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1.
The kinetic characteristics of NAD malic enzyme purified to homogeneity from cauliflower florets have been examined. Free NAD+ is the active form of this coenzyme. Double-reciprocal plots of data obtained by varying NAD+ and malate2? at a saturating concentration of Mg2+ or by varying Mg2+ and NAD+ at a saturating level of malate2? are of intersecting type. This indicates that NAD malic enzyme obeys a sequential mechanism. Analysis of these sets of data suggests that each of these substrate pairs binds randomly to the enzyme. However, each substrate binds tighter when others are already present on the enzyme. NAD malic enzyme cannot decarboxylate malate2? in the absence of either Mg2+ or NAD+. Arrhenius plots of the NAD-linked reaction are concave downward, indicating the existence of two rate-determining steps with activation energies of 26.5 and 14.2 kcal/mol, respectively. In addition to Mg2+, the enzyme can also use Mn2+ and Co2+. Using Co2+ in place of Mg2+ does not change Vmax or Km,malate2? but the Km for metal and NAD+ are greatly decreased. At pH 7.0 and above, Mn2+ isotherms and malate2? curves with Mn2+ are nonlinear and appear to be composed of two separate saturation curves. NAD malic enzyme is completely and irreversibly inactivated by N-ethylmaleimide. The enzyme is also irreversibly inactivated approximately 50% by KCNO. 相似文献
2.
Thanaa H. Ali Nadia H. Ali Bakry M. Haroun Amir E. Tantawy 《World journal of microbiology & biotechnology》2014,30(3):819-825
Aspergillus oryzae aminohydrolase free acid phosphodiesterase catalyzes nicotinamide adenine dinucleotide to deamino-NAD and ammonia. The enzyme was purified to homogeneity by a combination of acetone precipitation, anion exchange chromatography and gel filtration chromatography. The enzyme was purified 230.5 fold. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band of MW 94 kDa. The enzyme displayed maximum activity at pH 5 and 40 °C with NAD as substrate. The enzyme activity appeared to be stable up to 40 °C. The enzyme activity was enhanced slightly by addition of Na+ and K+, whereas inhibited strongly by addition of Ag+, Mn2+, Hg2+ and Cu2+ to the reaction mixtures. The enzyme hydrolyzes several substrates, suggesting a probable non-specific nature. The enzyme catalyzes the hydrolytic cleavage of amino group of NAD, adenosine, AMP, CMP, GMP, adenosine, cytidine and cytosine to the corresponding nucleotides, nucleosides or bases and ammonia. The substrate concentration–activity relationship is the hyperbolic type and the apparent Km and Kcat for the tested substrates were calculated. 相似文献
3.
Purification and Characterization of NAD Malic Enzyme from Leaves of Eleusine coracana and Panicum dichotomiflorum
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NAD malic enzyme (EC 1.1.1.39), which is involved in C4 photosynthesis, was purified to electrophoretic homogeneity from leaves of Eleusine coracana and to near homogeneity from leaves of Panicum dichotomiflorum. The enzyme from each C4 species was found to have only one type of subunit by SDS polyacrylamide gel electrophoresis. The Mr of subunits of the enzme from E. coracana and P. dichotommiflorum was 63 and 61 kilodaltons, respectively. The native Mr of the enzyme from each species was determined by gel filtration to be about 500 kilodaltons, indicating that the NAD malic enzyme from C4 species is an octamer of identical subunits. The purified NAD malic enzyme from each C4 species showed similar kinetic properties with respect to concentrations of malate and NAD; each had a requirement for Mn2+ and activation by fructose- 1,6-bisphosphate (FBP) or CoA. A cooperativity with respect to Mn2+ was apparent with both enzymes. The activator (FBP) did not change the Hill value but greatly decreased K0.5 (the concentration giving half-maximal activity) for Mn2+. The enzyme from E. coracana showed a very low level of activity when NADP was used as substrate, but this activity was also stimulated by FBP. Significant differences between the enzymes from E. coracana and P. dichotomiflorum were observed in their responses to the activators and their immunochemical properties. The enzyme from E. coracana was largely dependent on the activators FBP or CoA, regardless of concentration of Mn2+. In contrast, the enzyme from P. dichotomiflorum showed significant activity in the absence of the activator, especially at high concentrations of Mn2+. Both immunodiffusion and immunoprecipitation, using antiserum raised against the purified NAD malic enzyme from E. coracana, revealed partial antigenic differences between the enzymes from E. coracana and P. dichotomiflorum. The activity of the NAD malic enzyme from Amaranthus edulis, a typical NAD malic enzyme type C4 dicot, was not inhibited by the antiserum raised against the NAD malic enzyme from E. coracana. 相似文献
4.
