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1.
Photosynthetic carbon assimilation in plants is regulated by activity of the ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase. Although the carboxylase requires CO 2 to activate the enzyme, changes in CO 2 between 100 and 1,400 microliters per liter did not cause changes in activation of the leaf carboxylase in light. With these CO 2 levels and 21% O 2 or 1% or less O 2, the levels of ribulose bisphosphate were high and not limiting for CO 2 fixation. With high leaf ribulose bisphosphate, the Kact(CO 2) of the carboxylase must be lower than in dark, where RuBP is quite low in leaves. When leaves were illuminated in the absence of CO 2 and O 2, activation of the carboxylase dropped to zero while RuBP levels approached the binding site concentration of the carboxylase, probably by forming the inactive enzyme-RuBP complex. The mechanism for changing activation of the RuBP carboxylase in the light involves not only Mg2+ and pH changes in the chloroplast stroma, but also the effects of binding RuBP to the enzyme. In light when RuBP is greater than the binding site concentration of the carboxylase, Mg2+ and pH most likely determine the ratio of inactive enzyme-RuBP to active enzyme-CO2-Mg2+-RuBP forms. Higher irradiances favor more optimal Mg2+ and pH, with greater activation of the carboxylase and increased photosynthesis. 相似文献
2.
When the amount of activation of ribulose 1,5-bisphosphate carboxylase has been measured, two forms of the enzyme, not one, are actually determined experimentally. Only the enzyme-activator CO 2-Mg 2+ form can bind ribulose bisphosphate for reaction with substrate CO 2 or O 2. A method is presented which measures only this catalytically active form by stabilizing it with ribulose bisphosphate just before dilution and assay in Mg 2+-free reaction medium. 相似文献
3.
Leaf photosynthesis and ribulose bisphosphate carboxylase activation level were inhibited in several mutants of the C 3 crucifer Arabidopsis thaliana which possess lesions in the photorespiratory pathway. This inhibition occurred when leaves were illuminated under a photorespiratory atmosphere (50% O 2, 350 microliters per liter CO 2, balance N 2), but not in nonphotorespiratory conditions (2% O 2, 350 microliters per liter CO 2, balance N 2). Inhibition of carboxylase activation level was observed in strains with deficient glycine decarboxylase, serine transhydroxymethylase, serine-glyoxylate aminotransferase, glutamate synthase, and chloroplast dicarboxylate transport activities, but inhibition did not occur in a glycolate-P phosphatase-deficient strain. Also, the photorespiration inhibitor aminoacetonitrile produced a decline in leaf and protoplast ribulose bisphosphate carboxylase activation level, but was without effect on intact chloroplasts. Fructose bisphosphatase, a light-activated enzyme which is strongly dependent on stromal pH and Mg 2+ for regulation, was unaffected by conditions which caused inhibition of ribulose bisphosphate carboxylase. Thus, the mechanism of inhibition does not appear to involve changes in stromal Mg 2+ and pH but rather is associated with metabolite flux through the photorespiratory pathway. 相似文献
4.
Wheat ( Triticum aestivum L. cv Albis) was grown in open-top chambers in the field and fumigated daily with charcoal-filtered air (0.015 microliters per liter O 3), nonfiltered air (0.03 microliters per liter O 3), and air enriched with either 0.07 or 0.10 microliters per liter ozone (seasonal 8 hour/day [9 am-5 pm] mean ozone concentration from June 1 until July 10, 1987). Photosynthetic 14CO 2 uptake was measured in situ. Net photosynthesis, dark respiration, and CO 2 compensation concentration at 2 and 21% O 2 were measured in the laboratory. Leaf segments were freeze-clamped in situ for the determination of the steady state levels of ribulose 1,5-bisphosphate, 3-phosphoglycerate, triose-phosphate, ATP, ADP, AMP, and activity of ribulose, 1,5-bisphosphate carboxylase/oxygenase. Photosynthesis of flag leaves was highest in filtered air and decreased in response to increasing mean ozone concentration. CO 2 compensation concentration and the ratio of dark respiration to net photosynthesis increased with ozone concentration. The decrease in photosynthesis was associated with a decrease in chlorophyll, soluble protein, ribulose bisphosphate carboxylase/oxygenase activity, ribulose bisphosphate, and adenylates. No decrease was found for triose-phosphate and 3-phosphoglycerate. The ratio of ATP to ADP and of triosephosphate to 3-phosphoglycerate were increased suggesting that photosynthesis was limited by pentose phosphate reductive cycle activity. No limitation occurred due to decreased access of CO 2 to photosynthetic cells since the decrease in stomatal conductance with increasing ozone concentration did not account for the decrease in photosynthesis. Ozonestressed leaves showed an increased degree of activation of ribulose bisphosphate carboxylase/oxygenase and a decreased ratio of ribulose bisphosphate to initial activity of ribulose bisphosphate carboxylase/oxygenase. Nevertheless, it is suggested that photosynthesis in ozone stressed leaves is limited by ribulose bisphosphate carboxylation possibly due to an effect of ozone on the catalysis by ribulose bisphosphate carboxylase/oxygenase. 相似文献
5.
