Characterization of ATP binding sites of sheep kidney medulla (Na+ + K+)--ATPase using CrATP

The nucleotide substrate sites of sheep kidney medulla (NA+ + K+)-ATPase are characterized using CrATP, a paramagnetic, substitution-inert substrate analogue probe. The paramagnetic effect of CrATP on 1/T1 of water protons of water protons is enhanced upon complexation with the enzyme. Titrations of the enzyme with CrATP in the presence of Na+ and K+ yielded characteristic enhancements for the binary enzyme-CrATP and ternary enzyme-Mg2+-CrATP complexes of 3.3 and 3.6 and dissociation constants for CrATP of 5 and 12 microM, respectively. Substitution of Li+ for K+ in these titrations did not substantially alter the titration behavior. From the frequency dependence of 1/T1, the correlation time, tau c, for the dipolar water proton-CrATP interaction is 2.7 x 10(-10) sec, indicating that tau c is dominated by tau s, the electron spin relaxation time of Cr3+. The paramagnetic effect of enzyme-bound Mn2+ on 1/T1 of water protons decreases upon the addition of CrATP. Titration of the binary enzyme-Mn2+ complex with CrATP decreases the characteristic enhancement due to Mn2+ from 6.6-8.0 to 1.5. The failure to observe free Mn2+ epr signals in solutions of the ATPase, Mn2+, and CrATP demonstrate that this decrease in epsilon Mn is due to cross-relaxation between Mn2+ and Cr3+ bound simultaneously to the enzyme, and not to displacement of Mn2+ from the enzyme by CrATP. The relaxation rate, 1/T1, of 7Li+ is increased upon addition of CrATP to solutions of the ATPase, indicating that the sites for Li+ and CrATP are close on the enzyme. A Cr3+-Li+ distance of 4.8 +/- 0.5 angstrom is calculated from that data.     

Magnetic resonance and kinetic studies of the role of the divalent cation activator of RNA polymerase from Escherichia coli.

The interaction of Mn2+, substrates and initiators with RNA polymerase have been studied by kinetic and magnetic resonance methods. As determined by electron paramagnetic resonance, Mn2+ binds to RNA polymerase at one tight binding site with a dissociation constant less than 10 muM and at 6 +/- 1 weak binding sites with dissociation constants 100-fold greater. The binding of Mn2+ to RNA polymerase at both types of sites causes an order of magnitude enhancement of the paramagnetic effect of Mn2+ on the longitudinal relaxation rate of water protons, indicating the presence of residual water ligands on the enzyme-bound Mn2+. A kinetic analysis of the Mn2+-activated enzyme with poly(dT) as template indicates the substrate to be MnATP under steady-state conditions in the presence or absence of the initiator ApA. ATP and UTP interact with the tightly bound Mn2+ to form ternary complexes with approximately 50% greater enhancement factors. The dissociation constant of MnATP from the tight Mn2+ site as determined by longitudinal proton relaxation rate (PRR) titration (4.7 muM) is similar to the KM of MnATP in the ApA-initiated RNA polymerase reaction (10 +/- 3 muM) but not in the ATP-initiated reaction (160 +/- 30 muM). Similarly, the dissociation constant of the substrate MnUTP from the tight Mn2+ site (90 muM) is in agreement with the KM of MnUTP (101 +/- 13 muM) when poly[d(A-T)]-poly[d(A-T)] is used as template, indicating the tight Mn2+ site to be the catalytic site for RNA chain elongation. Manganese adenylyl imidodiphosphate (MnAMP-PNP) has been found to be a substrate for RNA polymerase. It has the same affinity as MnATP for the tight site but, unlike the results obtained with MnATP, the enhancement is decreased by 43% in the enzyme Mn-AMP-PNP complex. These results suggest that the enzyme-bound Mn2+ interacts with the leaving pyrophosphate group. The initiators ApA and ApU and the inhibitor rifamycin interact with the enzyme-Mn2+ complex producing small (15-20%) decreases in the enhancement. The dissociation constant of ApA estimated from PRR data (less than or equal to 1.5 muM) agrees with that determined kinetically (1.0 +/- 0.5 muM) as the concentration of ApA required to produce half-maximal change in the KM of MnATP. In the presence of the initiation specific reagents ApA, ApU, or rifamycin, the affinity of the enzyme-Mn complex for ATP or UTP shows little change. However, ATP and UTP no longer increase the enhancement factor of the tightly bound Mn2+ but decrease it by 30-55%, indicating a change in the environment of the Mn2+-substrate complex on the enzyme when the initiation site is either occupied or blocked. Although the role of the six weak Mn2+ binding sites is not clear, the presence of a single tightly bound Mn2+ at the catalytic site for chain elongation which interacts with the substrate reinforces the number of active sites as one per molecule of holoenzyme and provides a paramagnetic reference point for further structural studies.     

Mandelate racemase from Pseudomonas putida. Magnetic resonance and kinetic studies of the mechanism of catalysis.