Pyridoxine kinase purified from sheep liver was found to consist of a single polypeptide chain with a molecular weight of 60,000 as determined by gel filtration, sedimentation equilibrium ultracentrifugation, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric pH of the enzyme was 5.1, and the pH optimum was between 5.5 and 6.0. The enzyme required divalent cations for activity. At cation concentrations of 80 μm, the enzyme activity with each cation was in the order of Zn2+ > Mn2+ > Mg2+. At cation concentrations of 400 μm, the enzyme activity with each cation was in the order of Mn2+ > Zn2+ > Mg2+. Excess free divalent cation inhibited the enzyme. Pyridoxine kinase also required monovalent cations. The enzyme activation was greatest with K+, then Rb+ and NH4+, whereas the enzyme had very little activity with Na+, Li+, or Cs+. Na+ did not interfere with the activation by K+. The activation of the kinase by K+, NH4+, and Rb+ followed Michaelis-Menten kinetics, and the apparent Km values for the cations were 8.9, 3.7, and 5.3 mm, respectively. Increasing the potassium concentration lowered the apparent Km value of the enzyme for pyridoxine and had little or no effect on the Km for ZnATP2? or the V of the kinase-catalyzed reaction. 相似文献
5.
The screening of enzyme patterns in seeds ofAllium cepa cv. Všetatská revealed the presence of the following enzymes: alcohol dehydrogenase, lactate dehyd ogenase, NAD+- and NADP+-glyceraldehyde-3-phosphate
dehydrogenase, glucose-6-phosphate dehydrogenase NAD+- and NADP+-malate dehydrogenase, NADH2- and NADPH2-tetrazolium reductase catalase, Superoxide dismutase, acid and alkaline phosphatase, L-leucine aminopeptidase, glutamate
dehydrogenase, non-specific esterase, and cholinesterase. Altogether 17 enzymes were detected in onion seeds, nine of which
had more than three isoenzymes, NAD+-malate dehydrogenase had 8, and non-specific esterase 9 isoenzymes. The demonstration
of cholinesterase and Superoxide dismutase activities is remarkable. 相似文献
6.
Bush LP 《Plant physiology》1969,44(3):347-350
Succinyl CoA synthetase from Nicotiana tabacum exhibited a requirement for univalent and divalent cations. Mn2+ replaced Mg2+ in the assay medium and Co2+ and Ca2+ partially replaced Mg2+. Addition of Zn2+ resulted in no enzyme activity. The enzyme was activated by univalent cations K+, Rb+, NH4+, and Na+; Li+ showed little or no activation. Maximum enzyme activity varied significantly with potassium salts of different anions. Greatest activation was obtained with K3PO4 and, respectively, KCl, KNO3, K2SO4 and KF exhibited steadily decreasing enzyme activation. 相似文献
7.
Magnesium-dependent adenosine triphosphatase has been purified from sheep kidney medulla plasma membranes. The purification, which is based on treatment of a kidney plasma membrane fraction with 0.5% digitonin in 3 mm MgCl2, effectively separates the Mg2+-ATPase from (Na+ + K+)-ATPase present in the same tissue and yields the Mg2+-ATPase in soluble form. The purified enzyme is activated by a variety of divalent cations and trivalent cations, including Mg2+, Mn2+, Ca2+, Co2+, Fe2+, Zn2+, Eu3+, Gd3+, and VO2+. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme shows two bands with Rf values corresponding to molecular weights of 150,000 and 77,000. The larger peptide is phosphorylated by [γ-32P]ATP, suggesting that this peptide may contain the active site of the Mg2+-ATPase. The Mg2+-ATPase activity is unaffected by the specific (Na+ + K+)-ATPase inhibitor ouabain. 相似文献
8.
