves and roots (in vivo) or as PEP carboxylase (PEPCase) activity in desalted leaf and roof extracts (in vitro) from Pisum sativum L. cv. Kleine Rheinländerin. Its relation to the malate content and to the nitrogen source (nitrate or ammonium) was investigated. In tissue from nitrate-grown plants, PEP carboxylation varied diurnally, showing an increase upon illumination and a decrease upon darkening. Diurnal variations in roots were much lower than in leaves. Fixation rates in leaves remained constantly low in continuous darkness or high in continuous light. Dark CO 2 fixation of leaf slices also decreased when leaves were preilluminated for 1 h in CO 2-free air, suggesting that the modulation of dark CO2 fixation was related to assimilate availability in leaves and roots. Phosphoenolpyruvate carboxylase activity was also measured in vitro. However, no difference in maximum enzyme activity was found in extracts from illuminated or darkened leaves, and the response to substrate and effectors (PEP, malate, glucose-6-phosphate, pH) was also identical. The serine/threonine protein kinase inhibitors K252b, H7 and staurosporine, and the protein phosphatase 2A inhibitors okadaic acid and cantharidin, fed through the leaf petiole, did not have the effects on dark CO 2 fixation predicted by a regulatory system in which PEPCase is modulated via reversible protein phosphorylation. Therefore, it is suggested that the diurnal modulation of PEP carboxylation in vivo in leaves and roots of pea is not caused by protein phosphorylation, but rather by direct allosteric effects. Upon transfer of plants to ammonium-N or to an N-free nutrient solution, mean daily malate levels in leaves decreased drastically within 4–5 d. At that time, the diurnal oscillations of PEP carboxylation in vivo disappeared and rates remained at the high light-level. The coincidence of the two events suggests that PEPCase was de-regulated because malate levels became very low. The drastic decrease of leaf malate contents upon transfer of plants from nitrate to ammonium nutrition was apparently not caused by increased amino acid or protein synthesis, but probably by higher decarboxylation rates.Abbreviations CAM
crassulacean acid metabolism
- PEP
Phosphoenolpyruvate
- PEPCase
phosphoenolpyruvate carboxylase
- PP
protein phosphatase
- PK
protein kinase
This work was supported by the Deutsche Forschungsgemeinschaft. B. Baur was a recipient of a doctoral grant, and L. Leport recipient of a post-doctoral grant of the DFG. The skilled technical assistance of Eva Wirth and Maria Lesch is gratefully acknowledged. 相似文献
9.
Gas exchange in K. blossfeldiana shows a circadian rhythm in net CO 2 uptake and transpiration when measured under low and medium irradiances. The period length varies between 21.4 h at 60 W m -2 and 24.0 h at 10 W m -2. In bright light (80 W m -2) or darkness there are no rhythms. High leaf temperatures result in a fast dampening of the CO 2-uptake rhythm at moderate irradiances, but low leaf temperatures can not overcome the dampening in bright light. The rhythm in CO 2 uptake is accompanied by a less pronounced and more rapidly damped rhythm in transpiration and by oscillations in malate levels with the amplitude being highly reduced. The oscillations in starch content, usually observed to oscillate inversely to the acidification in light-dark cycles, disappear after the first cycle in continuous light. The balance between starch and malate levels depends in continuous light on the irradiance applied. Leaves show high malate and low starch content at low irradiance and high starch and low malate in bright light. During the first 12 h in continuous light replacing the usual dark period, malate synthesis decreases with the increasing irradiance. Up to 50 W m -2 starch content decreases; at higher irradiances it increases above the values usually measured at the end of the light period of the 12:12 h light-dark cycle.Abbreviations CAM
Crassulacean acid metabolism
- FW
fresh weight
- PEP
phosphoenolpyruvate 相似文献
10.
14CO 2 was applied repeatedly at 3- to 6-h intervals to Kalanchoë daigremontiana leaves during continuous light of differing irradiances. The circadian rhythm in net CO 2 uptake in gasexchange measurements and its disappearance at high irradiances was confirmed by oscillating rates of 14CO 2 incorporation. At 10–30 W m -2 a markedly circadian oscillation in the 14CO 2-uptake rate was measured; with increasing energy fluence rate the oscillation levelled off at a constant high uptake rate. The labelling patterns obtained during the 10 min of 14CO 2 fixation indicated that the rhythm of CO 2 exchange is the consequence of a rhythmic behaviour in the C 4 pathway of CO 2 fixation. During the mininum of 14CO 2 uptake no C 4 products were labelled; however, substantial amounts of label were transferred to C 4 products during the peaks of 14CO 2 uptake. Metabolism of C 3 and C 4 products was also studied in pulsechase experiments at different points of the circadian cycle. In bright light (100 W m -2), when the 14CO 2 uptake was constantly high, the transfer of label into C 4 products (malic acid) was high in spite of the fact that the malate pool is known to be reduced to a permanently low level under these conditions. This led us to the conclusion that it is not the capacity of the phosphoenolpyruvatecarboxylase-mediated CO 2 fixation but rather the storage of malic acid in the vacuole that is disturbed under bright-light conditions when the circadian oscillation levelled off.Abbreviations CAM
Crassulacean acid metabolism
- LL
continuous light
- PEP
phosphoenolpyruvate 相似文献
11.