The interactions of mandelate racemase with divalent metal ion, substrate, and competitive inhibitors were investigated. The enzyme was found by electron paramagnetic resonance (EPR) to bind 0.9 Mn2+ ion per subunit with a dissociation constant of 8 muM, in agreement with its kinetically determined activator constant. Also, six additional Mn2+ ions were found to bind to the enzyme, much more weakly, with a dissociation constant of 1.5 mM. Binding to the enzyme at the tight site enhances the effect of Mn2+ on the longitudinal relaxation rate (1/T1p) of water protons by a factor of 11.9 at 24.3 MHz. From the frequency dependence of 1/T1p, it was determined that there are similar to 3 water ligands on enzyme-bound Mn2+ which exchange at a rate larger than or equal to 10-7 sec-1. The correlation time for enzyme-bound Mn2+-water interaction is frequency-dependent, indicating it to be dominated by the electron spin relaxation time of Mn2+. Formation of the ternary enzyme-Mn2+-mandelate complex decreases the number of fast exchanging water ligands by similar to 1, but does not affect tau-c, suggesting the displacement or occlusion of a water ligand. The competitive inhibitors D,L-alpha-phenylglycerate and salicylate produce little or no change in the enzyme-Mn2+-H2O interaction, but ternary complexes are detected indirectly by changes in the dissociation constant of the enzyme-Mn2+ complex and by mutual competition experiments. In all cases the dissociation constants of substrates and competitive inhibitors from ternary complexes determined by magnetic resonance titrations agree with K-M and K-i values determined kinetically and therefore reflect kinetically active complexes. From the paramagnetic effects of Mn2+ on 1/T1 and 1/T2 of the 13C-enriched carbons of 1-[13C]-D,L-mandelate and 2-[13C]-D,L-mandelate, Mn2+ to carboxylate carbon and Mn2+ to carbinol carbon distances of 2.93 plus or minus 0.04 and 2.71 plus or minus 0.04 A, respectively, were calculated, indicating bidentate chelation in the binary Mn2+-mandelate complex. In the active ternary complex of enzyme, Mn2+, and D,L-mandelate, these distances increase to 5.5 plus or minus 0.2 and 7.2 plus or minus 0.2 A, respectively, indicating the presence of at least 98.9% of a second sphere complex in which Mn2+, and C1 and C2 carbon atoms are in a linear array. The water relaxation data suggest that a water ligand is immobilized between the enzyme-bound Mn2+ and the carboxylate of the bound substrate. This intervening water ligand may polarize or protonate the carboxyl group. From 1/T2p the rate of dissociation of the substrate from this ternary complex (larger than or equal to 5.6 times 10-4 sec-1) is at least 52 times greater than the maximal turnover number of the enzyme (1070 sec-1), indicating that the complex detected by nuclear magnetic resonance (NMR) is kinetically competent to participate in catalysis. Relationships among the microscopic rate constants are considered.     

Dual divalent cation requirement for activation of pyruvate kinase; essential roles of both enzyme- and nucleotide-bound metal ions.

Rabbit muscle pyruvate kinase requires two divalent cations per active site for catalysis of the enolization of pyruvate in the presence of adenosine 5'-triphosphate (ATP). One divalent cation is bound directly to the enzyme and forms a second sphere complex with the bound ATP (site 1). The second divalent cation is directly coordinated to the phosphoryl groups of ATP and does not interact with the enzyme (site 2). The essential role of the divalent cation at site 1 is shown by the requirement for Mg2+ or Mn2+ for the enolization of pyruvate in the presence of the substitution inert Cr3+-ATP complex. The rate of detritiation of pyruvate shows a hyperbolic dependence of Mn2+ concentration in the presence of high concentrations of enzyme and Cr3+-ATP. A dissociation constant for Mn2+ from the pyruvate kinase-Mn2+-ATP-Cr3+-pyruvate complex of 1.3 +/- 0.5 muM is determined by the kinetics of detritiation of pyruvate and by parallel Mn2+ binding studies using electron paramagnetic resonance. The essential role of the divalent cation at site 2 is shown by the sigmoidal dependence of the rate of detritiation of pyruvate on Mn2+ concentration in the presence of high concentrations of enzyme and ATP yielding a dissociation constant of 29 +/- 9 muM for Mn2+ from site 2. This value is similar to the dissociation constant of the binary Mn-ATP complex (14 +/- 6 muM) determined under similar conditions. The rate of detritiation of pyruvate is proportional to the concentration of the pyruvate kinase-Mn2+-ATP-Mn2+-pyruvate complex, as determined by parellel kinetic and binding studies. Variation of the nature of the divalent cation at site 1 in the presence of CrATP causes only a twofold change in the rate of detritiation of pyruvate which does not correlate with the pKa of the metal-bound water. Variation of the nature of the divalent cation at both sites in the presence of ATP causes a sevenfold variation in the rate of detritiation or pyruvate that correlates with the pKa of the metal-bound water. The greater rate of enolization observed with CrATP fits this correlation, indicating that the electrophilicity of the nucleotide bound metal (at site 2) determines the rate of enolization of pyruvate.     

Temperature dependent conformational changes at the active site of pyruvate kinase. A li nuclear magnetic resonance study.

