Effect of the structure of the prosthetic group on the catalytic properties of prostaglandin H synthetase

The effect of vinyl groups of protohemin IX on its cofactor properties with respect to prostaglandin H synthetase has been studied. It was shown that substitution of ethyl groups or a hydrogen for vinyl groups affects neither binding of the prosthetic group to the apoenzyme nor catalytic properties of holo-prostaglandin H synthetase. Replacement of vinyl groups with bulkier substituents (hydroxyethyl or acetyl groups) decreases holoenzyme stability and catalytic activity. By comparison of the cofactor properties of protoporphyrin and hematoporphyrin macrocycles with different central ions (Fe3+, Mn2+, 2H+ in the case of protoporphyrin, and Fe3+, Mg2+, Cd2+ and Cu2+ in the case of hematoporphyrin), the presence of Fe3+ ions was shown to be mandatory for prostaglandin H synthetase activity. It was demonstrated that the cofactor structure modifications do not affect the holo-prostaglandin H synthetase inactivation rate constant in a reaction.     

Doxorubicin inhibits the phosphate-transport protein reconstituted in liposomes. A study on the mechanism of the inhibition

Using Thr(P)-inhibitor-1 and Ser(P)-casein as substrates, studies on the activation of calcineurin purified from bovine brain have been carried out. The phosphatase requires the synergistic action of Ca2+, calmodulin and another divalent cation (Mg2+, Mn2+, Co2+ or Ni2+, but not Zn2+) for full expression of its activity. Ca2+ and Ca2+ X calmodulin act as allosteric activators to transform the phosphatase to a relaxed conformation, while Mg2+ acts solely as a cofactor for the catalytic action of the enzyme. In addition to their function as cofactors for catalysis, transition metal ions can also substitute for Ca2+ as allosteric activators. Ca2+ and calmodulin exert their activating effects mainly by increasing the Vm of the phosphatase reaction with little effect on the Km values for the substrates or on the KA values for the divalent cation cofactors. The predominant factor in dictating the catalytic properties of calcineurin is the divalent cation cofactor. For example, with Mg2+ as a cofactor, the phosphatase exhibits an optimum around pH 8.0-8.5; while with a transition metal ion as a cofactor, the optimum is around pH 7.0-7.5, regardless of whether Thr(P)-inhibitor-1 or Ser(P)-casein serves as a substrate, in the absence or the presence of Ca2+ X calmodulin.     

Phosphoryl Group Exchange between ATP and ADP Catalyzed by H+-ATPase from Oat Roots

ATP-ADP exchange was estimated in the presence of plasma membrane H+-ATPase of oat (Avena sativa) roots partially purified with Triton X-100 by measuring [14C]ATP formation from [14C]ADP. Most studies were done at 0[deg]C. At pH 6.0 the exchange showed: (a) Mg2+ requirement with a biphasic response giving maximal activity at 152 [mu]M and (b) insensitivity to ionic strength, [Na+], and [K+]. ATP and ADP dependence were analyzed with a model in which nucleotide-enzyme interactions are at rapid-random equilibrium, whereas E1ATP [left right arrow] E1P-ADP transitions occur in steady state. The results indicated competition between ADP and ATP for the catalytic site, whereas ATP interaction with the ADP site was extremely weak. At 0[deg]C the exchange showed a 3-fold pH increase, from pH 5.5 to 9.0. At an alkaline pH the reaction was not affected by sodium azide and carbonyl cyanide p-trifluometoxyphenyl-hydrazone, had a biphasic response to Mg2+ (maximal at 513 [mu]m), and was insensitive to ionic strength. At 20[deg]C ATP-ADP exchange was pH insensitive. At both temperatures ATP hydrolysis displayed a bell-shaped response, with a maximum around pH 6.0 to 6.5. Because no adenylate kinase activity was detected under any condition, these results demonstrate the existence of an ATP-ADP exchange reaction catalyzed by the plant H+-ATPase.     

Thermodynamics of the arginine kinase reaction.