Ribulose 1,5-bisphosphate carboxylase/oxygenase purified from malate-grown Thiocapsa roseopersicina required Mg 2+ for the activation of both carboxylase and oxygenase activities. Mg 2+ was either not required or required at very low concentrations for catalysis by both enzyme activities. EDTA and dithiothreitol had no effect on ribulose 1,5-biphosphate oxygenase. The K0.5 values with respect to Mg 2+ for activation of the carboxylase and oxygenase activities were 8.4 and 2 mm, respectively. Ribulose 1,5-biphosphate carboxylase and oxygenase activities revealed differential sensitivities to 6-phosphogluconate. This ligand at 1 mm inhibited the carboxylase activity 30%, whereas the oxygenase activity was inhibited by 69%. 相似文献
6.
Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO 2-free air for 3 hours with either 1% O 2 in N 2 or N 2-only and then returned to normal air of 340 microliters per liter CO 2, 21% O 2 in N 2. Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO 2-free treatments as was photosynthetic CO 2 fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N 2 only treatment. During the CO 2-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO 2-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO 2-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO 2 fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO 2 fixed per millogram protein per minute. This suggests that in these seedlings considerable inactive carboxylase must be stored in a form not activatable in extracts by CO 2 and Mg 2+. Possible mechanisms of regulation of photosynthesis by the ribulose bisphosphate carboxylase must consider not only the amount of active enzyme, but the amount of enzyme which the plant can make activatable upon demand. 相似文献
7.
Ribulose 1,5-bisphosphate in the chloroplast has been suggested to regulate the activity of the ribulose bisphosphate carboxylase/oxygenase. To generate high levels of ribulose bisphosphate, isolated and intact spinach chloroplasts were illuminated in the absence of CO 2. Under these conditions, chloroplasts generate internally up to 300 nanomoles ribulose 1,5-bisphosphate per milligram chlorophyll if O 2 is also absent. This is equivalent to 12 millimolar ribulose bisphosphate, while the enzyme, ribulose bisphosphate carboxylase, offers up to 3.0 millimolar binding sites for the bisphosphate in the chloroplast stroma. During illumination, the ribulose bisphosphate carboxylase is deactivated, due mostly to the absence of CO 2 required for activation. The rate of deactivation of the ribulose bisphosphate carboxylase was not affected by the chloroplast ribulose bisphosphate levels. Upon addition of CO 2, the carboxylase in the chloroplast was completely reactivated. Of interest, addition of 3-phosphoglycerate stopped deactivation of the carboxylase in the chloroplast while ribulose bisphosphate accumulated. With intact chloroplasts in light, no correlation between deactivation of the carboxylase and ribulose bisphosphate levels could be shown. 相似文献
8.
Equations are developed to describe the reactions of ribulose 1,5-biphosphate carboxylase—oxygenase with ribulose biphosphate (RuP 2), carbon dioxide, and oxygen. It is predicted that at the high concentrations of enzyme sites found in vivo there will be a large proportion of the total RuP 2 bound to the enzyme. The kinetic characteristics of the in vivo reactions with RuP 2 are predicted to be analogous to those which would occur in the presence of a tight-binding substrate. Equations are developed which are applicable when the enzyme is only partially activated by CO 2 and Mg 2+. The response of carboxylase velocity to CO 2 concentration is sigmoidal when Mg 2+ concentration is low. 相似文献
9.