The interaction of Li⁺, a weak activator of pyruvate kinase, with substrate and inhibitor complexes of the enzyme has been investigated by magnetic resonance techniques. Proton relaxation rate (PRR) titrations indicate that the dissociation constant of Li⁺ from the ternary enzyme-Mn(II)-phosphoenolpyruvate (P-enolpyruvate) complex is 15 mm at 5 °C and 17 mm at 30 °C. The electron paramagnetic resonance spectrum of the enzyme-Mn(II)-Li(I)-P-enolpyruvate complex is the superposition of spectra for two distinct species (Reed, G. H., and Cohn, M. (1973) J. Biol. Chem.248, 6436–6442). Low temperatures favor the form giving rise to the more nearly isotropic spectrum, whereas high temperatures favor the species giving rise to the anisotropic “K⁺-like” spectrum. ⁷Li nuclear magnetic resonance data are consistent with a model in which the two forms observed by epr correspond to differing Mn(II) to Li(I) distances. The form giving rise to the anisotropic spectrum is characterized by a Mn(II) to Li(I) distance of 4.7 Å, and in the more isotropic form this distance is approximately 9 Å. The 4.7 Å separation of the Mn(II) and Li(I) in the anisotropic form of the complex compares favorably with the 4.9 Å separation of Mn(II) and T1(I) (Reuben, J., and Kayne, F. J. (1971) J. Biol. Chem.246, 6227–6234) in the P-enolpyruvate complex, although T1⁺ is a much better activator of the pyruvate kinase reaction. Thus, a change in the distance between the monovalent and divalent cations does not account quantitatively for the lower activation by Li⁺, inasmuch as more than 50% of the enzyme-Mn(II)-Li(I)-P-enolpyruvate complex has the “active” conformation with respect to the separation of the cations and the epr spectrum of the complex. As reported previously (Reed, G. H., and Morgan, S. D. (1974) Biochemistry13, 3537–3541), the dissociation constant of oxalate and the epr spectrum for the ternary complex of pyruvate kinase with Mn(II) and oxalate are not influenced by the species of monovalent cation present. The nuclear relaxation rates of Li⁺ are increased in the presence of the ternary oxalate complex, although the separation of the Mn(II) and Li(I) appears to be much greater than for the “anisotropic” form of the P-enolpyruvate complex.     

Arrangement of the substrates at the active site of brain pyridoxal kinase

The distances between enzyme-bound paramagnetic CrATP (a stable, beta, gamma-bidentate complex of Cr3+ and ATP) at the active site of sheep brain pyridoxal kinase and the protons of bound inhibitor 4-dPyr (4-deoxypyridoxine) were determined in the ternary enzyme-CrATP.4-dPyr complex by measuring the paramagnetic effects of Cr3+ on the longitudinal relaxation rates (1/T1p) of the protons of 4-dPyr. The correlation time for the Cr(3+)-4-dPyr dipolar interaction on the enzyme was estimated as 1.59 ns by the frequency dependence of 1/T1p of water protons. Temperature dependence of 1/T1p values indicated the fast exchange of 4-dPyr from the paramagnetic enzyme.CrATP.4-dPyr complex; hence the measured 1/T1p values can be used for metalnucleus distance determinations. The distances from the Cr3+ of the enzyme-bound CrATP to the 2-methyl (7.19 A), 4-methyl (7.18 A), and H6 proton (6.18 A) of the 4-dPyr are too great to permit a direct coordination of any group from 4-dPyr. However, these distances can be built into a model in which phosphorus of the gamma-phosphoryl group of ATP is 4 A away from the oxygen atom of the 5-CH2OH group of the 4-dPyr. This suggests that phosphorylation of pyridoxal can occur via direct transfer of the phosphoryl group between the bound substrates at the active site of pyridoxal kinase.     

Structures of manganese(II) complexes with ATP, ADP, and phosphocreatine in the reactive central complexes with creatine kinase: electron paramagnetic resonance studies with oxygen-17-labeled ligands

Coordination of Mn(II) to the phosphate groups of the substrates and products in the central complexes of the creatine kinase reaction mixture has been investigated by electron paramagnetic resonance (EPR) spectroscopy with regiospecifically 17O-labeled substrates. The EPR pattern for the equilibrium mixture is a superposition of spectra for the two central complexes, and this pattern differs from those observed for the ternary enzyme-Mn(II)-nucleotide complexes and from that for the dead-end complex enzyme-Mn(II)ADP-creatine. In order to identify those signals that are associated with each of the central complexes of the equilibrium mixture, spectra were obtained for a complex of enzyme, Mn(II)ATP, and a nonreactive analogue of creatine, 1-(carboxymethyl)-2-iminoimidazolidin-4-one, which is a newly synthesized competitive inhibitor. This inhibitor permits an unobstructed view of the EPR spectrum for Mn(II)ATP in the closed conformation of the active site. The EPR spectrum for this nonreactive complex with Mn(II)ATP matches one subset of signals in the spectrum for the equilibrium mixture, i.e., those due to the enzyme-Mn(II)-ATP-creatine complex. Chemical quenching of the samples followed by chromatographic assays for both ATP and ADP indicates that the enzyme-Mn(II)ADP-phosphocreatine and the enzyme-Mn(II)ATP-creatine complexes are present in a ratio of approximately 0.7 to 1. A similar value for the equilibrium constant for enzyme-bound substrates is obtained directly from the EPR spectrum for the equilibrium mixture.(ABSTRACT TRUNCATED AT 250 WORDS)     

Equilibrium and water proton relaxation rate enhancement properties of formyltetrahydrofolate synthetase-manganous ion-substrate complexes.