The effect of temperature, pH, free [Mg(2+)], and ionic strength on the apparent equilibrium constant of arginine kinase (EC 2.7.3.3) was determined. At equilibrium, the apparent K' was defined as [see text] where each reactant represents the sum of all the ionic and metal complex species. The K' at pH 7.0, 1.0 mM free [Mg(2+)], and 0. 25 M ionic strength was 29.91 +/- 0.59, 33.44 +/- 0.46, 35.44 +/- 0. 71, 39.64 +/- 0.74, and 45.19 +/- 0.65 (n = 8) at 40, 33, 25, 15, and 5 degrees C, respectively. The standard apparent enthalpy (DeltaH degrees') is -8.19 kJ mol(-1), and the corresponding standard apparent entropy of the reaction (DeltaS degrees') is + 2. 2 J K(-1)mol(-1) in the direction of ATP formation at pH 7.0, free [Mg(2+)] =1.0 mM, ionic strength (I) =0.25 M at 25 degrees C. We further show that the magnitude of transformed Gibbs energy (DeltaG degrees ') of -8.89 kJ mol(-1) is mostly comprised of the enthalpy of the reaction, with 7.4% coming from the entropy TDeltaS degrees' term (+0.66 kJ mol(-1)). Our results are discussed in relation to the thermodynamic properties of its evolutionary successor, creatine kinase.     

Activation of brain calcineurin phosphatase towards nonprotein phosphoesters by Ca2+, calmodulin, and Mg2+

Calcineurin purified from bovine brain was found to be active towards beta-naphthyl phosphate greater than p-nitrophenyl phosphate greater than alpha-naphthyl phosphate much greater than phosphotyrosine. In its native state, calcineurin shows little activity. It requires the synergistic action of Ca2+, calmodulin, and Mg2+ for maximum activation. Ca2+ and Ca2+ X calmodulin exert their activating effects by transforming the enzyme into a potentially active form which requires Mg2+ to express the full activity. Ni2+, Mn2+, and Co2+, but not Ca2+ or Zn2+, can substitute for Mg2+. The pH optimum, and the Vm and Km values of the phosphatase reaction are characteristics of the divalent cation cofactor. Ca2+ plus calmodulin increases the Vm in the presence of a given divalent cation, but has little effect on the Km for p-nitrophenyl phosphate. The activating effects of Mg2+ are different from those of the transition metal ions in terms of effects on Km, Vm, pH optimum of the phosphatase reaction and their affinity for calcineurin. Based on the Vm values determined in their respective optimum conditions, the order of effectiveness is: Mg2+ greater than or equal to Ni2+ greater than Mn2+ much greater than Co2+. The catalytic properties of calcineurin are markedly similar to those of p-nitrophenyl phosphatase activity associated with protein phosphatase 3C and with its catalytic subunit of Mr = 35,000, suggesting that there are common features in the catalytic sites of these two different classes of phosphatase.     

Mycobacterium tuberculosis ketopantoate hydroxymethyltransferase: tetrahydrofolate-independent hydroxymethyltransferase and enolization reactions with alpha-keto acids

The panB gene that encodes ketopantoate hydroxymethyltransferase has been cloned from Mycobacterium tuberculosis, expressed, and purified to homogeneity. 1H NMR spectroscopy was used to determine the rate of (i) tetrahydrofolate-independent hydroxymethyltransferase chemistry between formaldehyde and alpha-ketoisovalerate and (ii) deuterium exchange in the methylenetetrahydrofolate-independent enolization of alpha-ketoisovalerate and other alpha-keto acids, catalyzed by PanB. These studies have demonstrated that substrate enolization by PanB is divalent metal-dependent with a preference of Mg2+ > Zn2+ > Co2+ > Ni2+ > Ca2+. The rate of enolization is pH-dependent with optimal activity in the range of 7.0-7.5. The pH profile was bell-shaped, depending on the ionization state of two ionizable groups with apparent pK values of 6.2 and 8.3. Enolization and isotope exchange occurs with some alpha-keto acids (e.g., pyruvate and alpha-ketobutyrate), resulting in the complete exchange of all beta-hydrogens. Enzyme-catalyzed enolization and isotope exchange occur with other long-chain and branched alpha-keto acids, resulting in the stereospecific exchange of only one of the beta-hydrogen atoms. These results are discussed in the context of steric restrictions present in the enzyme active site and the stereochemistry of base-catalyzed isotope exchange.     