C4 acid decarboxylation in one group of C4-pathway species is mediated by an NAD malic enzyme. This paper reports on the partial purification and properties of this enzyme from three species of this group, Atriplex spongiosa, Amaranthus edulis, and Panicum miliaceum. Depending upon the conditions, the Atriplex spongiosa enzyme was 5–30% as active with NADP compared with NAD but the enzyme from the other species was specific for NAD. The enzyme from each species had an absolute requirement for Mn2+ that could not be replaced by Mg2+, and activity was increased several fold by low concentrations of either CoA or acetyl CoA. For the enzyme from Atriplex spongiosa and Amaranthus edulis, there was cooperativity for malate binding and the activators CoA and acetyl CoA functioned to increase the affinity of malate for the enzyme. The Hill coefficients for malate binding were approximately 2 and 4, respectively. However, with the enzyme from Panicum miliaceum, cooperative binding of malate was not apparent and activators operated by increasing V rather than the affinity for malate. Bicarbonate inhibited the enzyme from Atriplex spongiosa and Amaranthus edulis and its effect was inversely related to the concentrations of malate, NAD, and activators. The possible significance of these various allosteric effects on the regulation of the enzyme in vivo is discussed. Reactant concentrations and other conditions required for maximum activity are reported. 相似文献
9.
Giek Far Chan Noor Aini Abdul Rashid Abdull Rahim Mohd Yusoff 《Annals of microbiology》2013,63(1):343-351
Flavin reductase plays an important biological role in catalyzing the reduction of flavin by NAD(P)H oxidation. The gene that codes for flavin reductase from Citrobacter freundii A1 was cloned and expressed in Escherichia coli BL21(DE3)pLysS. In this study, we aimed to characterize the purified recombinant flavin reductase of C. freundii A1. The recombinant enzyme was purified to homogeneity and the biochemical profiles, including the effect of pH, temperature, metal ions and anions on flavin reductase activity and stability, were determined. This enzyme exhibited optimum activity at 45 °C in a 10-min reaction at pH 7.5 and was stable at temperatures up to 30 °C. At 0.1 mM concentration of metal ions, flavin reductase activity was stimulated by divalent cations including Mn2+, Sr2+, Ni2+, Sn2+, Ba2+, Co2+, Mg2+, Ca2+ and Pb2+. Ag+ was noticeably the strongest inhibitor of recombinant flavin reductase of C. freundii A1. This enzyme should not be defined as a standard flavoprotein. This is the first attempt to characterize flavin reductase of C. freundii origin. 相似文献
10.
《Journal of steroid biochemistry》1982,16(4):427-433
A partial characterization of human term placental 3ß-HSDH in mitochondria is reported. Apparent KM of pregnenolone: 70 nM. A dose-dependent stimulation of 3ß-HSDH by NAD+ or NADP+ was observed in the range from 10−6 to 10−3 M (KM value of NAD+: 20 μM). At equimolar concentrations NAD+ is more than 10-fold as effective a cofactor of the 3ß-HSDH than NADP+. pH optimum: 9.5 (glycine-NaOH buffer). Temperature optimum 40–45°C. A rapid loss of 3ß-HSDH activity was found after preincubation of the enzyme at 37°C after 30 min: less than 50% of initial enzyme activity is present. No inhibition was obtained by Mg2+, Ca2+ Sr2+ and Ba2+ (1–100 mM). A strong inhibition was achieved with 1 mM Zn2+, Cd2+, Cu2+ and 10 mM and 100 mM Fe2+, Mn2+, Co2+ and Ni2+. 相似文献
11.
