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At least 919 records · Page 51Linked to original sources
PubMed1997

Temperature-sensitive mutants of Corynebacterium ammoniagenes ATCC 6872 with a defective large subunit of the manganese-containing ribonucleotide reductase.

Chemical mutagenesis of the nucleotide-producing strain Corynebacterium ammoniagenes ATCC 6872 with N-methyl-N-nitro-N-nitrosoguanidine followed by an enrichment protocol yielded 46 temperature-sensitive (ts) clones. A rapid assay for the allosterically regulated Mn-ribonucleotide reductase (RRase) was developed with nucleotide-permeable cells of C. ammoniagenes in order to screen for possible defects in DNA precursor biosynthesis at elevated temperature. Three mutants (CH 31, CH 32, and CH 33) grew well at 30 degrees C but did not proliferate at 40 degrees C because they did not reduce ribonucleotides to 2'-deoxyribonucleotides. They were designated nrdts (nucleotide reduction defective). When the cultures were shifted from 30 to 40 degrees C, the nrdts mutants immediately ceased to incorporate radiolabeled nucleic acid precursors into the DNA fraction, while DNA chain elongation was barely affected. Thus, exhaustion of the deoxyribonucleotide pool ultimately inhibited cell division, leading to a filamentous growth morphology. In contrast to the wild-type, all three nrdts mutants displayed a distinctly enhanced sensitivity of ribonucleotide reduction towards hydroxyurea (in permeabilized cells and in vitro) at 30 degrees C. The results from assays for biochemical complementation of heat-inactivated (2 min, 37 degrees C) mutant enzyme with either the small or the large subunit of wild-type Mn-RRase located the mutational defect on the large subunit.

Brevibacterium↗
PubMed2005

Allosteric site in M2 acetylcholine receptors: evidence for a major conformational change upon binding of an orthosteric agonist instead of an antagonist.

Muscarinic acetylcholine receptors contain two distinct ligand binding sites, i.e. the orthosteric site for acetylcholine and other conventional ligands, and an allosteric site located at the entrance of the ligand binding pocket. We used a set of allosteric agents to probe whether muscarinic M2 receptors whose orthosteric site is occupied by an agonist still reveal the common allosteric site that has been identified in M2 receptors being occupied by an orthosteric antagonist (N-methylscopolamine, NMS). Equilibrium and dissociation binding experiments were carried out in porcine heart homogenates using either the agonist [3H]oxotremorine M ([3H]OxoM) or the antagonist [3H]NMS. The affinities of the allosteric agents were determined for the radioligand-occupied receptor states and, additionally, for the radioligand-free (ground state) M2 receptor. The archetypal agent W84 (hexane-1,6-bis[dimethyl-3'-phthalimidopropyl-ammonium bromide] and its bispyridinio middle chain analogue WDuo3 (1,3-bis[4-(phthalimidomethoxyimino-methyl)-pyridinium-1-yl]propane dibromide) had a clearly lower affinity for [3H]OxoM-liganded receptors compared with [3H]NMS-liganded and ground state receptors. In contrast, a derivative resembling only one half of W84 had equal affinities for both radioligand-occupied receptor states. Also, the agents gallamine and obidoxime did not discriminate between [3H]OxoM- and [3H]NMS-occupied receptors. The allosteric antagonistic tool obidoxime inhibited WDuo3 action in [3H]OxoM-liganded receptors with the same potency as in [3H]NMS-liganded receptors. We conclude that the common allosteric site is still present in OxoM-liganded M2 receptors, but its spatial conformation is considerably altered compared with NMS-liganded receptors.

Allosteric Regulation↗
PubMed1999

Ion selectivity of the cytoplasmic binding sites of the Na,K-ATPase: I. Sodium binding is associated with a conformational rearrangement.

To investigate Na+ binding to the ion-binding sites presented on the cytoplasmic side of the Na,K-ATPase, equilibrium Na+-titration experiments were performed using two fluorescent dyes, RH421 and FITC, to detect protein-specific actions. Fluorescence changes upon addition of Na+ in the presence of various Mg2+ concentrations were similar and could be fitted with a Hill function. The half-saturating concentrations and Hill coefficients determined were almost identical. As RH421 responds to binding of a Na+ ion to the third neutral site whereas FITC monitors conformational changes in the ATP-binding site or its environment, this result implies that electrogenic binding of the third Na+ ion is the trigger for a structural rearrangement of the ATP-binding moiety. This enables enzyme phosphorylation, which is accompanied by a fast occlusion of the Na+ ions and followed by the conformational transition E1/E2 of the protein. The coordinated action both at the ion and the nucleotide binding sites allows for the first time a detailed formulation of the mechanism of enzyme phosphorylation that occurs only when three Na+ ions are bound.