Metal cofactors play a dual role in Mycobacterium tuberculosis inorganic pyrophosphatase

Inorganic pyrophosphatase from Mycobacterium tuberculosis (Mt-PPase) is one of the possible targets for the rational design of anti-tuberculosis agents. In this paper, functional properties of this enzyme are characterized in the presence of the most effective activators--Mg2+ and Mn2+. Dissociation constants of Mt-PPase complexed with Mg2+ or Mn2+ are essentially similar to those of Escherichia coli PPase. Stability of a hexameric form of Mt-PPase has been characterized as a function of pH both for the metal-free enzyme and for Mg2+- or Mn2+-enzyme. Hexameric metal-free Mt-PPase has been shown to dissociate, forming monomers at pH below 4 or trimers at pH from 8 to 10. Mg2+ or Mn2+ shift the hexamer-trimer equilibrium found for the apo-Mt-PPase at pH 8-10 toward the hexameric form by stabilizing intertrimeric contacts. The pK(a) values have been determined for groups that control the observed hexamer-monomer (pK(a) 5.4), hexamer-trimer (pK(a) 7.5), and trimer-monomer (pK(a) 9.8) transitions. Our results demonstrate that due to the non-conservative amino acid residues His21 and His86 in the active site of Mt-PPase, substrate specificity of this enzyme, in contrast to other typical PPases, does not depend on the nature of the metal cofactor.     

Human complement factor I does not require cofactors for cleavage of synthetic substrates

Complement factor I (fI) plays a major role in the regulation of the complement system. It circulates in an active form and has very restricted specificity, cleaving only C3b or C4b in the presence of a cofactor such as factor H (fH), complement receptor type 1, membrane cofactor protein, or C4-binding protein. Using peptide-7-amino-4-methylcoumarin derivatives, we investigated the substrate specificity of fI. There is no previous report of synthetic substrate cleavage by fI, but five substrates were found in this study. A survey of 15 substrates and a range of inhibitors showed that fI has specificity similar to that of thrombin, but with much lower catalytic activity than that of thrombin. fI amidolytic activity has a pH optimum of 8.25, typical of serine proteases and is insensitive to ionic strength. This is in contrast to its proteolytic activity within the fI-C3b-fH reaction, in which the pH optimum for C3b cleavage is <5.5 and the reaction rate is highly dependent on ionic strength. The rate of cleavage of tripeptide 7-amino-4-methylcoumarins by fI is unaffected by the presence of fH or C3(NH(3)). The amidolytic activity is inhibited by the synthetic thrombin inhibitor Z-D-Phe-Pro-methoxypropylboroglycinepinanediol ester, consistent with previous reports, and by benzenesulfonyl fluorides such as Pefabloc SC. Suramin inhibits fI directly at concentration of 1 mM. Within a range of metal ions tested, only Cr(2+) and Fe(3+) were found to inhibit both the proteolytic and amidolytic activity of fI.     

Mechanism of action of Zn2+ and Mg2+ on rat placenta alkaline phosphatase. II. Studies on membrane-bound phosphatase in tissue sections and in whole placenta.

Alkaline phosphatase (EC 3.1.3.1) bound to trophoblastic cells in rat placenta is activated by Mg2+ and inhibited by Zn2+ in the same way as is found with partially purified soluble alkaline phosphatase in the same tissue (PetitClerc, C., Delisle, M., Martel, M., Fecteau, C. & Brière, N. (1975) Can. J. Biochem. 53, 1089-1100). In studies done with tissue sections (6-10 micron), it is shown that alkaline phosphatase activity and labelling of active sites by orthophosphate are lost during incubation with ethanolamine at pH 9.0. Addition of Mg2+ causes total recovery of catalytic activity and active sites labelling. Zn2+ displaces and replaces at the Mg2+ binding sites. The affinity for both ions is similar, and dissociation of Zn2+ from the enzyme is a very slow process, even in the presence of Mg2+. The Zn2+-alkaline phosphatase and Mg2+-alkaline phosphatase, which only differ by the ion bound to an apparent modulator site, have the same catalytic activity at pH less than 7.0, but the Zn2+ species has little activity at alkaline pH. Phosphorylation of the enzyme by orthophosphate indicates that with both enzyme species phosphoryl intermediate does not accumulate at alkaline pH. These results suggest that with orthophosphate, the phosphorylation step is rate determining for both enzymes, and that Zn2+ affects this step to a much greater extent. It is proposed that Zn2+ and Mg2+ regulate alkaline phosphatase in rat placenta. The concentration of both ions in maternal serum and placenta suggest that such a mechanism could exist in vivo.     