Streptococcus suis, a Gram-positive coccus, is an emerging zoonotic pathogen for both humans and pigs, but little is known about the properties of its metabolic enzymes. Isocitrate dehydrogenase (IDH) is a key regulatory enzyme in the citric acid cycle that catalyzes the oxidative decarboxylation of isocitrate yielding α-ketoglutarate and NAD(P)H. Here, we report the overexpression and enzymatic characterization of IDH from S. suis Serotype 2 Chinese highly virulent strain 05ZYH33 (SsIDH). The molecular weight of SsIDH was estimated to be 74 kDa by gel filtration chromatography, suggesting a homodimeric structure. Additionally, SsIDH was divalent cation-dependent and Mg2+ was found to be the most effective cation. The optimal pH of SsIDH was 7.0 (Mn2+) and 8.5 (Mg2+), and the maximum activity was around 30 °C (Mn2+) and 50 °C (Mg2+), respectively. Heat inactivation studies showed that SsIDH retained 50% activity after 20 min of incubation at 49 °C. Sequence comparison revealed that SsIDH had a significantly homologous identity to bacterial homodimeric IDHs. The recombinant SsIDH displayed a 117-fold (kcat/Km) preference for NAD+ over NADP+ with Mg2+, and a 80-fold greater specificity for NAD+ than NADP+ with Mn2+. Therefore, SsIDH has remarkably high coenzyme preference toward NAD+. This current work is expected to shed light on the functions of metabolic enzymes in S. suis and provide useful information for SsIDH to be considered as a possible candidate for serological diagnostics and detection of S. suis infection. 相似文献
12.
Scott D. Grover Pedro F. Canellas Randolph T. Wedding 《Archives of biochemistry and biophysics》1981,209(2):396-407
A procedure is described for purification of NAD malic enzyme (EC 1.1.1.39) to near homogeneity from potato tuber mitochondria. The purified enzyme is active with either NAD or NADP, and functions with either Mg2+ or Mn2+. Vapp is greatest when the enzyme is assayed with Mg2+ and NAD. When Mn2+ replaces Mg2+ the Vapp of the NAD-linked reaction decreases but the Km values for all substrates drop substantially. When NADP is used in place of NAD, the Vapp of the Mg2+-linked reaction decreases and the Km values for most substrates increase. The pH optimum of the enzyme depends on the metal ion and cofactor used and varies between 6.4 and 6.8. At pH 6.8, with saturating levels of Mg2+ and NAD, the turnover number of the enzyme is 37,000 min?1. The shape of the pH profile indicates the involvement of two to three protons in the activation of the enzyme, whereas only one proton is involved in the inactivation process. The molecular weight of the enzyme in the presence of 5 mm dithiothreitol and 2 mm MgCl2 is 490,000 as determined by gel filtration. A lower molecular weight form of the enzyme predominates in gel filtration at lower levels of dithiothreitol and in native gel electrophoresis. Sodium dodecyl sulfate gel electrophoresis of the enzyme reveals two main bands with molecular weights of 61,000 and 58,000, suggesting that the subunit stoichiometry of the high-molecular-weight form may be α4β4. However, given the possibility that the smaller subunit may be a proteolytic artifact, the enzyme may prove to be an octamer of identical subunits. 相似文献
13.