Net CO 2 dark fixation of Kalanchoë daigremontiana varies with night temperature. We found an optimum of fixation at about 15° C; with increasing night temperature fixation decreased. We studied the temperature dependence of the activity of phosphoenolpyruvate (PEP)-carboxylase, the key enzyme for CO 2 dark fixation. We varied the pH, the substrate concentration (PEP), and the L-malate and glucose-6-phosphate (G-6-P) concentration in the assay. Generally, lowering the pH and reducing the amount of substrate resulted in an increase in activation by G-6-P and in an increase in malate inhibition of the enzyme. Furthermore, malate inhibition and G-6-P activation increased with increasing temperature. Activity measurements between 10° C and 45°C at a given concentration of the effectors revealed that the temperature optimum and maximum activities at that optimum varied with the effector applied. Under the influence of 5 mol m -3 L-malate the temperature optimum and maximum activity dropped drastically, especially when the substrate level was low (at 0.5 mol m -3 PEP from 32° C to 20° C). G-6-P raised the temperature optimum and maximum activity when the substrate level was low. If both malate and G-6-P were present, intermediate values were measured. We suggest that changes in metabolite levels in K. daigremontiana leaves can alter the temperature features of PEP-carboxylase so that the observed in vivo CO 2 dark fixation can be explained on the basis of PEP-carboxylase activity.Abbreviations PEP-c
phosphoenolpyruvate carboxylase
- CAM
crassulacean acid metabolism
- PEP
phosphoenolpyruvate
- G-6-P
glucose-6-phosphate 相似文献
12.
When Vicia faba guard cell protoplasts were treated with fusicoccin, dark 14CO 2 fixation rates increased by as much as 8-fold. Rate increase was saturated with less than 1 micromolar fusicoccin. Even after 6 minutes of dark 14CO 2 fixation, more than 95% of the incorporated radioactivity was in stable products derived from carboxylation of phosphoenolpyruvate (about 50% and 30% in malate and aspartate, respectively). The relative distribution of 14C among products and in the C-4 position of malate (initially more than 90% of [ 14C]malate) was independent of fusicoccin concentration. After incubation in the dark, malate content was higher in protoplasts treated with fusicoccin. A positive correlation was observed between the amounts of 14CO 2 fixed and malate content. It was concluded that (a) fusicoccin causes an increase in the rate of dark 14CO2 fixation without alteration of the relative fluxes through pathways by which it is metabolized, (b) fusicoccin causes an increase in malate synthesis, and (c) dark 14CO2 fixation and malate synthesis are mediated by phosphoenolpyruvate carboxylase. 相似文献
13.
Rat lung mitochondrial preparations were incubated in the presence of pyruvate and malate. The principal metabolic products measured were citrate and CO 2. Citrate formation from pyruvate was found to be dependent on the presence of malate. Significant citrate was formed in the presence of isocitrate and the rate of citrate formation was increased by the addition of pyruvate. Small amounts of citrate were formed by lung mitochondrial preparations in the presence of 2-oxoglutarate and succinate only after the addition of pyruvate. The level of acetyl-CoA was significantly greater in the presence of pyruvate than in the presence of pyruvate plus malate. The addition of malate to lung mitochondrial preparations increased 14CO 2 production from [U- 14C]- and [1- 14C] pyruvate but decreased its production from [2- 14C]- and [3- 14C]-pyruvate. However, malate increased the incorporation of [2- 14C] pyruvate into malate and citrate. A low level of pyruvate-dependent H 14CO 8-incorporation into acid-stable products was observed, principally citrate and malate, but this rate did not exceed 5% of the rate of net citrate formation in the presence of malate and pyruvate. The capacity of rat lung mitochondria to form oxaloacetate from pyruvate alone in vitro is very limited, and would appear to cast doubt on a major role of pyruvate carboxylase in citrate formation. It is concluded that the rate of citrate formation from pyruvate is limited by the availability of intramitochondrial oxaloacetate and the rate of citrate efflux across the mitochondrial membrane. 相似文献
14.
