Catalytic and regulatory site composition of yeast AMP deaminase by comparative binding and rate studies. Resolution of the cooperative mechanism

Yeast AMP deaminase is allosterically activated by ATP and MgATP and inhibited by GTP and PO4. The tetrameric enzyme binds 2 mol each of ATP, GTP, and PO4/subunit with Kd values of 8.4 +/- 4.0, 4.1 +/- 0.6, and 169 +/- 12 microM, respectively. At 0.7 M KCl, ATP binds to the enzyme, but no longer activates. Titration with coformycin 5'-monophosphate, a slow, tight-binding inhibitor, indicates a single catalytic site/subunit. ATP and GTP bind at regulatory sites distinct from the catalytic site and their binding is mutually exclusive. Inorganic phosphate competes poorly with ATP for the ATP sites (Kd = 20.1 +/- 4.1 mM). However, near-saturating ATP reduces the moles of phosphate bound per subunit to 1 PO4, which binds with a Kd = 275 +/- 22 microM. In the presence of ATP, PO4 cannot effectively compete with ATP for the nucleotide triphosphate sites. The PO4 which binds in the presence of ATP is competitive with AMP at the catalytic site since the Kd equals the kinetic inhibition constant for PO4. Initial reaction rate curves are a cooperative function of AMP concentration and activation by ATP is also cooperative. However, no cooperativity is observed in the binding of any of the regulator ligands and ATP binding and kinetic activation by ATP is independent of substrate analog concentration. Cooperativity in initial rate curves results, therefore, from altered rate constants for product formation from each (enzyme.substrate)n species and not from cooperative substrate binding. The traditional cooperative binding models of allosteric regulation do not apply to yeast AMP deaminase, which regulates catalytic activity by kinetic control of product formation. The data are used to estimate the rates of AMP hydrolysis under reported metabolite concentrations in yeast.     

8-Azido-2'-O-dansyl-ATP. A fluorescent photoaffinity reagent for ATP-binding proteins and its application to adenylate kinase

The photoaffinity reagent 8-azido-2'-O-[14C]dansyl-ATP (AD-ATP) has been synthesized for labeling and monitoring the active sites of ATPases and kinases. In its first application, the reagent is used to explore the active site of adenylate kinase from rabbit muscle. In the dark, AD-ATP inhibits adenylate kinase reversibly and competitively with KI = 0.25 +/- 0.01 microM. Under weak UV illumination, AD-ATP labels adenylate kinase irreversibly. The photoinactivation data also show KI = 0.25 +/- 0.02 microM. The ratio (r) of the specific activity of AD-ATP-labeled adenylate kinase to that of the unlabeled enzyme has been determined as a function of the number (n) of label/enzyme. The linear plot of r versus n with slope equal to -1 shows that the labeling is very specific, i.e. each label completely inactivates an enzyme molecule. After the labeled enzyme was partially hydrolyzed and the radioactive peptides analyzed and sequenced, it was found that Leu-115, Cys-25, and probably His-36 were labeled, in agreement with previous conclusions on the structure of the active site of this enzyme based on amino acid sequence, x-ray diffraction, and NMR studies. The environment-sensitive fluorescent dansyl group of AD-ATP can function as an in situ probe for monitoring ligand or conformation changes at the active site. The fluorescence of AD-ATP-labeled enzyme with n = 0.9 is not affected by ATP but increases with the concentration of AMP in solution. This observation is also in agreement with the previous conclusion that ATP does not bind to the AMP site of adenylate kinase. The observed enhancement of fluorescence indicates that binding of AMP by this enzyme causes environmental change at its ATP site. The possible usefulness of AD-ATP as an effective biological inhibitor or as a molecular probe for studying the structure and regulation of ATP-binding proteins is discussed.     