The role of divalent cations in structure and function of murine adenosine deaminase.

For murine adenosine deaminase, we have determined that a single zinc or cobalt cofactor bound in a high affinity site is required for catalytic function while metal ions bound at an additional site(s) inhibit the enzyme. A catalytically inactive apoenzyme of murine adenosine deaminase was produced by dialysis in the presence of specific zinc chelators in an acidic buffer. This represents the first production of the apoenzyme and demonstrates a rigorous method for removing the occult cofactor. Restoration to the holoenzyme is achieved with stoichiometric amounts of either Zn2+ or Co2+ yielding at least 95% of initial activity. Far UV CD and fluorescence spectra are the same for both the apo- and holoenzyme, providing evidence that removal of the cofactor does not alter secondary or tertiary structure. The substrate binding site remains functional as determined by similar quenching measured by tryptophan fluorescence of apo- or holoenzyme upon mixing with the transition state analog, deoxycoformycin. Excess levels of adenosine or N6- methyladenosine incubated with the apoenzyme prior to the addition of metal prevent restoration, suggesting that the cofactor adds through the substrate binding cleft. The cations Ca2+, Cd2+, Cr2+, Cu+, Cu2+, Mn2+, Fe2+, Fe3+, Pb2+, or Mg2+ did not restore adenosine deaminase activity to the apoenzyme. Mn2+, Cu2+, and Zn2+ were found to be competitive inhibitors of the holoenzyme with respect to substrate and Cd2+ and Co2+ were noncompetitive inhibitors. Weak inhibition (Ki > or = 1000 microM) was noted for Ca2+, Fe2+, and Fe3+.     

Interconversion of androgen receptor forms by divalent cations and 8 S androgen receptor-promoting factor. Effects of Zn2+, Cd2+, Ca2+, and Mg2+

The effects of divalent cations (Zn2+, Cd2+, Ca2+, Mg2+) on the cytosol androgen receptor were determined by sedimentation into sucrose gradients. At low ionic strength (25 mM KCl, 50 mM Tris, pH 7.4), Zn2+ (200 microM total, which calculates to 130 nM free Zn2+ in 10 mM mercaptoethanol) causes a shift in the sedimentation coefficient of the rat Dunning prostate tumor (R3327H) cytosol receptor and rat ventral prostate cytosol receptor from 7.5 +/- 0.3 S to 8.6 +/- 0.3 S. Zn2+ stabilizes the 8.6 S receptor form in salt concentrations up to 0.15 M KCl in 50 mM Tris, pH 7.2. In low ionic strength gradients containing Ca2+ (greater than or equal to 200 microM) or Mg2+ (greater than or equal to 1 mM), the receptor sediments as 4.7 +/- 0.3 S. The dissociating effects of Ca2+ and Mg2+ can be fully reversed by sedimentation into gradients containing Zn2+ (200 microM total) or Cd2+ (10 microM total). In the presence of Zn2+ (200 microM total), Ca2+ (10 microM to 3 mM) converts the receptor to an intermediate form with sedimentation coefficient 6.2 +/- 0.2 S, Stokes radius 73 A, and apparent Mr approximately 203,000. The potentiating effect of Zn2+ on formation of the 8.6 S receptor (in the absence of Ca2+) and the 6.2 S receptor (in the presence of Ca2+) requires both the 4.5 S receptor and the 8 S androgen receptor-promoting factor. Sodium molybdate stabilizes the untransformed cytosol receptor but, unlike Zn2+, does not promote reconstitution of the 8.6 S receptor from its partially purified components. These results indicate that divalent cations alter the molecular size of the androgen receptor in vitro and thus may have a role in altering the state of transformation of the receptor.     