《BBA》2020,1861(1):148087
Electron bifurcating, [FeFe]-hydrogenases are recently described members of the hydrogenase family and catalyze a combination of exergonic and endergonic electron exchanges between three carriers (2 ferredoxinred− + NAD(P)H + 3 H+ = 2 ferredoxinox + NAD(P)+ + 2 H2). A thermodynamic analysis of the bifurcating, [FeFe]-hydrogenase reaction, using electron path-independent variables, quantified potential biological roles of the reaction without requiring enzyme details. The bifurcating [FeFe]-hydrogenase reaction, like all bifurcating reactions, can be written as a sum of two non-bifurcating reactions. Therefore, the thermodynamic properties of the bifurcating reaction can never exceed the properties of the individual, non-bifurcating, reactions. The bifurcating [FeFe]-hydrogenase reaction has three competitive properties: 1) enabling NAD(P)H-driven proton reduction at pH2 higher than the concurrent operation of the two, non-bifurcating reactions, 2) oxidation of NAD(P)H and ferredoxin simultaneously in a 1:1 ratio, both are produced during typical glucose fermentations, and 3) enhanced energy conservation (~10 kJ mol−1 H2) relative to concurrent operation of the two, non-bifurcating reactions. Our analysis demonstrated ferredoxin E°′ largely determines the sensitivity of the bifurcating reaction to pH2, modulation of the reduced/oxidized electron carrier ratios contributed less to equilibria shifts. Hydrogenase thermodynamics data were integrated with typical and non-typical glycolysis pathways to evaluate achieving the ‘Thauer limit’ (4 H2 per glucose) as a function of temperature and pH2. For instance, the bifurcating [FeFe]-hydrogenase reaction permits the Thauer limit at 60 °C if pH 2 ≤ ~10 mbar. The results also predict Archaea, expressing a non-typical glycolysis pathway, would not benefit from a bifurcating [FeFe]-hydrogenase reaction; interestingly, no Archaea have been observed experimentally with a [FeFe]-hydrogenase enzyme. 相似文献
14.
Characterization of a partially purified adenosine triphosphatase from a corn root plasma membrane fraction 总被引:19,自引:18,他引:1
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The (K+,Mg2+)-ATPase was partially purified from a plasma membrane fraction from corn roots (WF9 × Mol7) and stored in liquid N2 without loss of activity. Specific activity was increased 4-fold over that of the plasma membrane fraction. ATPase activity resembled that of the plasma membrane fraction with certain alterations in cation sensitivity. The enzyme required a divalent cation for activity (Co2+ > Mg2+ > Mn2+ > Zn2+ > Ca2+) when assayed at 3 millimolar ATP and 3 millimolar divalent cation at pH 6.3. When assayed in the presence of 3 millimolar Mg2+, the enzyme was further activated by monovalent cations (K+, NH4+, Rb+ Na+, Cs+, Li+). The pH optima were 6.5 and 6.3 in the absence and presence of 50 millimolar KCl, respectively. The enzyme showed simple Michaelis-Menten kinetics for the substrate ATP-Mg, with a Km of 1.3 millimolar in the absence and 0.7 millimolar in the presence of 50 millimolar KCl. Stimulation by K+ approached simple Michaelis-Menten kinetics, with a Km of approximately 4 millimolar KCl. ATPase activity was inhibited by sodium orthovanadate. Half-maximal inhibition was at 150 and 35 micromolar in the absence and presence of 50 millimolar KCl. The enzyme required the substrate ATP. The rate of hydrolysis of other substrates, except UDP, IDP, and GDP, was less than 20% of ATP hydrolysis. Nucleoside diphosphatase activity was less than 30% of ATPase activity, was not inhibited by vanadate, was not stimulated by K+, and preferred Mn2+ to Mg2+. The results demonstrate that the (K+,Mg2+)-ATPase can be clearly distinguished from nonspecific phosphohydrolase and nucleoside diphosphatase activities of plasma membrane fractions prepared from corn roots. 相似文献
15.
A new simple procedure has been developed for the purification of plasma membranes from rabbit kidney microsomes which yields a three- to fourfold increase in the specific activity of Na+-K+-adenosine triphosphatase (ATPase). The procedure differs from previous methods with deoxycholate or other detergents and does not change the molecular activity of the ATPase. The K+-dependent p-nitrophenylphosphatase activity of the native Na+-K+-ATPase is controlled more effectively by Mg2+ in the presence of K+ at concentrations higher than that of Mg2+, and by K+ in the presence of Mg2+ at concentrations higher than that of K+. The enzyme in its Mg2+-regulating state, which shows K+-saturation curves with a Hill coefficient of 1, is less sensitive to ouabain (I0.5 = 90 μM) and corresponds to the enzyme conformation reported previously which is inhibited by the concurrent presence of Na+ and ATP or of Na+ and oligomycin (I0.5 is the midpoint of the saturation curve). The enzyme in its K+-regulating state, which shows K+-saturation curves with a Hill coefficient of 2, is more sensitive to ouabain inhibition (I05 = 8 μM) and corresponds to the enzyme conformation which is stimulated by the concurrent presence of Na+ and ATP or of Na+ and oligomycin. There appear to be two conformations of the enzyme that are regulated by Mg2+ binding on the inhibitory sites of the enzyme. 相似文献
16.