Photoaffinity labeling of the allosteric AMP site of biodegradative threonine dehydratase of Escherichia coli with 8-azido-AMP

The photoreactive AMP analog, 8-azido-AMP, stimulated the activity of biodegradative threonine dehydratase of Escherichia coli in a reversible manner and, like AMP, decreased the Km for threonine. The concentrations required for half-maximal stimulation by AMP and 8-azido-AMP were 40 microM and 1.5 microM, respectively, and the maximum stimulation by 8-azido-AMP was 25% of that seen with AMP. Gel-filtration experiments revealed that 8-azido-AMP stabilized a dimeric form of the enzyme, whereas AMP promoted a tetrameric species. When present together, AMP and 8-azido-AMP showed mutual competition in influencing catalytic activity as well as the conformational state of the protein. Photolabeling of AMP-free dehydratase with 8-azido-[2-3H]AMP resulted in a time and concentration-dependent enzyme inactivation and concomitant incorporation of 8-azido-AMP into protein. At low 8-azido-AMP concentrations, incorporation of about 1 mol 8-azido-AMP/mol dehydratase tetramer was correlated with almost complete inactivation of the enzyme. The presence of AMP in the photolabeling reaction greatly reduced the extent of enzyme inactivation and 8-azido-AMP binding. Ultraviolet irradiation with 20 microM 3H-labeled 8-azido-AMP revealed one tryptic peptide, Thr230-Thr-Gly-Thr-Leu-Ala-Asp-Gly-Cys-Asp-Val-Ser-Arg242, with bound radioactivity. This peptide, labeled at low concentration of 8-azido-AMP, most likely represents the AMP-binding region on the dehydratase molecule.     

Catalytic and allosteric mechanism of AMP nucleosidase from primary, beta-secondary, and multiple heavy atom kinetic isotope effects

Adenosine 5'-phosphate was synthesized with specific heavy atom substitutions to permit measurement of V/K kinetic isotope effects for the N-glycohydrolase activity of the allosteric AMP nucleosidase and the acid-catalyzed solvolysis of these compounds. The effects of allosteric activation on the kinetic isotope effects together with the kinetic mechanism of AMP nucleosidase [DeWolf, W. E., Jr., Emig, F. A., & Schramm, V. L. (1986) Biochemistry 25, 4132-4140] indicate that the kinetic isotope effects are fully expressed. Comparison of individual primary and secondary kinetic isotope effects with combined isotope effects and the isotope effect of the reverse reaction indicated that kinetic isotope effects in AMP nucleosidase arise from a single step in the reaction mechanism. Under these conditions, kinetic isotope effects can be used to interpret transition-state structure for AMP nucleosidase. Changes in kinetic isotope effects occurred as a function of allosteric activator, demonstrating that allosteric activation alters transition-state structure for AMP nucleosidase. Kinetic isotope effects, expressed as [V/K(normal isotope]/[V/K(heavy isotope)], were observed with [2'-2H]AMP (1.061 +/- 0.002), [9-15N]AMP (1.030 +/- 0.003), [1'-2H]AMP (1.045 +/- 0.002), and [1'-14C]AMP (1.035 +/- 0.002) when hydrolyzed by AMP nucleosidase in the absence of MgATP. Addition of MgATP altered the [2'-2H]AMP effect (1.043 +/- 0.002) and the [1'-2H]AMP effect (1.030 +/- 0.003) and caused a smaller decrease of the 14C and 15N effects. Multiple heavy atom substitutions into AMP caused an increase in observed isotope effects to 1.084 +/- 0.004 for [1'-2H,1'-14C]AMP and to 1.058 +/- 0.002 for [9-15N,1'-14C]AMP with the enzyme in the absence of ATP.(ABSTRACT TRUNCATED AT 250 WORDS)     