Adenosine Triphosphate↗
PubMed1997

Alteration of the quaternary structure of glutamate dehydrogenase from Clostridium symbiosum by a single mutation distant from the subunit interfaces.

X-ray crystallographic studies have previously shown that glutamate dehydrogenase from Clostridium symbiosum is a homohexamer. Mutation of the active-site aspartate-165 to histidine causes an alteration in the structural properties of the enzyme. The mutant enzyme, D165H exists predominantly as a single species of lower molecular mass than the wild-type enzyme as indicated by gel filtration and sedimentation velocity analysis. The latter technique gives an S20,w value for D165H of (6.07 +/- 0.01)S which compares with (11.08 +/- 0.01)S for the wild-type, indicative of alteration of the homohexameric quaternary structure of the native enzyme to a dimeric form, a result confirmed by sedimentation equilibrium experiments. Further support for this is provided by chemical modification by Ellman's reagent of cysteine-144 in the mutant, a residue which is buried at the dimer-dimer interface in the wild-type enzyme and is normally inaccessible to modification. The results suggest a possible structural route for communication between the active sites and subunit interfaces which may be important for relaying signals between subunits in allosteric regulation of the enzyme.

Chromatography, Gel↗
PubMed2003

ADP-dependent 6-phosphofructokinase, an extremely thermophilic, non-allosteric enzyme from the hyperthermophilic, sulfate-reducing archaeon Archaeoglobus fulgidus strain 7324.

The hyperthermophilic, sulfate-reducing archaeon Archaeoglobus fulgidus strain 7324 has been shown to degrade starch via glucose using a modified Embden-Meyerhof pathway. In this pathway phosphorylation of fructose-6-phosphate to fructose-1,6 bisphosphate is catalyzed by an ADP-dependent 6-phosphofructokinase (ADP-PFK), which was purified 1,800-fold to homogeneity. The enzyme is composed of 50 kDa subunits and is eluted from gel filtration as both a homotetramer and a homodimer. It had a temperature optimum at 85 degrees C and showed significant thermostability up to 95 degrees C. Kinetic constants were determined for both reaction directions at pH 6.6 and 80 degrees C. Rate dependence for all substrates followed Michaelis Menten kinetics. The apparent Km for ADP and fructose-6-phosphate (forward reaction) was 0.6 mM and 2.2 mM, respectively; the apparent Vmax was 1,200 U/mg. ADP-PFK catalyzed in vitro the reverse reaction, with apparent Km for fructose-1,6-bisophosphate and AMP of 5.7 and 1.4 mM, respectively, and a Vmax value of 85 U/mg. The enzyme did not use ATP, PPi, or acetyl phosphate as phosphoryl donor and was highly specific for fructose-6-phosphate as substrate. The A. fulgidus ADP-PFK did not phosphorylate glucose and thus differs from the bifunctional ADP-PFK/GLK from Methanococcus jannaschii. Divalent cations were required for catalytic activity; Mg2+, which was most effective, could be partially replaced by Mn2+, Ni2+, and Co2+. Enzyme activity was not allosterically regulated by classical effectors of bacterial and eukaryal ATP-PFKs, such as ADP, AMP, phosphoenolpyruvate, or citrate. N-terminal amino acid sequence showed high similarity to known ADP-PFKs. In the genome of Archaeoglobus fulgidus strain VC 16, which is closely related to strain 7324, no homologous gene for ADP-PFK could be identified.

Amino Acid Sequence↗
PubMedDate not supplied

Towards a high-affinity allosteric enhancer at muscarinic M1 receptors.