Ribulose 1,5-diphosphate-dependent oxygenase activity was demonstrated for crystallized Fraction 1 protein (RuDP 2 carboxylase EC 4.1.1.39) from tobacco. The kinetic properties of this oxygenase function were examined polarographically in air-equilibrated medium. Optimum activity was obtained at pH 8.4–8.6, and required 4–8 mm MgCl 2. Higher Mg 2+ concentrations decreased activity and slightly shifted the pH optimum to 8.2–8.3. The apparent Km (RuDP) and Km (Mg 2+) were 22 μm and 0.5 mm, respectively. Oxygenase activity was inhibited by bicarbonate and indirectly by KCN. Kinetic studies suggest that the active inhibitory substance is the cyanohydrin derivative formed from the reaction of KCN with RuDP.Changes in oxygenase kinetics were observed upon addition of RuDP, as previously reported for the carboxylase function of this enzyme. Oxygenase activity required preincubation of the enzyme with both Mg 2+ and low concentrations of bicarbonate. Activities were enhanced about 20 and 70% when FDP (0.1 mm) and NADPH (0.5 mm), respectively, were included during preincubation. 相似文献
10.
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
11.
The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase displayed near-maximal activity in isolated, intact barley ( Hordeum vulgare L. cv. Pennrad) mesophyll protoplasts. The carboxylase deactivated 40 to 50% in situ when protoplasts were dark-incubated 20 minutes in air-equilibrated solutions. Enzyme activity was fully restored after 1 to 2 minutes of light. Addition of 5 millimolar NaHCO 3 to the incubation medium prevented dark-inactivation of the carboxylase. There was no permanent CO 2-dependent activation of the protoplast carboxylase either in light or dark. Activation of the carboxylase from ruptured protoplasts was not increased significantly by in vitro preincubation with CO 2 and Mg 2+. In contrast to the enzyme in protoplasts, the carboxylase in intact barley chloroplasts was not fully reactivated by light at atmospheric CO 2 levels. The lag phase in carbon assimilation was not lengthened by dark-adapting protoplasts to low CO 2 demonstrating that light-activation of the carboxylase was not involved in photosynthetic induction. Irradiance response curves for reactivation of the the carboxylase and for CO 2 fixation by isolated barley protoplasts were similar. The above results show that there was a fully reversible light-activation of the carboxylase in isolated barley protoplasts at physiologically significant CO 2 levels. 相似文献
12.
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
13.
Ribulose bisphosphate carboxylase (rubisco) is the first enzyme in photosynthetic CO 2 assimilation. It is also the single largest sink for nitrogen in plants. Several parameters of rubisco activity are often measured including initial activity upon extraction, degree of carbamylation, catalytic constant of the enzyme (k cat), and the total amount of enzyme present in a leaf. We report here improvements of the photometric assay of rubisco in which rubisco activity is coupled to NADH oxidation which is continuously monitored in a photometer. The initial lag usually found in this assay was eliminated by assaying rubisco activity at pH 8.0 instead of 8.2, using a large amount of phosphoglycerate kinase, and adding monovalent cations to the assay buffer. We found that when using the photometric assay, the ratio of activity found initially upon extraction divided by the activity after incubating with CO 2 and Mg 2+ reflects the degree of carbamylation as determined by 14carboxyarabinitol bisphosphate/ 12carboxyarabinitol bisphosphate competition. We developed methods for measuring the catalytic constant of rubisco as well as the total amount of enzyme present using the photometric assay and carboxyarabinitol 1,5-bisphosphate. We believe that the photometric assay for activity will prove more useful than the 14CO 2 assay in many studies.Abbreviations CA1P
2-carboxyarabinitol 1-phosphate
- GAP
glyceraldehyde 3-phosphate
- OD
optical density
- PGA
3-phosphoglycerate
- rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate 相似文献
14.