AMP nucleosidase: kinetic mechanism and thermodynamics

The kinetic mechanism of AMP nucleosidase (EC 3.2.2.4; AMP + H2O----adenine + ribose 5-phosphate) from Azotobacter vinelandii is rapid-equilibrium random by initial rate studies of the forward and reverse reactions in the presence of MgATP, the allosteric activator. Inactivation-protection studies have established the binding of adenine to AMP nucleosidase in the absence of ribose 5-phosphate. Product inhibition by adenine suggests a dead-end complex of enzyme, AMP, and adenine. Methanol does not act as a nucleophile to replace H2O in the reaction, and products do not exchange into substrate during AMP hydrolysis. Thus, the reactive complex has the properties of concerted hydrolysis by an enzyme-directed water molecule rather than by formation of a covalent intermediate with ribose 5-phosphate. The Vmax in the forward reaction (AMP hydrolysis) is 300-fold greater than that in the reverse reaction. The Keq for AMP hydrolysis has been experimentally determined to be 170 M and is in reasonable agreement with Keq values of 77 and 36 M calculated from Haldane relationships. The equilibrium for enzyme-bound substrate and products strongly favors the enzyme-product ternary complex ([enzyme-adenine ribose 5-phosphate]/[enzyme-AMP] = 480). The temperature dependence of the kinetic constants gave Arrhenius plots with a distinct break between 20 and 25 degrees C. Above 25 degrees C, AMP binding demonstrates a strong entropic effect consistent with increased order in the Michaelis complex. Below 20 degrees C, binding is tighter and the entropic component is lost, indicating distinct enzyme conformations above and below 25 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of glyceraldehyde-3-phosphate dehydrogenase with AMP as studied by means of a spin-labeled analog

The binding of a spin-labeled AMP analog to tetrameric glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle is described. The spin label, perdeuterated and 15N-substituted 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, was attached to C-8 of AMP (C8-SL-AMP). Up to 8 equivalents of C8-SL-AMP bind per enzyme tetramer, i.e., 2 per monomer. Combining sites are the adenine subsite of the coenzyme-binding domain and the phosphate site. Glyceraldehyde 3-phosphate causes a conformational change in the enzyme that brings C8-SL-AMP molecules bound to adjacent R-axis-related subunits closer to one another by 0.2-0.3 nm and allows for spin-spin interaction between the nitroxide radicals. Similar, but less pronounced structural changes take place upon lowering the pH from 8 to 7. Addition of a single equivalent of NAD+ to a complex of the enzyme with 7.6 equivalents of C8-SL-AMP leads to the release of almost 4 C8-SL-AMP molecules. This supports our previous findings that binding of just one NAD+ molecule induces conformational changes in all four subunits.     

Insulin Binding to Human Astrocytoma Cells and Its Effect on Uridine Incorporation into Nucleic Acid

Binding of [125I]monoiodoinsulin to human astrocytoma cells (U-373 MG) was time dependent, reaching equilibrium after 1 h at 22 degrees C with equilibrium binding corresponding to 2.2 fmol/mg protein: this represents approximately 2,000 occupied binding sites per cell. The t1/2 of 125I-insulin dissociation at 22 degrees C was 10 min; the dissociation rate constant of 1.1 X 10(-2) s-1 was unaffected by a high concentration of unlabeled insulin (16.7 microM). Porcine insulin competed for specific 125I-insulin binding in a dose-dependent manner and Scatchard analysis suggested multiple affinity binding sites (higher affinity Ka = 4.4 X 10(8) M-1 and lower affinity Ka = 7.4 X 10(6) M-1). Glucagon and somatostatin did not compete for specific insulin binding. Incubation of cells with insulin (0.5 microM) for 2 h at 37 degrees C increased [2-14C]uridine incorporation into nucleic acid by 62 +/- 2% (n = 3) above basal. Cyclic AMP, in the absence of insulin, also stimulated nucleoside incorporation into nucleic acid [65 +/- 1% (n = 3)] above basal. Preincubation with cyclic AMP followed by insulin had an additive effect on nucleoside incorporation [160 +/- 4% (n = 3) above basal]. Dipyridamole (50 microM), a nucleoside transport inhibitor, blocked both basal and stimulated uridine incorporation. These studies confirm that human astrocytoma cells possess specific insulin receptors with a demonstrable effect of ligand binding on uridine incorporation into nucleic acid.     