Loss of forebrain acetylcholine (ACh) is an early neurochemical lesion in Alzheimer's Disease (AD), and muscarinic receptors for ACh are involved in memory and cognition, so a muscarinic agonist could provide 'replacement therapy' in this disease. Muscarinic receptors, which couple to G-proteins, occur throughout the CNS, and in the periphery they mediate the responses of the parasympathetic nervous system, so selectivity is crucial. The five subtypes of muscarinic receptor, M1-M5, have a distinct regional distribution, with M2 and M3 mediating most of the peripheral effects, M2 predominating in hindbrain areas, and M1 predominating in the cortex and hippocampus--the brain regions most associated with memory and cognition, which has lead to a search for a truly M1-selective muscarinic agonist. That search has so far been unsuccessful, but acetylcholinesterase inhibitors such as donepezil (Aricept), which potentiate cholinergic neurotransmission, have a therapeutic role in the management of AD; so the M1 receptor remains a therapeutic target. Our approach is to develop allosteric enhancers--compounds which bind to the receptor at an 'allosteric' site which is distinct from the 'primary' site to which the endogenous ligand binds, and which enhance the affinity (or efficacy) of the endogenous ligand. We have developed radioligand binding assays and analyses for the detection and quantitatitation of allosteric interactions of a test agent with labelled and unlabelled 'primary' ligands, and we report here some results of the initial phase of a chemical synthesis project to develop potent and selective allosteric enhancers at muscarinic M1 receptors.

Acetylcholine↗
PubMed1987

Is rat skeletal muscle hexokinase an allosteric enzyme?

A recent report by M. Gregoriou, I. P. Trayer, and A. Cornish-Bowden (1986, Eur. J. Biochem. 161, 171-176) showed that the mechanism for rat skeletal muscle hexokinase contains two allosteric sites: one for ATP and one for glucose 6-phosphate. In this report, we show that the allosteric mechanism is at variance with a large amount of kinetic data for the skeletal muscle hexokinase reaction in the literature. In addition, the allosteric mechanism conflicts with isotope exchange at chemical equilibrium data reported by M. Gregoriou, I. P. Trayer, and A. Cornish-Bowden (1983, Eur. J. Biochem. 134, 283-288).

Allosteric Regulation↗
PubMed1990

Escherichia coli E-39 ADPglucose synthetase has different activation kinetics from the wild-type allosteric enzyme.

Kinetic and binding studies have shown that Lys39 of Escherichia coli ADPglucose synthetase is involved in binding of the allosteric activator. In order to study structure-function relationships at the activator binding site, this lysine residue was substituted by glutamic acid (Lys39----Glu) by site-directed mutagenesis. The resultant mutant enzyme (E-39) showed activation kinetics different from those of the wild-type enzyme. The level of activation of the E-39 enzyme by the major activators of E. coli ADPglucose synthetase, 2-phosphoglycerate, pyridoxal phosphate, and fructose-1,6-phosphatase was only approximately 2-fold compared to activation of 15- to 28-fold respectively, for the wild-type enzyme. NADPH, an activator of the wild-type enzyme, was unable to activate the mutant enzyme. In addition, the concentrations of the above activators necessary to obtain 50% of the maximal stimulation of enzyme activity (A0.5) were 5-, 9-, and 23-fold higher, respectively, than those for the wild-type enzyme. The E-39 enzyme also had a lower apparent affinity (S0.5) for the substrates ATP and MgCl2 than the wild-type enzyme and the values obtained in the presence or absence of activator were similar. The concentration of inhibitor giving 50% of enzyme activity (I0.5) was also similar for the E-39 enzyme in the presence or absence of activator. These results indicate that the E-39 mutant enzyme is not effectively activated by the major activators of the E. coli ADPglucose synthetase wild-type enzyme, and that this amino acid substitution also prevents the allosteric effect that the activator has on the wild-type enzyme kinetics, either increasing its apparent affinity for the substrates or modulating the enzyme's sensitivity to inhibition.

Allosteric Regulation↗
PubMed1991

Hill coefficient ratios give binding ratios of allosteric enzyme effectors; inhibition, activation, and squatting in deoxycytidylate aminohydrolase (EC 3.5.4.12)

The ratio of the steady-state kinetic Hill coefficients of two different effectors equals (under some rather weak general assumptions) the ratio in which the effectors displace each other from an enzyme. This principle can make implications of experimental allosteric enzyme kinetic data immediately apparent. We can use it to find that one molecule of the allosteric inhibitor of dCMP aminohydrolase, at moderately high effector concentrations, displaces one molecule of substrate, or one molecule of activator, whereas at very high concentrations, one molecule of inhibitor displaces two of substrate. Further use of the principle suggests that substrate, at high concentrations, binds binds to activator sites. However, ratios of substrate, activator, and inhibitor Hill coefficients are incompatible with a simple model of activation in which substrate and activator are bound to the same conformation.