Binding of Mn(pi)-nucleotide complexes to the enzyme formyltertrahydrofolate synthetase (EC 6.3.4.3) from Clostridium cylindrosporum has been examined in the presence and absence of other substrates by solvent proton relaxation mearurements. MnADP and MnATP form ternary complexes with the enzyme with highly enhanced proton relaxation rates for water. The enhancement parameters, epsilont, for the MnADP and MnATP ternary complexes are 19.8 and 12.5, respectively at 24.3 MHZ and 25 degrees. Titration curves with constant total concentrations of enzyme and Mn(pi) with variable nucleotide concentration are similar to those observed in similar titrations with the endp and MnATP are 175 muM and 64 muM, respectively at 25 degrees. Addition of tetrahydrofolate to solutions of the MnADP OR MnATP ternary complexes lowers the observed relaxation enhancement markedly. An analysis of titration curves with constant total concentrations of enzyme, Mn(pi), and nucleotide with variable tetrahydrofolate concentration gives the dissociation constant for tetrahydrofolate from the respective quaternary complexes. The affinity of the enzyme for tetrahydrofolate is increased 6-fold when MnADP is present at the active site whereas a 3-fold increase is observed with MnATP present. Furthermore, there is a 20-fold increase in the enzyme's affinity for tetrahydrofolate when both MnADP and the third substrate, formate, are present. The observed relaxation rate of water for solutions of the complex, enzyme-MnADP-tetrahydrofolate-formate, is deenhanced with respect to the rate observed for the simple aquo-Mn(pi) solution. Addition of nitrate to solutions of the above complex increases the affinity of the enzyme for tetrahydrofolate and MnADP by an additional factor of 5 and lowers the relaxation rate further to a value which approaches that for solutions of the enzyme and substrates which lack the paramagnetic cation.     

Manganese (II) and substrate interaction with unadenylylated glutamine synthetase (Escherichia coli w). II. Electron paramagnetic resonance and nuclear magnetic resonance studies of enzyme-bound manganese(II) with substrates and a potential transition-state analogue, methionine sulfoximine.

The enhancement of the longitudinal proton relaxation rate of solvent water protons which occurs when Mn(II) is bound to the "tight" metal ion site of unadenylylated glutamine synthetase (GS) was used to determine the binding constant of L-methionine (SR)-sulfoximine to GS-Mn(II) complexes. The binary enhancement for GS-Mn(II) is 22 at 24 MHz, 25 degrees C. The enhancement is lowered in the presence of the sulfoximine and the computed dissociation constant is 30 muM with epsilont, the enhancement for the ternary complex, equal to 3.0. Titration curves for the sulfoximine were also obtained in the presence of Mg-ADP, Mg-ADP plus Pi, and Mg-ATP. The dissociation constants were 9, 5, and 0.8 muM, respectively. The progressive tightening of the dissociation constants is symptomatic of conformational changes at the active site as the total subsite occupied by ATP is filled. The number of rapidly exchanging water molecules drops from 2 to approximately 0.1 when saturating concentrations of L-methionine (SR)-sulfoximine and nucleotide are present. The kinetically determined KI value of approximately 4 muM for the sulfoximine is about three orders of magnitude tighter than thee Km' value of approximately 3 mM for L-glutamate. The previously mentioned dissociation constants obtained by enhancement titrations are also orders of magnitude tighter than Km'. These data suggest that L-methionine (SR)-sulfoximine is a "transition-state" analogue for the glutamine synthetase reaction. ...     

Magnetic resonance studies of the interaction of Co2+ and phosphoenolpyruvate with pyruvate kinase.

Co2+, which activates rabbit muscle pyruvate kinase, competes with Mn2+ for the active site of the enzyme with a KD of 46 muM. Co2+ binds to phosphoenolpyruvate with a KD of 4.1 mM. The structures of the binary Co2+/P-enolpyruvate, and quaternary pyruvate kinase/Co2+/K+/P-enolpyruvate complexes were studied using EPR and the effects of Co2+ on the longitudinal (T1) and transverse (T2) relaxation times of the protons of water and P-enolpyruvate and the phosphorus of P-enolpyruvate. The EPR spectra of all complexes at 6 K, disappear above 40 K and reveal principal g values between 2 and 7 indicating high spin Co2+. For free Co2+ and for the binary Co2+/P-enolpyruvate complex, the T1 of water protons was independent of frequency in the range 8, 15, 24.3, 100, and 220 MHz. Assuming coordination numbers (q) of 6 and 5 for free Co2+ and Co2+/P-enolpyruvate, respectively, correlation times (tauc) of 1.3 times 10(-13) and 1.7 times 10(-13) s, were calculated. The distances from Co2+ and phosphorus and to the cis and trans protons in the binary Co2+/P-enolpyruvate complex calculated from their T1 values were 2.7 A, 4.1 A, AND 5.3 A, respectively, indicating an inner sphere phosphoryl complex. Consistent with direct phosphoryl coordination, a large Co2+ to phosphorus hyperfine contact coupling constant (A/h) of 5 times 10(5) Hz was determined by the frequency dependence of the T2 of phosphorus at 25.1, 40.5, and 101.5 MHz. For both enzyme complexes, the dipolar correlation time tauc was 2 times 10(-12) s and the number of rapidly exchanging water ligands (q) was 0.6 as determined from the frequency dependence of the T1 of water protons. In the quaternary enzyme/Co2+/K+/P-enolyruvate complex this tauc value was consistent with the frequency dependence of the T1 of the phosphorus of enzyme-bound P-enolpyruvate at 25.1 and 40.5 MHz. Distances from enzyme-bound C02+ to the phosphorus and protons of P-enolpyruvate, from their T1 values, were 5.0 A and 8 to 10 A, respectively, indicating a predominantly (greater than or equal to 98%) second spere complex and less than 2% inner sphere complex. Consistent with a second sphere complex on the enzyme, an A/h value of less than 10(3) Hz was determined from the frequency dependence of the T2 of phosphorus. In all complexes the exchange reates were found to be faster than the paramagnetic relaxation rates and the hyperfine contact interaction was found to be small compared to the dipolar interaction. The results thus indicate that the interaction of C02+ with P-enolpyruvate is greatly decreased upon binding to the active site of pyruvate kinase.     