Potentiometric measurements of stoichiometric and apparent affinity constants of EGTA for protons and divalent ions including calcium

The stoichiometric affinity constants of H+, Ca2+, Mg2+ and Sr+ for the ligand EGTA were determined using a modified version of the pH metric method developed by Moisescu and Pusch (Moisescu, D.G. and Pusch, H. (1975) Pfluegers Arch. 355, 243). The values obtained were slightly higher than those previously published. In addition, the shift in the H+ and Ca2+ stoichiometric constants with ionic strength was found to fit an empirical relationship if the total ionic content of the titration solutions was measured in terms of ionic equivalents, Ie (Johansson, L. (1975) Acta Chem. Scand. A29, 365-373), rather than the formal ionic strength, If. Finally, the apparent affinity of EGTA for Ca2+ ions was measured using an abbreviated form of the titration technique. The measured apparent affinity constant agreed with published results only if calculated with respect to pH (measured) of 7.0, rather than pH (concentration).     

Crystal structure of cis-biphenyl-2,3-dihydrodiol-2,3-dehydrogenase from a PCB degrader at 2.0 A resolution.

cis-Biphenyl-2,3-dihydrodiol-2,3-dehydrogenase (BphB) is involved in the aerobic biodegradation of polychlorinated biphenyls (PCBs). The crystal structure of the NAD+-enzyme complex was determined by molecular replacement and refined to an R-value of 17.9% at 2.0 A. As a member of the short-chain alcohol dehydrogenase/reductase (SDR) family, the overall protein fold and positioning of the catalytic triad in BphB are very similar to those observed in other SDR enzymes, although small differences occur in the cofactor binding site. Modeling studies indicate that the substrate is bound in a deep hydrophobic cleft close to the nicotinamide moiety of the NAD+ cofactor. These studies further suggest that Asn143 is a key determinant of substrate specificity. A two-step reaction mechanism is proposed for cis-dihydrodiol dehydrogenases.     

Crystal structure of human E1 enzyme and its complex with a substrate analog reveals the mechanism of its phosphatase/enolase activity

Enolase-phosphatase E1 (MASA) is a bifunctional enzyme in the ubiquitous methionine salvage pathway that catalyzes the continuous reactions of 2,3-diketo-5-methylthio-1-phosphopentane to yield the aci-reductone metabolite using Mg2+ as cofactor. In this study, we have determined the crystal structure of MASA and its complex with a substrate analog to 1.7A resolution by multi-wavelength anomalous diffraction and molecular replacement techniques, respectively. The structures support the proposed mechanism of phosphatase activity and further suggest the probable mechanism of enolization. We establish a model for substrate binding to describe in detail the enzymatic reaction and the formation of the transition state, which will provide insight into the reaction mechanisms of other enzymes in the same family.     

Phosphoenolpyruvate carboxykinase (guanosine 5'-triphosphate) from rat liver cytosol. Divalent cation involvement in the decarboxylation reactions

The presence of a divalent metal ion together with a catalytic amount of inosine 5'-diphosphate (IDP) is essential for the formation of pyruvate from oxalacetate catalyzed by purified rat liver cytosol phosphoenolpyruvate carboxykinase (PEPCK). With decreasing order of effectiveness, this pyruvate-forming activity was supported by micromolar levels of Cd2+, Zn2+, Mn2+, and Co2+. At the same concentrations, Mg2+ or Ca2+ was not effective. Combinations of Cd2+ with either Zn2+, Mn2+ or Co2+ were not additive with respect to the pyruvate-forming activity of PEPCK. Kinetic determination, with Cd2+ as the supporting cation, showed a 1:1 stoichiometry of interaction between each enzyme molecule and the nonconsumable substrate IDP. With 10 muM added Cd2+, the apparent Km for oxalacetate was 41 muM, and the apparent Ka for IDP was 0.25 muM. With Zn2+ or Mn2+, the apparent Ka for IDP was 0.2 or 0.13 muM, respectively. The effect of divalent transition-metal ions on PEPCK-catalyzed formation of phosphoenolpyruvate from oxalacetate was also investigated. Under steady-state conditions, the basal activity with MgITP was effectively enhanced with micromolar levels of Mn2+, Cd2+, or Co2+ included in the assay. The Vm increased 7- and 3.6-fold, and the apparent Km for MgITP changed by about a factor of 2 with the optimal concentrations of Mn2+ and Co2+, respectively. The most striking changes were in the apparent Km values for oxalacetate, which decreased to one-third and one-tenth when either Mn2+ or Co2+ was present in the assay together with Mg2+. The possible physiological importance of this kinetic effect is discussed.     