UTP binding and phosphoinositidase C activation in ampulla from frog semicircular canal

Pyrimidine nucleotide-sensitive phosphoinositidase C activity (PLC), previously identified in frog semicircular canal ampulla, was pharmacologically characterized. Binding of [(3)H]UTP and abilities of unlabeled nucleotide analogs to inhibit binding and to stimulate PLC in myo-[(3)H]inositol-loaded ampullas were determined. Specific [(3)H]UTP binding was competitively inhibited by UTP [apparent dissociation binding constant = 0.8 microM; Hill coefficient = 0.7]. Scatchard analysis revealed a minor class of high-affinity binding sites [45 fmol UTP bound/microgram protein; dissociation constant (K(D1)) = 0.4 microM] and a major class of moderate-affinity binding sites (365 fmol UTP bound/microgram protein; K(D2) = 10 microM). The stereospecificity pattern for UTP analog recognition was UMP > UDP >/= ADP = UTP = dTTP > adenosine 5'-O-(3-thiotriphosphate) = ATP = CTP = 2'-and 3'-O-4-(benzoylbenzoyl)-ATP (Bz-ATP) >/= AMP >/= 2-methylthio-ATP = alpha,beta-methylene-ATP > uridine = diadenosine tetraphosphate (Ap(4)A); cAMP and adenosine were inactive. Antagonist recognition pattern was DIDS = pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) = reactive blue 2 > suramin. The rank order of potencies for agonist-induced PLC activation was UDP >/= UTP >/= Ap(4)A >/= UMP = Bz-ATP; uridine was inactive. UTP-stimulated PLC activity was inhibited by DIDS = reactive blue 2 = PPADS > suramin. These results suggest that the population of [(3)H]UTP-labeled binding sites is heterogeneous, with a low number of high-affinity UTP receptors whose function(s) need to be determined and a large number of moderate-affinity receptors triggering PLC activation.     

Regulation of rabbit liver fructose-1,6-bisphosphatase by metals, nucleotides, and fructose 2,6-bisphosphate as determined from fluorescence studies

The fluorescent nucleotide analogue formycin 5'-monophosphate (FMP) inhibits rabbit liver fructose-1,6-bisphosphatase (I50 = 17 microM, Hill coefficient = 1.2), as does the natural regulator AMP (I50 = 13 microM, Hill coefficient = 2.3), but exhibits little or no cooperativity of inhibition. Binding of FMP to fructose-1,6-bisphosphatase can be monitored by the increased fluorescence emission intensity (a 2.7-fold enhancement) or the increased fluorescence polarization of the probe. A single dissociation constant for FMP binding of 6.6 microM (4 sites per tetramer) was determined by monitoring fluorescence intensity. AMP displaces FMP from the enzyme as evidenced by a decrease in FMP fluorescence and polarization. The substrates, fructose 6-phosphate and fructose 1,6-bisphosphate, and inhibitors, methyl alpha-D-fructofuranoside 1,6-bisphosphate and fructose 2,6-bisphosphate, all increase the maximal fluorescence of enzyme-bound FMP but have little or no effect on FMP binding. Weak metal binding sites on rabbit liver fructose-1,6-bisphosphatase have been detected by the effect of Zn2+, Mn2+, and Mg2+ in displacing FMP from the enzyme. This is observed as a decrease in FMP fluorescence intensity and polarization in the presence of enzyme as a function of divalent cation concentration. The order of binding by divalent cations is Zn2+ = Mn2+ greater than Mg2+, and the Kd for Mn2+ displacement of FMP is 91 microM. Methyl alpha-D-fructofuranoside 1,6-bisphosphate, as well as fructose 6-phosphate and inorganic phosphate, enhances metal-mediated FMP displacement from rabbit liver fructose-1,6-bisphosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis and properties of 2-azido-NAD+. A study of interaction with glutamate dehydrogenase

A photoactive coenzyme analog of NAD+ has been synthesized by chemically coupling [32P]2-azido-AMP and NMN to produce [32P]nicotinamide 2-azidoadenosine dinucleotide (2-azido-NAD+). The utility of 2-azido-NAD+ as an effective active-site-directed photoprobe was demonstrated using bovine liver glutamate dehydrogenase as a model enzyme. In the absence of ultraviolet light, 2-azido-NAD+ is a substrate for this enzyme. Photoincorporation of probe was saturable with two different apparent dissociation constants of 10 microM and 40 microM. Protection of photoinsertion was seen with the natural substrate NAD+ with apparent dissociation constants of less than 5 microM and 25 microM. This observation may be explained on the basis of negative cooperative interaction between the subunits. The photoinsertion of 2-azido-NAD+ was increased by GTP and decreased by ADP in accordance with their known effects on NAD+ binding. When the enzyme was covalently modified by photolysis in the presence of saturating amounts of photoprobe, an approximately 40% inhibition of the enzyme activity was observed. These results demonstrate that the photoaffinity coenzyme analog has potential application as a probe to characterize NAD(+)-binding proteins and to identify the active sites of these proteins.     