Allosteric Regulation↗
PubMed1991

Allosteric modifier and substrate binding of donkey deoxycytidylate aminohydrolase (EC 3.5.4.12)

The hexameric allosteric enzyme deoxycytidylate aminohydrolase from donkey spleen is shown by equilibrium dialysis to bind specifically the allosteric inhibitor, dTTP, the activator dCTP, and the substrate analog dAMP each at six sites (the dTTP and dCTP sites may or may not be identical). These conclusions contrast with earlier ones that there were four sites for each effector; reasons for the discrepancy are discussed. With the knowledge of site numbers and the kinetic information from the accompanying paper it is concluded that the kinetic cooperativity of the enzyme excludes a concerted conformational transition mechanism. Amino acid analysis gives a molecular weight of 18,842 Da per subunit, i.e., 113,052 for the hexamer. A new simplified purification of homogeneous enzyme from donkey spleen probably useful for dCMP aminohydrolase from other sources is described.

Adamantane↗
PubMed1986

Rapid incorporation of the solubilized dihydropyridine receptor into phospholipid vesicles.

We describe the rapid incorporation of the CHAPS solubilized dihydropyridine receptor into phospholipid vesicles. A series of sucrose gradient sedimentation experiments demonstrate that the (+)-[3H]PN200-110-labeled dihydropyridine receptor is associated with lipid vesicles following detergent removal by Extracti-gel chromatography. Solubilization of the receptor results in a loss of (+)-[3H]PN200-110 binding affinity relative to that observed in native membranes; the high affinity binding of (+)-[3H]PN200-110 can be restored upon reincorporation of the receptor into phospholipid vesicles. Similarly, the incorporation of the receptor restores its stability to incubation at 37 degrees C relative to that of the detergent solubilized receptor, thereby mimicking the properties of the membrane bound form of the receptor. The dissociation rate of (+)-[3H]PN200-110 from the reconstituted receptor is shown to be allosterically regulated by verapamil and diltiazem, indicating that the binding sites for these calcium antagonists have been inserted along with the dihydropyridine receptor into phospholipid vesicles. The results presented in this report, thus demonstrate the successful reconstitution of the dihydropyridine receptor into phospholipid vesicles by a variety of criteria. The reconstitution method described here is rapid and efficient, and should now facilitate structure-function studies of this receptor and its interrelationships with other regulatory components of the voltage-sensitive calcium channel system.

Animals↗
PubMed1975

Interactions between K+ and ATP binding to the (Na+ + K+)-dependent ATPase.

K+ appears to decrease the affinity of the (Na+ + K+)-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) for its substrate, Mg2+ - ATP, and Mg2+ - ATP, in turn, appears to decrease the affinity of the enzyme for K+. These antagonisms have been investigated in terms of a quantitative model defining the magnitude of the effects as well as identifying the class of K+ sites on the enzyme involved. K+ increased the apparent Km for Mg2+ - ATP, an effect that was antagonized competitively by Na+. The data can be fitted to a model in which Mg2+ - ATP binding is prevented by occupancy of alpha-sites on the enzyme by K+ (i.e. sites of moderate affinity for K+ accessible on the "free" non-phosphorylated enzyme, in situ on the external membrane surface). By contrast, occupancy of these alpha-sites by Na+ has no effect on Mg2+ - ATP binding to the enzyme. On the other hand, Mg2+ - ATP decreased the apparent affinity of the enzyme for K+ at the alpha-sites, in terms of (i) the KD for K+ measured by K+-accelerated inactivation of the enzyme by F-, and (ii) the concentration of K+ for half-maximal activation of the K+-dependent phosphatase reaction (which reflects the terminal hydrolytic steps of the overall ATPase reaction). These data fit the same quantitative model. Although this formulation does not support schemes in which ATP binding effects the release of transported K+ from discharge sites, it is consistent with observations that K+ can inhibit the enzyme at low substrate concentrations, and that Li+, which has poor efficacy when occupying these alpha-sites, can stimulate enzymatic activity at high K+ concentrations by displacing the inhibitory K+.