Kinetic and magnetic resonance studies of effects of genetic substitution of a Ca2+-liganding amino acid in staphylococcal nuclease

The X-ray structure of staphylococcal nuclease suggests octahedral coordination of the essential Ca2+, with Asp-21, Asp-40, and Thr-41 of the enzyme providing three of the six ligands [Cotton, F. A., Hazen, E. E., Jr., & Legg, M. J. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2551-2555]. The Asp-40 codon was mutated to Gly-40 on the gene that had been cloned into Escherichia coli, and the mutant (D40G) and wild-type enzymes were both purified from E. coli by a simple procedure. The D40G mutant forms a (5 +/- 2)-fold weaker binary complex with Ca2+ as found by kinetic analysis and by Ca2+ binding studies in competition with Mn2+, a linear competitive inhibitor. Similarly, as found by electron paramagnetic resonance (EPR), Mn2+ binds to the D40G mutant with a 3-fold greater KD than that found with the wild-type enzyme. These differences in KD are increased by saturation of staphylococcal nuclease with the DNA substrate such that KmCa is 10-fold greater and KIMn is 15-fold greater for the mutant than for the wild-type enzyme, although KMDNA is only 1.5-fold greater in the mutant. The six dissociation constants of the ternary enzyme-Mn2+-nucleotide complexes of 3',5'-pdTp and 5'-TMP were determined by EPR and by paramagnetic effects on 1/T1 of water protons, and the dissociation constants of the corresponding Ca2+ complexes were determined by competition with Mn2+. Only small differences between the mutant and wild-type enzymes are noted in K3, the dissociation constant of the nucleotides from their respective ternary complexes. 3',5'-pdTp raises the affinities of both wild-type and mutant enzymes for Mn2+ by factors of 47 and 31, respectively, while 5'-TMP raises the affinities of the enzymes for Mn2+ by smaller factors of 6.8 and 4.4, respectively. Conversely, Mn2+ raises the affinities of both wild-type and mutant enzymes for the nucleotides by 1-2 orders of magnitude. Analogous effects are observed in the ternary Ca2+ complexes. Dissociation constants of Ca2+ and Mn2+ from binary and ternary complexes, measured by direct binding studies, show reasonable agreement with those obtained by kinetic analysis. Structural differences in the ternary metal complexes of the D40G mutant are revealed by a 31-fold decrease in Vmax with Ca2+ and by 1.4-3.1-fold decreases in the enhancement of 1/T1 of water protons with Mn2+.(ABSTRACT TRUNCATED AT 400 WORDS)     

25Mg NMR studies of yeast enolase and rabbit muscle pyruvate kinase.

25Mg NMR spectroscopy was used to study the interactions of the activating cations with their respective binding sites in the enzymes yeast enolase and rabbit muscle pyruvate kinase (PK). Titration of Mg2+ with enolase allows for the calculation of 1/T2 for Mg2+ bound at site I of 1510 s-1 and a quadrupolar coupling constant chi = 0.30 MHz. Titration of Mg2+ with enolase in the presence of 2-phosphoglycerate (PGA) and Zn2+, where Zn2+ binds specifically at site I, gives a 1/T2 for Mg2+ bound at site II of 4000 s-1 (chi = 0.49 MHz). The Mg2+ at site II appears to be more anisotropic than Mg2+ at site I. The titration of site I of the enolase-Mg-PGA-Mg complex with Zn2+ or Mn2+ shows a simple displacement of the Mg2+. No paramagnetic effects by Mn2+ on 25Mg relaxation were observed. Temperature studies of the 25Mg resonance show that fast exchange of the Mg2+ occurs under these conditions. From the lack of a paramagnetic effect, the distance between the cations at sites I and II must be more than 6-9 A. This distance limits the location, hence the function, of the cation at site II for catalytic activity. Titration of Mg2+ with PK gives a 1/T2 for bound Mg2+ of 2200 s-1 (chi = 0.24 MHz). A titration of Mg2+ with PK in the presence of the inhibitor oxalate gives a 1/T2 of 400 s-1. The temperature dependence of 25Mg relaxation in the PK-Mg-oxalate complex is consistent with slow exchange (Ea = 6.1 +/- 1.6 kcal/mol). The enzyme-bound cation is more tightly sequestered by the addition of a ligand that binds directly to the cation. An investigation of the 25Mg relaxation in the PK-Mn-oxalate-Mg-ATP complex, where the Mg2+ is bound to the nucleotide and the Mn2+ was enzyme bound, was not successful due to precipitation of PK under experimental conditions and the short T2 relaxation for 25Mg in this complex. The applications of 25Mg NMR have been useful in partially describing the properties of the bound Mg2+ in these two metal-requiring enzymes.     