Detection of conformational changes in chloroplast coupling factor 1 by 8-anilino-1-naphthalene-sulphonate fluorescence changes

Chloroplast coupling factor 1 (CF1) contains a high-affinity binding site for 8-anilino-1-napthalene sulphonate (ANS,Kd = 5-6 microM). The binding of ANS to the enzyme is associated with a fluorescence enhancement and a blue-shift in the emission spectrum. ANS only slightly inhibits ATP hydrolysis by CF1. Adenine nucleotides and inorganic phosphate induce a fast ANS fluorescence quenching of about 50% which is due to a decrease in the affinity of the enzyme for ANS (Kd increases from 6 microM to 22 microM) and in the fluorescence quantum yield of the bound probe (by 33%) but not in the number of ANS sites (n = 1). Conversely, Mg and Ca ions induce a fluorescence enhancement of bound ANS. Inactivation of the enzyme enhances ANS fluorescence, eliminates the response to adenine nucleotides and inorganic phosphate but increases the response to divalent metals. The affinity of latent CF1 for ADP (Kd = 12 microM) is considerably higher than for ATP (Kd = 95 microM) in buffer containing EDTA. The Kd for inorganic phosphate is 140 microM. Mg increases the apparent affinity for ATP (Kd = 28 microM) but not for ADP or Pi. Binding of ATP to the tight-sites does not inhibit the ADP or Pi-induced fluorescence quenching but decreases the affinity for ADP (Kd = 34 microM) and for inorganic phosphate (Kd = 320 microM). These results suggest that the ADP and phosphate binding sites are different but not independent from the tight sites. Activation of a Mg-specific ATPase in CF1 by octyl glucoside decreases the affinity for ADP and inorganic phosphate by about threefold but increases the affinity for ATP. ATPase activation of CF1 also increases the Ki for ADP inhibition of ATP hydrolysis. ATPase activation also influences the ANS responses to Ca and Mg. Ca-ATPase activation increases the fluorescence enhancement and the apparent affinity for Ca whereas Mg-ATPase activation specifically increases the Mg-induced fluorescence enhancement. The fluorescence of CF1-bound ANS is enhanced by Dio-9 and quenched by phloridzin, quercetin, Nbf-Cl and FITC. Nbf-Cl and FITC completely inhibit the ADP-induced fluorescence quenching whereas Dio-9 inhibits the Mg-induced fluorescence enhancement. ANS does not relieve the quercetin or phloridzin inhibition of ATP hydrolysis indicating that these inhibitors do not compete with ANS for a common binding site. ANS may be used, therefore, as a sensitive probe to detect conformational changes in CF1 in response to activation or inactivation and to binding of substrates and of inhibitors.     

Covalent labelling of ligand binding sites of human placental S-adenosylhomocysteine hydrolase with 8-azido derivatives of adenosine and cyclic AMP.

S-Adenosylhomocysteine hydrolase (AdoHcyase) has previously been identified as a cytoplasmic adenosine and cyclic AMP binding protein. In order to examine the relationship between the adenosine and cyclic AMP binding sites on this enzyme we have explored the use of 8-azido analogues of adenosine and cyclic AMP as photoaffinity reagents for covalently labelling AdoHcyase purified from human placenta. 8-Azidoadenosine (8-N3-Ado), like adenosine, inactivated AdoHcyase, and the rate of inactivation was greatly increased by periodate oxidation. In addition, 8-N3-Ado was found to participate in the first step in the catalytic mechanism for AdoHcyase, resulting in conversion of enzyme-bound NAD+ to NADH, although it was not a substrate for the full enzyme-catalysed reaction. Radioactively labelled 8-N3-Ado, its periodate-oxidized derivative and 8-azidoadenosine 3', 5'-phosphate (8-N3-cAMP) bound specifically to adenosine binding sites on AdoHcyase and, after irradiation, became covalently linked to the enzyme. Photoaffinity-labelled enzyme could be precipitated by monoclonal antibody to human AdoHcyase. Two observations suggested that cyclic AMP and adenosine bind to the same sites on AdoHcyase. First cyclic AMP and adenosine each blocked binding of both radioactively labelled 8-N3-Ado and 8-N3-cAMP, and second, digestion with V8 proteinase generated identical patterns of peptides from AdoHcyase that had been photolabelled with [32P]8-N3-cAMP and [3H]8-N3-Ado. Binding sites for cyclic AMP on AdoHcyase were found to differ functionally and structurally from cyclic AMP binding sites on the R1 regulatory subunit of cyclic AMP-dependent protein kinase.     