Ca2+-sensitivity of actomyosin ATPase purified from Physarum polycephalum.

A contractile protein closely resembling natural actomyosin (myosin B) of rabbit skeletal muscle was extracted from plasmodia of the slime mold, Physarum polycephalum, by protecting the SH-groups with beta-mercaptoethanol or dithiothreitol. Superprecipitation of the protein induced by Mg2+-ATP at low ionic strength was observed only in the presence of very low concentrations of free Ca2+ ions, and the Mg2+-ATPase [EC 3.6.1.3] reaction was activated 2- to 6-fold by 1 muM of free Ca2+ ions. Crude myosin and actin fractions were separated by centrifuging plasmodium myosin B in the presence of Mg2+-PPi at high ionic strength. The crude myosin showed both EDTA- and Ca2+-activated ATPase activities. The Mg2+-ATPase activity of crude myosin from plasmodia was markedly activated by the addition of pure F-actin from rabbit skeletal muscle. Addition of the F-action-regulatory protein complex prepared from rabbit skeletal muscle as well as the actin fraction of plasmodium caused the same degree of activation as the addition of pure F-actin only in the presence of very low concentrations of Ca2+ ion     

The role of biotin and sodium in the decarboxylation of oxaloacetate by the membrane-bound oxaloacetate decarboxylase from Klebsiella aerogenes

The biotin-containing oxaloacetate decarboxylase from Klebsiella aerogenes catalyzed the Na+-dependent decarboxylation of oxaloacetate to pyruvate and bicarbonate (or CO2) but not the reversal of this reaction, not even in the presence of an oxaloacetate trapping system. The enzyme catalyzed an avidin-sensitive isotopic exchange between [1-14C]pyruvate and oxaloacetate, which indicated the intermediate formation of a carboxybiotin enzyme. Sodium ions were not required for this partial reaction, but promoted the second partial reaction, the decarboxylation of the carboxybiotin enzyme, thus accounting for the Na+ requirement of the overall reaction. Therefore, the 14CO2-enzyme which was formed upon incubation of the decarboxylase with [4-15C]oxaloacetate, could only be isolated if Na+ ions were excluded. Preincubation of the decarboxylase with avidin also prevented its labelling with 14CO2. The isolated 14CO2-labelled oxaloacetate decarboxylase revealed the following properties. It was slowly decarboxylated at neutral pH and rapidly upon acidification. The 14CO2 residues of the 14CO2-enzyme could be transferred to pyruvate yielding [4-14C]oxaloacetate. In the presence of Na+ this 14CO2 transfer was repressed by the simultaneous decarboxylation of the 14CO2-enzyme. However, Na+ alone was insufficient as a cofactor for the decarboxylation of the isolated 14CO2-enzyme, since this required pyruvate in addition to Na+. It is therefore concluded that the decarboxylation of oxaloacetate proceeds over a CO2-enzyme--pyruvate complex and that free CO2-enzyme is an abortive reaction intermediate. The activation energy of the enzymic decarboxylation of oxaloacetate changed with temperature and was about 113 kJ below 11 degrees C, 60 kJ between 11 degrees C and 31 degrees C and 36 kJ between 31--45 degrees C.     

Mechanistic studies of glutamine synthetase from Escherichia coli. An integrated mechanism for biosynthesis, transferase, ATPase reaction.