Conformations and arrangement of substrates at active sites of ATP-utilizing enzymes

The phosphoryl transferring enzymes pyruvate kinase, cAMP-dependent protein kinase and the pyrophosphoryl transferring enzyme PP-Rib-P synthetase utilize the beta, gamma bidentate metal--ATP chelate (delta-isomer) as substrate, as determined with substitution-insert CrIIIATP or CoIII(NH3)4ATP complexes. In addition, these enzymes bind a second divalent cation, which is an essential activator for pyruvate kinase and PP-Rib-P synthetase and an inhibitor of protein kinase. The enzyme-bound metal has been used as a paramagnetic reference point in T1 measurements to determine distances to the protons and phosphorus atoms of the bound nucleotide and acceptor substrates. These distances have been used to construct models of the conformations of the bound substrates. The activating metal forms a second sphere complex of the metal-nucleotide substrate on pyruvate kinase and PP-Rib-P synthetase while the inhibitory metal directly coordinates the polyphosphate chain of the metal-nucleotide substrate on protein kinase. Essentially no change is found in the dihedral angle at the glycosidic bond of ATP upon binding to pyruvate kinase (chi = 30 degrees), an enzyme of low base specificity, but significant changes in the torsional angle of ATP occur on binding to protein kinase (chi = 84 degrees) and PP-Rib-P synthetase (chi = 62 degrees), enzymes with high adenine-base specificity. Intersubstrate distances, measured with tridentate CrATP or beta, gamma bidentate CrAMPPCP as paramagnetic reference points, have been used to deduce the distance along the reaction coordinate on each enzyme. The reaction coordinate distances on pyruvate kinase (# +/- 1 A) and PP-Rib-P synthetase (not less than 3.8 A) are consistent with associative mechanisms, while that on protein kinase (5 +/- 0.7 A) allows room for a dissociative mechanism.     

Magnetic resonance studies of the manganese guanosine di- and triphosphate complexes with elongation factor Tu.

Analysis of titration data of EF-Tu-GDP with Mn(II) where free and bound Mn(II) were determined by proton relaxation rate of water (PRR) yields one tight Mn(II) binding site and a value of 2 muM for the dissociation constant of Mn(II) from the EF-Tu-MnGDP complex, K'A. The dissociation constant of manganese nucleotide from the ternary EF-Tu-MnGDP complex, K2, 0.2 muM, was derived from the known value of Ks, the dissociation constant for the binary EF-Tu-GDP complex, and the titration data of the ternary complex with excess GDP as titrant. The apparent number, n, of rapidly exchanging water ligands coordinated to bound Mn(II) in the ternary complex EF-Tu-MnGDP is estimated from the frequency dependence of the PRR of the complex to be approximately 1. The value of n and the values of PRR enhancements, epsilont = 4.3 for EF-Tu-MnGDP at 21 degrees, 24.3 MHZ and epsilont = 4.1 for the ternary GTP complex, are unusually low for protein-Mn-nucleotide complexes. The antibiotic X5108 which induces GTPase activity in EF-Tu-MgGTP was shown to bind stoichiometrically to EF-Tu-MnGDP and thereby change the PRR enhancement of the complex from 4.3 to 7.4. The characteristic broad lines in the EPR spectra of Mn(II) nucleotides are strikingly narrowed upon binding of Mn(II) nucleotides to EF-Tu. The long electron spin relaxation times inferred from the EPR spectra indicate a limited access of solvent water to the first coordination sphere of Mn(II) in its EF-Tu-nucleotide complexes. The frequency dependence of the PRR indicates that the electron spin relaxation time, T1e, is the dominant process modulating the Mn(II)-H2O interaction of the EF-Tu-MnGDP complex and consequently determines the correlation time. The value of T1e, estimated from the PRR experiments to be 2.5 ns at 21 degrees, is consistent with the lower limit of T1e obtained from the line widths of the EPR spectrum of the complex. Upon binding of a stoichiometric quantity of the antibiotic X5108, the EPR spectrum of EF-Tu-MnGDP is severely broadened indicating greater access of solvent water to the manganese coordination sphere, i.e. an opening of the nucleotide binding site as already suggested by the increased PRR enhancement.     

Magnetic resonance studies of the spatial arrangement of glucose-6-phosphate and chromium (III)-adenosine diphosphate at the catalytic site of hexokinase.

The interaction of CrADP, an exchange-inert paramagnetic analogue of Mg-ADP, with yeast hexokinase has been studied by measuring the effects of CrADP on the longitudinal nuclear relaxation rate (1/T1) of the protons of water and the protons and phosphorus atom of enzyme-bound glucose-6-P. The paramagnetic effect of CrADP on 1/T1 of water protons is enhanced upon complexation with the enzyme. Titrations measuring this paramagnetic effect at several enzyme concentrations in the presence of glucose-6-P yielded a characteristic enhancement factor for 1/T1 of water protons and the dissociation constant of CrADP from the ternary enzyme . ADPCr . glucose-6-P complex. The latter value (2 mM) is similar to that obtained from kinetic inhibition studies (Danenberg and Cleland [1975]. Biochemistry. 14:28). The presence of glucose-6-P increased the enhancement of the water relaxation rate by enzyme-bound CrADP, suggesting the formation of an enzyme . CrADP . glucose-6-P complex. The existence of such a complex was confirmed by the observation of a paramagnetic effect of enzyme-bound CrADP on the l/T1 of the 31P-nucleus and protons of enzyme-bound glucose-6-P. From the paramagnetic effects of enzyme-bound CrADP on the relaxation rates of the 31P-nucleus and the carbon-bound protons of glucose-6-P in the enzyme . ADPCr . glucose-6-P complex, using the correlation time of approximately 0.7 ns, determined from the magnetic field-dependence of 1/T1 of water protons over the range 24.3-360 MHz, a Cr3+ to phosphorus distance of 6.6 +/- 0.7 A and Cr3+ to alpha- and beta-anomeric proton distances of 8.9 and 9.7 A were calculated. These results imply the absence of a direct coordination of the phosphoryl group of glucose-6-P by the nucleotide-bound metal on hexokinase but indicate van der Waals contact between a phosphoryl oxygen of glucose-6-P and the hydration sphere of the nucleotide-bound metal. The distances are consistent with a model that assumes molecular contact between the phosphorus of glucose-6-P and a beta-phosphoryl oxygen of ADP suggesting an associative phosphoryl transfer. Because after phosphorylation of ADP, the metal ion is coordinated to the transferred phosphoryl group, the overall migration of the phosphoryl group during the phosphoryl transfer is approximately 3.6 A toward the nucleotide-bound metal. Little or no catalysis of phosphoryl transfer from glucose-6-P to alpha, beta-bidentate or beta-monodentate CrADP ( less than or equal to 0.05% of the rate found with MgADP) occurred in the presence of hexokinase, as monitored by glucose formation in a coupled assay system using glucose oxidase and peroxidase. The ability of beta, gamma-bidentate CrATP to act as a substrate (Danenberg and Cleland [1975].     