The pathway of adenylate catabolism in Azotobacter vinelandii. Evidence for adenosine monophosphate nucleosidase as the regulatory enzyme.

Cell-free, dialyzed extracts from Azotobacter vinelandii rapidly dephosphorylate [U-14C]ATP to labeled ADP and AMP, which is then degraded to hypoxanthine, the end product of AMP catabolism under the experimental conditions which were used. The intermediates of the pathway from ATP to hypoxanthine have been identified by thin layer chromatography and quantitated by the 14-C content. The concentrations of intermediates present during the production of hypoxanthine are consistent with AMP nucleosidase being responsible for AMP degradation in these extracts. This result was confirmed in experiments which utilized rabbit antibody prepared against purified AMP nucleosidase. The antibody inhibited AMP nucleosidase activity in cell-free extracts but did not inhibit adenine demanase or adenosine deaminase from the same extracts. In the presence of antibody prepared against purified AMP nucleosidase, the dialyzed extracts showed a marked reduction in the production of hypoxanthine from ATP. Other enzymes which could be responsible theoretically for the conversion of AMP to hypoxanthine were not detected by standard assay procedures. These results are consistent with AMP degradation proceeding by way of AMP nucleosidase to yield adenine and ribose 5-phosphate. The adenine is then converted to hypoxanthine by adenine deaminase. Both of these enzymes were present in sufficient quantities to account for the observed rates of hypoxanthine formation. The rate of hypoxanthine formation decreases during the time course of the [U-14-C]ATP degradation experiments, even though the concentration of AMP remains high. This decrease in the rate of hypoxanthine formation as a function of time is attributed to the decreasing ATP and increasing P0-4 concentrations, since ATP is an activator of AMP nucleosidase and P0-4 is an inhibitor. These observations suggest that the in vivo activity of AMP nucleosidase could also be regulated by changes in the relative ratios of ATP:AMP:P0-4.     

The interaction of ligands with chemically modified phosphorylase b.

Phosphorylase b which had been inactivated with 5-diazo1H-tetrazole was specifically labelled with 4-iodoacetamidosalicylic acid (a fluorescent probe) or with N-(1-oxyl-2,2,6,6,-tetramethyl-4-piperidinyl)iodoacetamide (a spin label probe) so that the binding of ligands and accompanying conformational changes could be determined by fluorescence or electron spin resonance changes, respectively. The allosteric effector, AMP, causes conformational changes similar to those caused in the native enzyme. The affinity of binding of phosphate or AMP to the inhibited protein is the same as for the unmodified protein. The heterotropic interactions between glucose-1-phosphate or glycogen and AMP are much less in the inactivated enzyme than in unmodified phosphorylase. Using a light scattering assay, it is shown that the modified enzyme binds to glycogen less strongly than the native protein. Phosphorylase b which had been inactivated by carbodimide in the presence of glycine ethyl ester, resulting in the modification of one or more carboxyl groups, was labelled with the spin label probe described above. The modified enzyme has an affinity for AMP similar to that of the native enzyme. AMP binding to the modified enzyme is tightened by glycogen, weakened by glucose-6-phosphate and is unaffected by glucose-1-phosphate. The actions of 5-diazo-1H-tetrazole and carbodimide on phosphorylase are discussed in the light of the above observation.     

Molecular characterization of angiotensin II type II receptors in rat pheochromocytoma cells.