The mechanism of biosynthetic, transferase, ATPase, and transphosphorylation reactions catalyzed by unadenylylated glutamine synthetase from E. coli was studied. Activation complex(es) involved in the biosynthetic reaction are produced in the presence of either Mg2+ or Mn2+ ; however, with the Mn2+-enzyme inhibition by the product, ADP, is so great that the overall forward biosynthetic reaction cannot be detected with the known assay methods. Binding studies show that substrates (except for NH3 and NH2OH which are not reported here) can bind to the enzyme in a random manner and that binding of the ATP-glutamate, ADP-Pi or ADP-arsenate pairs is strongly synergistic. Inhibition and binding studies show that the same binding site is utilized for glutamate and glutamine in biosynthetic and transferase reactions, respectively, and that a common nucleotide binding site is used for all reactions studied. Studies of the reverse biosynthetic reaction and results of fluorescent titration experiments suggest that both arsenate and orthophosphate bind at a site which overlaps the gamma-phosphate site of nucleoside triphosphate. In the reverse biosynthetic and transferase reactions, ATP serves as a substrate for the Mn2+-enzyme but not for the Mg2+-enzyme. The ATP supported transferase activity of Mn2+-enzyme is probably facilitated by the generation of ADP through ATP hydrolysis. When AMP was the only nucleotide substrate added, it was converted to ATP with concomitant formation of two equivalents of glutamate, under the reverse biosynthetic reaction conditions, and no ADP was detected. The reversibility of 180 transfer between orthophosphate and gamma-acyl group of glutamate was confirmed. ATPase activity of Mg2+ and Mn2+ unadenylylated enzymes is about the same. Both enzymes forms catalyze transphosphorylation reactions between various purine nucleoside triphosphates and nucleoside diphosphates under biosynthetic reaction conditions. The data are consistent with the hypothesis that a single active center is utilized for all reactions studied. Two stepwise mecanisms that could explain the results are discussed.     

The steady-state kinetic mechanism of ATP hydrolysis catalyzed by membrane-bound (Na+ + K+)-ATPase from ox brain. I. Substrate identity

A detailed steady-state kinetic investigation of the hydrolysis of ATP catalyzed by (Na+ + K+)-ATPase is reported. The activity was studied in the presence of (i) Na+ (130 mM), K+ (20 mM) and micromolar ATP concentrations and Na+ (150 mM) the ('Na+-enzyme'). The data obtained lead to the following results: 1. The action of each enzyme may be described by a simple kinetic mechanism with one (Na+-enzyme) or two ((Na+ + K+)-enzyme) dead-end Mg complexes. 2. For both enzymes, both MgATP and free ATP are substrates, with Mg2+, in the latter case, as the second substrate. 3. For each enzyme, the complete set of kinetic constants (seven for the Na+-enzyme, eight for the (Na+ + K+)-enzyme) are determined from the data. 4. For each enzyme it is shown that, in the alternate substrate mechanism obtained, the ratio of net steady-state flux along the 'MgATP pathway' to that of the 'ATP-Mg pathway' increases linearly with the concentration of free Mg2+. The parameters of this function are determined from the data. As a result of this, at high (greater than 3 mM) free Mg2+ concentrations the alternate substrate mechanism degenerates into a 'limiting' kinetic mechanism, with MgATP as the (essentially) sole substrate, and Mg2+ as an uncompetitive (Na+-enzyme) or non-competitive ((Na+ + K+)-enzyme) inhibitor.     

Interaction of Cd2+ with the calmodulin-activated (Ca2+ + Mg2+)-ATPase activity of human erythrocyte ghosts

Treatment of erythrocyte ghosts with micromolar concentrations of Cd2+ results in a noncompetitive inhibition of the calmodulin-dependent (Ca2+ + Mg2+)-ATPase activity. Higher concentrations of Cd2+ are required for inhibition of the (Ca2+ + Mg2+)-ATPase activity of calmodulin-depleted ghosts. The interaction of Cd2+ is time-dependent with an apparent rate constant around 0.12/min. The inhibition is relieved by addition of EGTA with a rate constant around 0.15/min. If Cd2+ is allowed to interact with calmodulin prior to the association of the protein with the ghosts, the inhibition is mainly competitive. The results suggest that the inhibitory effect caused by Cd2+ is due to an interaction with calmodulin. The slow interaction of Cd2+ suggests that calmodulin bound to the (Ca2+ + Mg2+)-ATPase is inaccessible to Cd2+.