Activities of key enzymes of Calvin cycle and C 4 metabolism, rate of 14CO 2 fixation in light and dark and the initial products of photosynthetic 14CO 2 fixation were determined in flag leaf and different ear parts of wheat viz. pericarp, awn and glumes. Compared to the activities of RuBP carboxylase and other Calvin cycle enzymes viz. NADP-glyceraldehyde-3-phosphate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, the levels of PEP carboxylase and other enzymes of C 4 metabolism viz. NADP-malate dehydrogenase, NAD-malate dehydrogenase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase genase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase, were generally greater in ear parts than in the flag leaf. In contrast to CO 2 fixation in light, the various ear parts incorporated CO 2 in darkness at much higher rates than flag leaf. In short term assimilation of 14CO 2 by illuminated ear parts, most of the 14C was in malate with less in 3-phosphoglyceric acid, whereas flag leaves incorporated most into 3-phosphoglyceric acid. It seems likely that ear parts have the capability of assimilating CO 2 by the C 4 pathway of photosynthesis and utilise PEP carboxylase for recapturing the respired CO 2. 相似文献
15.
Comparative 14CO 2 pulse- 12CO 2 chase studies performed at CO 2 compensation ()-versus air-concentrations of CO 2 demonstrated a four-to eightfold increase in assimilation of 14CO 2 into the C 4 acids malate and aspartate by leaves of the C 3-C 4 intermediate species Panicum milioides Nees ex Trin., P. decipiens Nees ex Trin., Moricandia arvensis (L.) DC., and M. spinosa Pomel at . Specifically, the distribution of 14C in malate and aspartate following a 10-s pulse with 14CO 2 increases from 2% to 17% ( P. milioides) and 4% to 16% ( M. arvensis) when leaves are illuminated at the CO 2 compensation concentration (20 l CO 2/l, 21% O 2) versus air (340 l CO 2/l, 21% O 2). Chasing recently incorporated 14C for up to 5 min with 12CO 2 failed to show any substantial turnover of label in the C 4 acids or in carbon-4 of malate. The C 4-acid labeling patterns of leaves of the closely related C 3 species, P. laxum Sw. and M. moricandioides (Boiss.) Heywood, were found to be relatively unresponsive to changes in pCO 2 from air to . These data demonstrate that the C 3-C 4 intermediate species of Panicum and Moricandia possess an inherently greater capacity for CO 2 assimilation via phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) at the CO 2 compensation concentration than closely related C 3 species. However, even at , CO 2 fixation by PEP carboxylase is minor compared to that via ribulosebisphosphate carboxylase (EC 4.1.1.39) and the C 3 cycle, and it is, therefore, unlikely to contribute in a major way to the mechanism(s) facilitating reduced photorespiration in the C 3-C 4 intermediate species of Panicum and Moricandia.Abbreviations Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- PEP
phosphoenolpyruvate
-
CO 2 compensation concentration
- 3PGA
3-phosphoglycerate
- SuP
sugar monophosphates
- SuP 2
sugar bisphosphates
Published as Paper No. 8249, Journal Series, Nebraska Agricultural Research Division 相似文献
16.
Properties of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) obtained from isolated guard-cell protoplasts of Vicia faba L. were determined following rapidly desalting of the extract on a Sephadex G 25 column. The activity of PEP carboxylase was measured as a function of PEP and malate concentration, pH and K + concentration within 2–3 min after homogenization of the guard-cell protoplasts. The activity of this enzyme was stimulated by PEP concentrations of 0.1 to 0.75 mM and by K + ions (12 mM), but inhibited by PEP concentrations above 1 mM and by malate. Changes in the K m(PEP) and V max values with increasing malate concentrations (2.5 and 5 mM) indicate that the malate level, varying in relation to the physiological state of guard cells, plays an important role in regulating the properties of phosphoenolpyruvate carboxylase.Abbreviations CAM
Crassulacean acid metabolism
- GCP
guard-cell protoplast
- PEP
phosphoenolpyruvate
Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday 相似文献
17.