The activity of liver alcohol dehydrogenase with nicotinamide–adenine dinucleotide phosphate as coenzyme
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1. The separation of nucleotide impurities from commercial NADP preparations by chromatography is described. All the preparations studied contained 0·1–0·2% of NAD. 2. The activity of pure crystalline liver alcohol dehydrogenase with NADP as coenzyme has been confirmed. Initial-rate data are reported for the reaction at pH 6·0 and 7·0 with ethanol and acetaldehyde as substrates. With NADP and NADPH2 of high purity, the maximal specific rates were similar to those obtained with NAD and NADH2, but the Michaelis constants for the former coenzymes were much greater than those for the latter. 3. The oxidation of ethanol by NADP is greatly inhibited by NADH2, and this accounts for low values of certain initial-rate parameters obtained with commercial NADP preparations containing NAD. The kinetics of the inhibition are consistent with competitive inhibition in a compulsory-order mechanism. 4. Initial-rate data with NAD and NADPH2 do not conform to the requirements of the mechanism proposed by Theorell & Chance (1951), in contrast with results previously obtained with NAD and NADH2. The possibility that the deviations are due to competing nucleotide impurity in the oxidized coenzyme cannot be excluded. The data show that the enzyme reacts more slowly with, and has a smaller affinity for, NADP and NADPH2 than NAD and NADH2. 5. Phosphate behaves as a competitive inhibitor towards NADP. 相似文献
17.
Zusammenfassung Ruhende Zellen von Hydrogenomonas H 16 enthalten je Gramm Trockengewicht 0,7 mg ATP und 0,9 mg NAD. Bei der Fixierung von Kohlendioxyd und der Synthese des Speicherstoffs Poly--hydroxybuttersäure sinkt die intracelluläre ATP-Konzentration um 30% und das Redoxverhältnis NADH2/NAD von 1,4 auf 0,46. Die NAD-abhängige Hydrogenase enthält NAD als Coenzym relativ fest gebunden. In Gegenwart von Wasserstoff wird dieses zu NADH2 reduziert und das Enzym in eine aktive, reaktionsfähige Form umgewandelt. Die Geschwindigkeit der NAD-Reduktion ist infolge einer allosterischen Hemmung der NAD-abhängigen Hydrogenase durch ihr Reaktionsprodukt NADH2 von dem Redoxverhältnis NADH2/NAD abhängig. Hierdurch erhält das Enzym eine regulatorische Funktion für den von Folgereaktionen abhängigen Wasserstofftransport.
Summary Resting cells of Hydrogenomonas strain H 16 contain 0.7 mg ATP and 0.9 mg NAD/g dry weight. During the fixation of carbon dioxide and the synthesis of the storage product poly--hydroxybutyric acid, the intracellular concentration of ATP decreases by 30% and the redox-ratio (NADH2/NAD) decreases from 1.4 to 0.46.In the NAD-dependent hydrogenase the coenzyme NAD is bound to the enzyme. In the presence of hydrogen NAD is reduced to NADH2 and the enzyme is converted to a reactive state. The velocity of NAD-reduction is related to the concentration of NADH2 and the redox-ratio NADH2/NAD. This allosteric inhibition of the NAD-dependent hydrogenase by its reaction product NADH2 is responsible for the control of hydrogen transport exerted by consecutive hydrogen requiring reactions.相似文献
18.