The binding sites and biochemical effects of angiotensin (A) II were investigated in rat pheochromocytoma (PC12W) cells. Sarcosine1, [125I]-tyrosine4, isoleucine8-AII ([125I]-SI-AII) bound to a saturable population of sites on membranes with an equilibrium dissociation constant (Kd) of 0.4 nM and a binding site maximum of 254 fmol/mg protein. Competitive displacement of [125I]-SI-AII by agonists and antagonists elucidated a rank order of potency of AIII greater than or equal to AII greater than PD 123177 greater than AI greater than [des-Phe]AII [AII(1-7)] much greater than DuP 753. The stable guanine nucleotide analog 5'-guanylyl imidodiphosphate did not alter the binding affinity or slope of the inhibition curves for AI, AII, AIII, or AII(1-7). Treatment of PC12W cells with AII or AIII did not affect the free intracellular calcium concentration, phosphoinositide metabolism, arachidonate release, cyclic GMP, or cyclic AMP concentrations. [125I]-AII binding sites remained on the cell surface and were not internalized after 2 h at 37 degrees C. Angiotensin II did not stimulate tyrosine, serine, or threonine phosphorylation. Northern analysis of PC12W mRNA with an AT1 receptor gene probe failed to produce an RNA:DNA hybrid at low stringency. These data indicate that PC12W cells express a homogeneous population of AT2 binding sites which differ significantly from AT1 receptors in signal transduction and molecular structure. AT2 sites may act via potentially novel, biochemical pathways or, alternatively, be vestigial receptors.     

Structure of Escherichia coli 5'-methylthioadenosine/ S-adenosylhomocysteine nucleosidase inhibitor complexes provide insight into the conformational changes required for substrate binding and catalysis

5'-Methylthioadenosine/S-adenosylhomocysteine (MTA/AdoHcy) nucleosidase is a key enzyme in a number of critical biological processes in many microbes. This nucleosidase catalyzes the irreversible hydrolysis of the N(9)-C(1') bond of MTA or AdoHcy to form adenine and the corresponding thioribose. The key role of the MTA/AdoHcy nucleosidase in biological methylation, polyamine biosynthesis, methionine recycling, and bacterial quorum sensing has made it an important antimicrobial drug target. The crystal structures of Escherichia coli MTA/AdoHcy nucleosidase complexed with the transition state analog, formycin A (FMA), and the nonhydrolyzable substrate analog, 5'-methylthiotubercidin (MTT) have been solved to 2.2- and 2.0-A resolution, respectively. These are the first MTA/AdoHcy nucleosidase structures to be solved in the presence of inhibitors. These structures clearly identify the residues involved in substrate binding and catalysis in the active site. Comparisons of the inhibitor complexes to the adenine-bound MTA/AdoHcy nucleosidase (Lee, J. E., Cornell, K. A., Riscoe, M. K., and Howell, P. L. (2001) Structure (Camb.) 9, 941-953) structure provide evidence for a ligand-induced conformational change in the active site and the substrate preference of the enzyme. The enzymatic mechanism has been re-examined.     

Localization of the tight ADP-binding site on the membrane-bound chloroplast coupling factor one

The photoaffinity analog 2-azido-ADP (2-azidoadenosine 5'-diphosphate) was used as a probe of the spinach chloroplast ATP synthase. The analog acted as a substrate for photophosphorylation. Several observations suggested that 2-azido-ADP and ADP bound to the same class of tight nucleotide binding sites: (a) 2-azido-ADP competitively inhibited ADP tight binding (Ki = 1.4 microM); (b) the concentration giving 50% maximum binding, K0.5 for analog tight binding (1 microM) was similar to that observed for ADP (2 microM); (c) nucleotide tight binding required prior membrane energization and was completely reversed by re-energization; (d) the tight binding of 2-azido-[beta-32P]ADP was completely prevented by ADP; (e) the analog inhibited the light-triggered ATPase activity at micromolar concentrations. Ultraviolet irradiation of washed thylakoid membranes containing tightly bound 2-azido-[beta-32P]ADP resulted in the covalent incorporation of the label into the membranes. Denaturing polyacrylamide gel electrophoresis of the labeled membranes demonstrated that the beta subunit of the coupling factor one complex was the only polypeptide in the thylakoid membranes which was labeled. These results identify the beta subunit of the coupling factor as the location of the tightly bound ADP on the thylakoid membranes.     