Adenosine Triphosphatases↗
PubMed1976

Phosphoenolpyruvate carboxylase of Escherichia coli. Studies on multiple conformational states elicited by allosteric effectors with a fluorescent probe, 1-anilinonaphthalene-8-sulfonate.

Conformational change of phosphoenolpyruvate carboxylase (orthophosphate: oxaloacetate carboxy-lyase (phosphorylating), EC 4.1.1.31) induced by allosteric effectors was investigated using a hydrophobic probe, 1-anilinonaphthalene-8-sulfonate (ANS). Kinetic experiments suggested that ANS binds with the enzyme at the sites which are not involved in the catalytic and regulatory functions, though it partially inhibits the enzyme activity with half-saturation concentration (S0.5) of 38.5 muM. Binding experiments showed that a maximum of 2 mol of ANS are able to bind with 1 mol of the enzyme subunit presumably with an equal dissociation constant to each other (34.5 muM). Flourescence emission of ANS was markedly increased by binding with the enzyme. L-Aspartate, the allosteric inhibitor, and CoASAc and fructose 1,6-bisphosphate (Fru-1,6-P2) the allosteric activators, produced various degrees of change in fluorescence, when added singly or in combinations. The changes were shown to be attributable to the allosteric interactions between the enzyme and effectors from some criteria such as structural specificity, half-saturation concentrations, and heterotropic-homotropic interactions of the ligands. It was concluded from these analyses that the enzyme can be in at least four conformational states which are distinct from each other. Especially noteworthy is the finding that the enzyme, upon simultaneous binding of CoASAc and Fru-1,6-P2, takes a new conformation which is enterely different from those induced by sole binding of each effector. In addition, the heterotropic interaction between the activator and the inhibitor was observed through conformational change by the ANS method, as observed in the kinetic studies.

Acetyl Coenzyme A↗
PubMed1976

Purification and properties of tyrosine-sensitive 3-deoxy-D-arabino-heptolosonate-7-phosphate synthetase of Escherichia coli K12.

1. The tyrosine-sensitive allosteric first enzyme of the aromatic amino acid biosynthetic pathway, 3-deoxy-D-arabinoheptulosonate 7-phosphate synthetase (7-Phospho-2-keto-3-deoxy-D-arabino-heptonate D-erythrose 4-phosphate-lyase (pyruvate phosphorylating), EC 4.1.2.15) has been purified from a mutant strain of Escherichia coli. 2. The enzyme activity was inhibited to 50% at 2-10(-5) M tyrosine and to 90% at 2-10(-4) M tyrosine concentration. At tyrosine concentrations lower than 2-10(-5) M a cooperative interaction between tyrosine binding sites was observed. 3. Co2+ increased the enzyme activity about 2-2.5-fold. The presence of Co2+ ions stabilized the enzyme. EDTA inhibited the enzyme activity, and this inhibition was reversed by Co2+. Tyrosine-sensitive DAHP synthetase seems to be a metal containing enzyme. 4. Kinetic experiments were carried out to study the catalytic action. Contrary to earlier suggestions it is concluded, that the reaction mechanism appears to be more complex--with either the ping-pong or sequential type predominating, depending on conditions.

Aldehyde-Lyases↗
PubMed1978

Allosteric effects of some antifibrinolytic amino acids on the catalytic activity of human plasmin.

The effects of L-lysine, 6-aminohexanoic acid, and trans-4-aminomethylcy-clohexane-1-carboxylic acid on the catalytic activity of plasmin (EC 3.4.21.7) have been investigated. The kinetics of the plasmin-catlysed hydrolysis of alpha-N-benzoyl-L-arginine ethyl ester in the presence of these compounds have been studied at a number of different concentrations of the three modifiers. They each exert two effects on the reaction, an activation and an inhibition, the concentration dependencies of which are markedly different. They must therefore arise from two different interactions between plasmin and the modifier. The inhibition is competitive, so that it most probably results from direct interaction at the catalytic site. The activation is kinetically non-competitive. The experimental observations seem to be explained best by assuming that L-lysine and certain analogous compounds function as both allosteric modifiers and competitive inhibitors of plasmin.

Allosteric Regulation↗