《Biochemical medicine and metabolic biology》1986,35(2):115-119
Activities and some properties of microsomal ATPases have been studied in developing human placenta. The enzyme activities (Na+ + K+ + Mg2+, Mg2+, and Ca2+ dependent) in the placenta increase steadily with gestational age until the 18th to 21st week, and decrease in the second half of pregnancy. Mg2+-dependent and Na+ + K+ + Mg2+-dependent ATPases possess nearly the same Km (apparent) for ATP, while the Ca2+-dependent enzyme shows a different one. Mg2+-dependent ATPase shows higher substrate affinity than Ca2+-dependent ATPase, although the Vmax of the Mg2+-dependent enzyme is lower than that of the latter. However, for each enzyme, the Km remains almost constant and Vmax varies during ontogenic development. Vmax of the enzymes decline at term. The enzymes are heat-labile, unaffected by amino acids, namely, l-phenylalanine, l-leucine, and l-tryptophan, and deoxycholate inhibits the enzyme activities by about 50%. 相似文献
19.
F.M.A.H. Schuurmans Stekhoven H.G.P. Swarts J.J.H.H.M. De Pont S.L. Bonting 《生物化学与生物物理学报:生物膜》1983,736(1):73-78
(1) Contrary to what has usually been assumed, (Na+ + K+)-ATPase slowly hydrolyses AdoPP[NH]P in the presence of Na+ + Mg2+ to ADP-NH2 and Pi. The activity is ouabain-sensitive and is not detected in the absence of either Mg2+ or Na2+. The specific activity of the Na+ + Mg2+ dependent AdoPP[NH]P hydrolysis at 37°C and pH 7.0 is 4% of that for ATP under identical conditions and only 0.07% of that for ATP in the presence of K+. The activity is not stimulated by K+, nor can K+ replace Na+ in its stimulatory action. This suggests that phosphorylation is rate-limiting. Stimulation by Na+ is positively cooperative with a Hill coefficient of 2.4; half-maximal stimulation occurs at 5–9 mM. The Km value for AdoPP[NH]P is 17 μM. At 0°C and 21°C the specific activity is 2 and 14%, respectively, of that at 37°C. AMP, ADP and AdoPP[CH2]P are not detectably hydrolysed by (Na+ + K+)-ATPase in the presence of Na+ + Mg2+. (2) In addition, AdoPP[NH]P undergoes spontaneous, non-enzymatic hydrolysis at pH 7.0 with rate constants at 0, 21 and 37°C of 0.0006, 0.006 and 0.07 h?1, respectively. This effect is small compared to the effect of enzymatic hydrolysis under comparable conditions. Mg2+ present in excess of AdoPP[NH]P reduces the rate constant of the spontaneous hydrolysis to 0.005 h?1 at 37°C, indicating that the MgAdoPP[NH]P complex is virtually stable to spontaneous hydrolysis, as is also the case for its enzymatic hydrolysis. (3) A practical consequence of these findings is that AdoPP[NH]P binding studies in the presence of Na+ + Mg2+ with enzyme concentrations in the mg/ml range are not possible at temperatures above 0°C. On the other hand, determination of affinity in the (Na+ + K+)-ATPase reaction by competition with ATP at low protein concentrations (μg/ml range) remains possible without significant hydrolysis of AdoPP[NH]P even at 37°C. 相似文献
20.
Deoxyribokinase, which catalyzes the ATP-dependent phosphorylation of 2-deoxy-d-ribose to 2-deoxy-d-ribose-5-P as the first step in the inducible fermentation pathway for this sugar in Salmonella typhimurium, was purified approximately 600-fold from deoxyribose-grown cells. Apparent Km′s for 2-deoxy-d-ribose and ATP were 0.1 and 0.5 mm, respectively. The enzyme had an absolute requirement for divalent cations which was best satisfied by Mg2+. Optimal activity was obtained in the presence of 0.5 m NH4+ or Cs+. Rb+ and K+ also stimulated enzyme activity whereas Na+ and Li+ inhibited. d-Ribose and 2-deoxy-d-ribitol could replace 2-deoxy-d-ribose as phosphoryl acceptor, and several ribo- and deoxyribonucleotides could replace ATP as phosphoryl donor. Molecular weight determinations gave values of 67,800 for the native enzyme and 33,500 for the dissociated enzyme, suggesting the probable existence of two subunits of similar size. 相似文献