The low activity of ribulose bisphosphate carboxylase from darkened soybean ( Glycine max [L.] Merr. cv. Bragg) leaves was not raised to the level of that from leaves in the light by CO 2 and Mg 2+, even after a 4-h incubation. The extract of darkened leaves, unlike the extract from illuminated leaves, was not fully CO 2/Mg 2+-activatable after Sephadex gel filtration in the absence of Mg 2+. (NH 4) 2SO 4 fractionation eliminated the inhibition effect found in the dark extracts resulting in similar rates for the extracts obtained from leaves in the dark and light. Although the Vmax values of the gel-filtered extracts from dark and light leaves differed by 3-fold, the Km(CO 2)-values were the same (12.7 μM), as were the Km(RuBP)-values (250 μM). These data support the hypothesis that for soybean leaves in the dark a tightly-binding inhibitor renders much of the ribulose bisphosphate carboxylase enzyme catalytically non-functional. 相似文献
15.
Ribulose diphosphate carboxylase was found to exist in two distinct kinetic forms in spinach leaf extracts. One form displayed an apparent Km for CO 2 in excess of 200 μm and is likely to be the form purified and studied by many previous workers. However, if leaf extracts were prepared in the presence of Mg 2+ and atmospheric levels of CO 2, the recently described high-affinity form was obtained. It had a Km for CO 2 of about 20 μm, was quite stable even at 25 °C, and its properties were consistent with it being the form which operates in photosynthesis in vivo. Mg 2+ was also able to convert the high- Km (CO 2) form to the low- Km (CO 2) form when it was added to an extract which had been prepared in its absence. Mg 2+ was more effective in causing this conversion if bicarbonate was added as well. This activating effect of bicarbonate is a probable cause of previously reported apparent homotropic effects of bicarbonate on ribulose diphosphate carboxylase activity. It is possible that the apparently high- Km (CO 2) form is not intrinsically active and appears to have activity only by virtue of the low- Km (CO 2) form produced by contact with Mg 2+ and bicarbonate (or CO 2) during the course of the assay. Extracts prepared with ribose 5-phosphate in the absence of Mg 2+ also showed low- Km (CO 2) carboxylase activity initially, but the presence of this sugar phosphate was deleterious during storage at 25 °C, where it promoted conversion to the apparently high- Km (CO 2) form.Effects on the affinity of ribulose diphosphate carboxylase for CO 2 were paralleled by effects on the activity of the associated ribulose diphosphate oxygenase. Treatments which produced the low- Km (CO 2) form of the carboxylase also resulted in high oxygenase activity, and it is possible that the apparently high- Km (CO 2) form of the carboxylase has little, if any, oxygenase activity associated with it.The carboxylase and oxygenase activities of the low- Km (CO 2) form showed broad and quite similar responses to pH variation, and the oxygenase had a Km for O 2 of 0.22 mm.The stability of the low- Km (CO 2) form in the presence of Mg 2+ and bicarbonate was quite sufficient for it to be partially purified by Sepharose chromatography. The significance of the low- Km (CO 2) form is discussed with respect to activation of photosynthesis by Mg 2+. 相似文献
16.
Limited tryptic proteolysis of spinach ( Spinacia oleracea) ribulose bisphosphate carboxylase/oxygenase (ribulose-P 2 carboxylase) resulted in the ordered release of two adjacent N-terminal peptides from the large subunit, and an irreversible, partial inactivation of catalysis. The two peptides were identified as the N-terminal tryptic peptide (acetylated Pro-3 to Lys-8) and the penultimate tryptic peptide (Ala-9 to Lys-14). Kinetic comparison of hydrolysis at Lys-8 and Lys-14, enzyme inactivation, and changes in the molecular weight of the large subunit, indicated that proteolysis at Lys-14 correlated with inactivation, while proteolysis at Lys-8 occurred much more rapidly. Thus, enzyme inactivation is primarily the result of proteolysis at Lys-14. Proteolysis of ribulose-P 2 carboxylase under catalytic conditions (in the presence of CO 2, Mg 2+, and ribulose-P 2) also resulted in ordered release of these tryptic peptides; however, the rate of proteolysis at lysyl residues 8 and 14 was reduced to approximately one-third of the rate of proteolysis of these lysyl residues under noncatalytic conditions (in the presence of CO 2 and Mg 2+ only). The protection of these lysyl residues from proteolysis under catalytic conditions could reflect conformational changes in the N-terminal domain of the large subunit which occur during the catalytic cycle. 相似文献
17.
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
18.