A pathway for conversion of the metabolic intermediate phosphoenolpyruvate (PEP) and the formation of acetate, succinate, formate, and H 2 in the anaerobic cellulolytic bacterium Ruminococcus flavefaciens FD-1 was constructed on the basis of enzyme activities detected in extracts of cells grown in cellulose- or cellobiose-limited continuous culture. PEP was converted to acetate and CO 2 (via pyruvate kinase, pyruvate dehydrogenase, and acetate kinase) or carboxylated to form succinate (via PEP carboxykinase, malate dehydrogenase, fumarase, and fumarate reductase). Lactate was not formed even during rapid growth (batch culture, µ = 0.35/h). H 2 was formed by a hydrogenase rather than by cleavage of formate, and 13C-NMR and 14 C-exchange reaction data indicated that formate was produced by CO 2 reduction, not by a cleavage of pyruvate. The distribution of PEP into the acetate and succinate pathways was not affected by changing extracellular pH and growth rates within the normal growth range. However, increasing growth rate from 0.017/h to 0.244/h resulted in a shift toward formate production, presumably at the presence of H 2. This shift suggested that reducing equivalents could be balanced through formate or H 2 production without affecting the yields of the major carbon-containing fermentation endproducts. 相似文献
18.
Summary
14CO 2 fixation and radioactive products in Chlorella cells were examined during the induction period. Chlorella vulgaris (strain 211-11f) grown under low CO 2 concentration (0.03 vol.-%) was used in our experiments. No transients in 14CO 2 fixation could be observed under the conditions used (0.7 vol.-% CO 2). Autoradiographic studies of the kinetics of appearance of labelled products showed that at the beginning of the illumination period radioactivity appeared first in intermediates of the Calvin cycle (e.g. phosphate esters). After 2 min smaller amounts of radioactivity were incorporated in malate and aspartate. In another series of experiments the influence of iodoacetamide (0.001 M) on 14CO 2 fixation was examined. In Chlorella cells inhibited with iodoacetamide radioactivity was located in malate, aspartate and 3-phosphoglycerate only. Results were discussed with reference to the participation of two different carboxylation pathways in the formation of the transients. Also the possible role of photorespiration in these transients was discussed. 相似文献
19.
Phosphoenolpyruvate carboxylase partially purified from leaves of Crassula and rendered insensitive to malate by storage without adjuvants can be altered to the form sensitive to malate inhibition by brief, 5-minute preincubation with 5 millimolar malate. The induction of malate sensitivity is reversible by lowering the malate 2− concentration. Of the reaction components only HCO 3− increases the sensitivity to malate in subsequent assay. Phosphoenolpyruvate (PEP), which itself tends to lower sensitivity to subsequent malate inhibition, also reduces the effect of malate in the assay, as does glucose-6-phosphate. PEP isotherms showed that the insensitive or unpreincubated enzyme, responds to the presence of 5 millimolar malate during assay with a 3-fold increase in Km, but no effect on Vmax. Enzyme preincubated with malate shows the same effect of malate on Km, but in addition Vmax is inhibited 72%. It thus appears that both sensitive and insensitive forms of PEP carboxylase are subject to K-type inhibition by malate, but only the sensitive form also shows V-type inhibition. Preincubation with malate at different pH values showed that at pH 6.15, the inhibition by malate in subsequent assay at pH 7 was much lower than at pH 7 or 8. When the reaction is prerun for 30 minutes with increasing concentrations of PEP, subsequent assay with malate shows progressively less inhibition due to malate. When 0.3 millimolar PEP either alone or with 0.1 millimolar ATP and 0.3 millimolar NaF is present during preincubation, the effect of malate in a following assay is to activate the reaction. These results may indicate an effect of phosphorylation of the enzyme on sensitivity to malate. 相似文献
20.
Stomatal opening on Vicia faba can be induced by high CO 2 partial pressures (10.2%) in dark as well as in light. Stomatal aperture was measured in both cases with a hydrogen porometer. The distribution of 14C among early products of photosynthesis was studied. Comparisons are made with carboxylations occurring when stomata were open in the dark with CO 2-free air and in light with 0.034% CO 2. Results showed that in high CO 2 partial pressure in light, less radioactivity was incorporated in Calvin cycle intermediates and more in sucrose. carboxylations and photorespiration seemed to be inhibited. In the dark in both CO 2 conditions, 14C incorporation was found in malate and aspartate but also in serine and glycerate in high CO 2 conditions. In light these changes in metabolic pathways may be related with the deleterious effects recorded on leaves after long-term expositions to high partial pressure of CO 2.Abbreviations DHAP
dihydroxyacetone phosphate
- PEP
phosphonenolpyruvate
- PEPCK
phosphonenolpyruvatecarboxykinase
- PGA
3-phosphoglyceric acid
- RUBPc
ribulose 1,5-bisphosphate carboxylase 相似文献
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