Metal ion and substrate binding to bovine galactosyltransferase

Bovine milk galactosyltransferase was examined by ESR and NMR proton relaxation measurements to determine the stoichiometry and nature of manganese and UDP-Gal substrate binding. The ESR and NMR data clearly showed the binding of two (Mn(II) per mol of enzyme in the ternary complex (enzyme-manganese-UDP-Gal). The affinity of the enzyme for manganese is much higher in the presence of UDP-Gal than in its absence. A deenhancement was observed in both water and UDP-Gal proton relaxation rates upon ternary complex formation [enzyme-Mn(II)-UDP-Gal] relative to the metal-substrate [Mn(II)-UDP-Gal] binary complex, yet the temperature dependence of the water proton relaxation rate was consistent with fast exchange. A simple model was proposed which accounted for the pronounced deenhancement, involving a slow conformational interconversion of an initially formed, rapidly exchanging conformer of the enzyme-Mn(II)-UDP-Gal complex to a second form which contributes negligibly to the relaxation.     

Conformational changes required for pyruvate kinase activity as modulated by monovalent cations.

The interaction of a series of alkylamines with muscle pyruvate kinase was investigated by kinetic and physical studies in order to understand the mechanisms by which certain monovalent cations can activate the enzyme and to define several of the important conformational changes necessary for catalytic activity. Monomethylammonium ion interacts with pyruvate kinase to activate the enzyme. Dimethyland trimethylammonium ions do not activate, but are competitive inhibitors against activating cations. Tetramethylammonium ion neither activates nor inhibits pyruvate kinase activity. When the enzyme is in the presence of monomethylammonium ion or dimethylammonium ion, a conformational change is observed by ultraviolet difference spectroscopy. This conformational change is similar to that observed with other activating cations and appears to be a necessary but no sufficient conformational change in the formation of an active complex. The interaction of the substrate phosphoenolpyruvate with the pyruvate kinase-Mn2+ complex in the presence of these cations was studied by water proton relaxation rate measurements. The affinity of the enzyme-Mn2+ complex for phosphoenolpyruvate is decreased by a factor of 5 in the presence of any of the alkylamines compared to the affinity measured in the presence of K+ or NH4+. No change in the Km of phosphoenolpyruvate is observed however when it is measured in the presence of monomethylammonium ion, suggesting that the decrease in affinity for the substrate is not the reason for lack of enzymic activity. The conformation of the ternary enzyme-Mn2+-phosphoenolpyruvate complex about the bound Mn2+, as reflected by the enhancement values (epsilont) measured, differs depending upon the nature of the monovalent cation. The epsilon t values measured in the presence of the alkylamines are larger (epsilont - 5.7 +/- 0.2) than those measured in the presence of K+ or NH4+ (epsilont = 1.9 +/- 0.1).     

Proton magnetic relaxation studies of the interaction of D-xylose and xylitol with D-xylose isomerase. Characterization of metal-enzyme-substrate interactions.

The interaction of D-xylose isomerase purified from two sources with Mn2+ and D-xylose or the competitive inhibitor xylitol has been examined by nuclear magnetic resonance. A greater paramagnetic effect of enzyme-bound Mn2+ on the alpha anomer of D-xylose than on the beta anomer was observed, providing independent evidence for the specificity of D-xylose isomerase for the alpha anomeric form of D-xylose. The exchange rate of alpha-D-xylose into the ternary complex, determined from the normalized paramagnetic contribution to the transverse relaxation rate (1/fT2p) of the carbon 1 proton of alpha-D-xylose, exceeds Vmax for the enzymatic reaction by 3 orders of magnitude. The amount of xylitol necessary to displace alpha-D-xylose from the substrate-enzyme-Mn2+ complex is consistent with the Km value for alpha-D-xylose and the inhibitor constant Ki for xylitol previously determined by the methods of enzyme kinetics. These results suggest that the NMR experiments observe complexes of D-xylose isomerase which are kinetically and thermodynamically competent to participate in catalysis. From the frequency dependence of the paramagnetic contribution to the longitudinal relaxation rate (1/T1p) of the carbon 1 proton of alpha-D-xylose, the correlation time (tauc) which modulates the dipolar interaction between enzyme-bound Mn2+ and alpha-D-xylose has been determined (5.1 x 1o(-10) s). From these observations a range of calculated distances between enzyme-bound Mn2+ and the carbon 1 proton of alpha-D-xylose (9.1 +/- 0.7 A) has been found. The enzyme-bound Mn2+ has comparable effects on the carbon 1, carbon 2, and carbon 5 protons of alpha-D-xylose, suggesting that these protons of the enzyme-bound substrate are equidistant from the bound Mn2+. A similar distance (9.4 +/- 0.7 A) between the enzyme-bound Mn2+ and the terminal methylene protons of xylitol, an analog of the open chain intermediate in the reaction, has been determined. The results of the present substrate relaxation and previous water relaxation studies suggest that two small ligands such as water molecules or a large portion of the protein intervene between the bound metal ion and the bound substrate in the active ternary complex.     