14C-Labeled 2-carboxyarabinitol-1,5-bisphosphate was bound to both nonactivated and CO 2and Mg 2+ activated forms of ribulose bisphosphate carboxylase/oxygenase. The complex could be precipitated with 20% polyethylene glycol and 20 mm MgCl 2 for quantitation of the moles of the affinity label bound per mole of enzyme. The [ 14C]carboxyarabinitol- P2 bound to the nonactivated enzyme could be exchanged with a 100-fold excess of the unlabeled compound. With the activated enzyme the binding of [ 14C]carboxyarabinitol- P2 was so tight that it did not exchange with the unlabeled compound and a binding stoichiometry of one molecule per active site was assumed. This tight binding was dependent upon pretreatment of the enzyme with both CO 2 and MgCl 2 in the same manner that enzyme activation depended on CO 2 and Mg 2+ concentrations. Various enzyme preparations from spinach leaves tightly bound [ 14C]carboxyarabinitol- P2 in proportion to their specific activities. By extrapolating to a maximum binding of 8 mol of [ 14C]carboxyarabinitol- P2 per mole of this A 8B 8 enzyme a theoretical specific activity of 2.8 μmol · min ?1 · mg protein ?1 was indicated. Enzyme preparations purified from spinach leaves generally have a specific activity in the range of 1.0 to 2.3. 相似文献
19.
Because glyoxylate inhibits CO 2 fixation by intact chloroplasts and purified ribulose bisphosphate carboxylase/oxygenase, glyoxylate might be expected to exert some regulatory effect on photosynthesis. However, ribulose bisphosphate carboxylase activity and activation in intact chloroplasts from Spinacia oleracea L. leaves were not substantially inhibited by 10 millimolar glyoxylate. In the light, the ribulose bisphosphate pool decreased to half when 10 millimolar glyoxylate was present, whereas this pool doubled in the control. When 10 millimolar glyoxylate or formate was present during photosynthesis, the fructose bisphosphate pool in the chloroplasts doubled. Thus, glyoxylate appeared to inhibit the regeneration of ribulose bisphosphate, but not its utilization. The fixation of CO2 by intact chloroplasts was inhibited by salts of several weak acids, and the inhibition was more severe at pH 6.0 than at pH 8.0. At pH 6.0, glyoxylate inhibited CO2 fixation by 50% at 50 micromolar, and glycolate caused 50% inhibition at 150 micromolar. This inhibition of CO2 fixation seems to be a general effect of salts of weak acids. Radioactive glyoxylate was reduced to glycolate by chloroplasts more rapidly in the light than in the dark. Glyoxylate reductase (NADP+) from intact chloroplast preparations had an apparent Km (glyoxylate) of 140 micromolar and a Vmax of 3 micromoles per minute per milligram chlorophyll. 相似文献
20.
Free ribulose bisphosphate (RuBP 4?) rather than its magnesium complex (RuBP-Mg 2?) was the apparent substrate for spinach ribulose bisphosphate carboxylase/oxygenase. The apparent K m for total RuBP (pH 8.0 at 30° C) increased with increasing Mg 2+ concentrations from 11.6 μM at 13.33 mM Mg 2+ to 32.6 μM at 40.33 mM Mg 2+. Similarly the apparent K m for RuBP-Mg 2? complex increased with increasing Mg 2+ from 9.4 μM at 13.33 mM Mg 2+ to 29.7 μM at 40.33 mM Mg 2+. However, the K m values for uncomplexed RuBP 4? were independent of the (saturating) concentration of Mg 2+ (K m=2.2 μM). The V max did not vary with the changing concentrations of Mg 2+. In contrast, the K m for total RuBP remained constant with varying Mg 2+ concentrations (K m=59.5 μM) for the enzyme from R. rubrum. The apparent K m for the RuBP-Mg 2? complex decreased with increasing Mg 2+ concentrations from 16.0 μM at 7.5 mM Mg 2+ to 5.9 μM at 27.5 mM Mg 2+. The initial velocity for the C. vinosum enzyme was also found to be independent of the (saturating) concentration of Mg 2+ when total RuBP was varied in the assay. Thus the response to total RuBP by these two bacterial enzymes, which markedly differ in structure, was closely similar. 相似文献
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