Stoichiometric photolabeling of two distinct low and high affinity nucleotide sites in sarcoplasmic reticulum ATPase

Highly purified 3'-arylazido-ATP (aATP) was obtained by high performance liquid chromatography. In the dark, this photoactivatable ATP analog was a competitive inhibitor of ATP hydrolysis catalyzed by purified sarcoplasmic reticulum (SR) ATPase with a Ki of 10 microM. The analog itself was hydrolyzed by the enzyme in the dark. A biphasic curve of velocity of hydrolysis of the analog versus aATP concentration was obtained, indicating the presence of high and low affinity sites with K0.5 of approximately 10 microM and 300 microM, respectively. Upon irradiation with visible light, a biphasic curve was obtained for the level of covalent photolabeling of the enzyme versus [beta-32P]aATP concentrations. Levels of 6.5-9 nmol of analog/mg of protein and 20-22 nmol of analog/mg of protein were obtained when labeling with 20-30 or with 400 microM aATP, respectively, showing the existence of 1 mol of high affinity sites/mol of ATPase and 1-1.5 mol of low affinity sites/mol of enzyme. The rate of light-dependent incorporation of [beta-32P]aATP was decreased by the presence of ATP, Pi, 2',3'-O-(2,4,6-trinitrocyclohexadienylidene-ATP, or Ca2+ in the illumination media. Photolabeling of the high affinity sites had little effect on the velocity of ATP hydrolysis but significantly inhibited the splitting of additional aATP added in the dark. Photolabeling the low affinity sites caused irreversible inhibition of the ATPase activity. The inhibition was prevented by having ATP in the illumination medium, which protected it from labeling. Gel filtration chromatography in the presence of detergent showed that radioactive photolabel was incorporated in the SR ATPase protein. The results indicate that aATP is a useful tool for stoichiometrically labeling and probing the nucleotide binding domains of the SR ATPase.     

Properties of the cyclic GMP phosphodiesterase from rat lung. Inhibition by unsaturated fatty acids

Catalytic and regulatory properties of the major form of cyclic GMP phosphodiesterase (3':5'-cyclic-GMP 5'-nucleotidohydrolase, EC 3.1.4.35) from rat lung were studied. The enzyme partially purified by a DEAE-Sepharose chromatography displayed a much higher affinity toward cyclic GMP than toward cyclic AMP, the apparent Km values being 5.7 microM and 482 microM for the guanylic and the adenylic cyclic nucleotide, respectively. In contrast, the V value for cyclic AMP was about 3-times higher than the V value for cyclic GMP. Linear double reciprocal plots of initial velocity were observed with each cyclic nucleotide. From 10(-8) to 3.3 X 10(-6) M, cyclic GMP did not change the hydrolysis of 1 or 10 microM cyclic [3H]AMP, while it became inhibitory at higher concentrations. In contrast with a calmodulin-sensitive phosphodiesterase prepared from rat brain, the lung enzyme was not stimulated by a heat-stable Ca2+-dependent factor from rat lung or by rat brain calmodulin or by lipids including fatty acids and lysophosphatidylcholine. Various unsaturated 18- and 20-carbon fatty acids inhibited at varying degrees the cyclic GMP phosphodiesterase from rat lung. The inhibitory potency increased with the number of double bonds in the hydrocarbon chain. In contrast, the methyl esters of the unsaturated fatty acids and the saturated fatty acids of variable hydrocarbon chain lengths had no appreciable effects. A linear Hill plot of phosphodiesterase inhibition with a slope of unity was obtained with arachidonic acid up to 30 microM, suggesting only one type of inhibitory site. In this range of concentrations the inhibition was entirely reversible. Kinetics analysis demonstrated that up to 30 microM arachidonic acid was a purely competitive inhibitor with an apparent Ki of 20 microM. Over 30 microM, the Hill coefficient increased progressively, indicating the binding to other inhibitory sites, while the reversibility disappeared.