Involvement of a divalent cation in the binding of fructose 6-phosphate to Trypanosoma cruzi phosphofructokinase: kinetic and magnetic resonance studies

When Mg2+ ions were replaced by Mn2+ in the assay of Trypanosoma (Schizotrypanum) cruzi phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) the Km for D-fructose 6-phosphate (F6P) was reduced threefold while the corresponding constant for ATP was essentially unaffected. A detailed kinetic investigation showed that the apparent Km for F6P decreased monotonically with increasing free Mn2+ concentrations, from a limiting value of 5.7 mM in its absence to a limiting value of 1.1 mM in the presence of saturating concentrations of the ion; the Vmax of the enzyme was, on the other hand, not affected by the concentration of Mn2+. Conversely, it was shown that the apparent Km for Mn2+ at fixed MnATP concentrations decreased with increasing F6P concentrations, from a limiting value of 30 microM in the absence of the sugar phosphate to 9 microM at saturating concentrations of the substrate, while the apparent Vmax increased monotonically from zero to its limiting value. Both electron paramagnetic resonance and water proton longitudinal relaxation studies showed binding of one Mn2+ ion per 18,000 Da catalytic subunit of enzyme in the absence of F6P, with a dissociation constant of 57 +/- 4 microM, comparable to the apparent Km for the ion in the absence of F6P. The presence of saturating level of F6P decreases the value of the dissociation constant of Mn2+ to a limiting value of 7.9 microM in agreement with the results of the kinetic analysis. The substrate F6P decreases the enhancement of the water proton longitudinal relaxation rate in a saturable fashion, suggesting displacement of water molecules coordinated to the enzyme-bound Mn2+ ion by the sugar phosphate. Computer fitting of the several dissociation constants and relaxation enhancements for binary and ternary complexes gives a value of 7.9 mM for the dissociation constant of the enzyme-F6P complex in the absence of Mn2+ and 1.1 mM in the presence of saturating concentrations of the ion, in excellent agreement with the respective Km values of F6P extrapolated to zero and saturating Mn2+, respectively. Studies of the frequency dependence of the water proton longitudinal relaxation rate enhancements in the presence of both binary (enzyme-Mn2+) and ternary (enzyme-Mn2(+)-F6P) complexes, are most simply explained by assuming two exchangeable water molecules in the coordination sphere of the enzyme-bound Mn2+ in the binary complex, while in the ternary complex the data are consistent with the displacement of one of the water molecule from the coordination sphere with no significant alteration of the correlation time. Overall, the kinetic and binding data are consistent with the formation of an enzyme-metal-F6P bridge complex at the active site of T. cruzi phosphofructokinase, a coordination scheme which is unique among the phosphofructokinases.     

Magnetic resonance studies of the proximity and spatial arrangement of propionyl coenzyme A and pyruvate on a biotin-metalloenzyme, transcarboxylase.

A spin-labeled ester of CoA, R-CoA (3-carboxy-2,2,5,5-tetramethyl-1-pyrolidinyl-1-oxy CoA thioester), has been shown by competition studies using electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) to bind specifically to the propionyl-CoA binding sites of transcarboxylase. Titrations indicate 0.7 +/- 0.2 binding site for R-CoA per enzyme-bound biotin with a dissociation constant of 0.33 +/- 0.12 mM. Propionyl-CoA binds to this site with a 1.3-fold lower disonable agreement with kinetically determined inhibitor constants of CoA and propionyl-CoA and propionyl-CoA (D. B. Northrop (1969), J. Biol. Chem. 244, 5808). The bit of this spin-label on 1/T1 of water protons. The formation of a ternary transcarboxylase-R-CoA-pyruvate complex is suggested by the failure of pyruvate to displace R-CoA from the tight site and is established by the paramagnetic effects of enzyme-bound R-CoA on the relaxation rates of the protons and 13C atoms of enzyme-bound pyruvate. From the paramagnetic effects of R-CoA on the relaxation rates of the methyl protons of pyruvate at 40.5 and 100 MHz, and on the 13C-enriched carbonyl and carboxyl carbon atoms of pyruvate at 25.1 MHz, a correlation time of 7 nsec and distances from the bound nitroxide radical to the methyl protons, the carbonyl, and carboxyl carbon atoms of bound pyruvate of 7.9 +/- 0.7, 10.3 +/- 0.8, and 12.1 +/- 0.9 A, respectively, are calculated. These distances establish the close proximity of the CoA ester and keto acid sites on transcarboxylase. Together with the previously determined distances from the enzyme-bound (Co(II) to the methyl protons and 2 carbon atoms of bound pyruvate and to 12 protons and 3 phosphorus atoms of bound propionyl-CoA, the present distances are used to derive a composite model of the bound substrates in the overall transcarboxylation reaction. In this model the distance from the methyl carbon of pyruvate and the methylene carbon of propionyl-CoA, between which the carboxyl transfer takes place is only approximately 7 A. Depending on the detailed mechanism of the carboxyl transfer, the distance through which the carboxybiotin must migrate is therefore between 0 and 7 A. Hence the major role of the 14-A arm of carboxybiotin is not to permit a large carboxyl migration but, rather to permit carboxybiotin to traverse the gap which occurs at the interface of three subunits and to insinuate itself between the CoA and keto acid sites.