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At least 91 records · Page 5Linked to original sources
PubMed1975

Complementarity in the regulation of phosphoglucomutase, phosphofructokinase and hexokinase; the role of glucose 1,6-bisphosphate.

ATP and citrate, the well known inhibitors of phosphofructokinase (ATP: D-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.11), were found to inhibit the activities of the multiple forms of phosphoglucomutase (alpha-D-glucose 1,6-bisphosphate: alpha-D-glucose 1-phosphate phosphotransferase, EC 2.7.5.1) from rat muscle and adipose tissue. This inhibition could be reversed by an increase in the glucose 1,6-bisphosphate (Glc-1,6-P2) concentration. Other known activators (deinhibitors) of phosphofructokinase, viz. cyclic AMP, AMP, ADP or Pi, had no direct deinhibitory action on the ATP or citrate inhibited multiple phosphoglucomutases. Cyclic AMP and AMP, could however lead indirectly to deinhibition of the phosphoglucomutases, by activating phosphofructokinase which catalyzes the ATP-dependent phosphorylation of glucose 1-phosphate to form Glc-1,6-P2, the la-ter then released the multiple phosphoglucomutases from ATP or citrate inhibition. The Glc-1,6-P2 was also found to exert a selective inhibitory effect on hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) type II, the predominant form in skeletal muscle. This selective inhibition by Glc-1,6-P2 was demonstrated on the multiple hexokinases which were resolved by cellogel electrophoresis or isolated by chromatography on DEAE-cellulose. Based on the in vitro studies it is suggested that during periods of highly active epinephrine-induced glycogenolysis in muscle, the Glc-1,6-P2, produced by the cyclic AMP-stimulated reaction of phosphofructokinase with glucose 1-phosphate, will release the phosphoglucomutases from ATP or citrate inhibition, and will depress the activity of muscle type II hexokinase.

Adenosine Triphosphate↗
PubMed1976

Stabilization of glucosaminephosphate synthase from rat liver by hexose 6-phosphates. Properties and interconversion of two molecular forms.

Glucosaminephosphate synthase (glucosaminephosphate isomerase (glutamine-forming), EC 5.3.1.19) prepared from rat liver by extraction in the presence of glucose 6-phosphate (Glc-6-P) followed by precipitation with (NH4)2SO4 is susceptible to digestion by trypsin. This enzyme, designated form A, can be converted to tryptic-insusceptible form B upon incubation with Glc-6-P or fructose 6-phosphate (Fru-6-P) at 37 degrees C. The two forms also differ in the degree of activation by dithiothreitol, the degree of inhibition by methyl-glyoxal and the behavior on DEAE-Sephadex and Sephadex G-200 column chromatography. During purification with DEAE-Sephadex followed by hydroxyapatite, form B is converted to form A if Fru-6-P is absent and form A to form B if Fru-6-P is present. The two forms are therefore intercovertible. Under the conditions of purification, form B is more stable than form A, since the purity and yield of the final product are greater with form B than with form A. These findings suggest that the two forms of glucosaminephosphate synthase differ conformationally and that the equilibrium position depends on the concentration of Fru-6-P. Glc-6-P is effective only when it gives rise to Fru-6-P by mediation of glucose-phosphate isomerase.

Animals↗
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

Comparative kinetic studies on the L-type pyruvate kinase from rat liver and the enzyme phosphorylated by cyclic 3', 5'-AMP-stimulated protein kinase.

The kinetics of rat liver L-type pyruvate kinase (EC 2.7.1.40), phosphorylated with cyclic AMP-stimulated protein kinase from the same source, and the unphosphorylated enzyme have been compared. The effects of pH and various concentrations of substrates, Mg2+, K+ and modifiers were studied. In the absence of fructose 1, 6-diphosphate at pH 7.3, the phosphorylated pyruvate kinase appeared to have a lower affinity for phosphoenolpyruvate (K0.5=0.8 mM) than the unphosphorylated enzyme (K0.5=0.3 mM). The enzyme activity vs. phosphoenolpyruvate concentration curve was more sigmoidal for the phosphorylated enzyme with a Hill coefficient of 2.6 compared to 1.6 for the unphosphorylated enzyme. Fructose 1, 6-diphosphate increased the apparent affinity of both enzyme forms for phosphoenolpyruvate. At saturating concentrations of this activator, the kinetics of both enzyme forms were transformed to approximately the same hyperbolic curve, with a Hill coefficient of 1.0 and K0.5 of about 0.04 mM for phosphoenolpyruvate. The apparent affinity of the enzyme for fructose 1, 6-diphosphate was high at 0.2 mM phosphoenolpyruvate with a K0.5=0.06 muM for the unphosphorylated pyruvate kinase and 0.13 muM for the phosphorylated enzyme. However, in the presence of 0.5 mM alanine plus 1.5 mM ATP, a higher fructose 1, 6-diphosphate concentration was needed for activation, with K0.5 of 0.4 muM for the unphosphorylated enzyme and of 1.4 muM for the phosphorylated enzyme. The results obtained strongly indicate that phosphorylation of pyruvate kinase may also inhibit the enzyme in vivo. Such an inhibition should be important during gluconeogenesis.

Adenosine Diphosphate↗
PubMed1977

Purification and molecular properties of the AMP-activated pyruvate kinase from Escherichia coli.

The AMP-activated pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) from Escherichia coli has been purified 200 times through a three-step procedure which gives a homogeneous preparation with a specific activity of 110. The enzyme appears to be a tetramer of molecular weight 190 000. Subunits (molecular weight 51 000) show a single amino-terminal amino acid (serine) and appear as a single band in polyacrylamide gel electrophoresis in sodium dodecyl sulphate. The enzyme crystallizes in conditions of reduced dielectric constant of the solvent in the pH range 6.5-7.5. Kinetic and regulatory properties of the purified enzyme are similar to those described for crude preparations of the enzyme.

Amino Acids↗
PubMed1980

Studies on the regulation of chloroplast fructose-1,6-bisphosphatase. Activation by fructose 1,6-bisphosphate.

Chloroplast fructose-1,6-bisphosphatase (D-fructose 1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) isolated from spinach leaves, was activated by preincubation with fructose 1,6-bisphosphate. The rate of activation was slower than the rate of catalysis, and dependent upon the temperature and the concentration of fructose 1,6-bisphosphate. The addition of other sugar diphosphates, sugar monophosphates or intermediates of the reductive pentose phosphate cycle neither replaced fructose 1,6-bisphosphate nor modified the activation process. Upon activation with the effector the enzyme was less sensitive to trypsin digestion and insensitive to mercurials. The activity of chloroplast fructose-1,6-bisphosphatase, preincubated with fructose 1,6-bisphosphate, returned to its basal activity after the concentration of the effector was lowered in the preincubation mixture. The results provide evidence that fructose-1,6-bisphosphatase resembles other regulatory enzymes involved in photosynthetic CO2 assimilation in its activation by chloroplast metabolites.

Chloroplasts↗
PubMed1975

Inhibition of peptide chain initiation in lysates from ATP-depleted cells. II. Studies on the mechanism of the lesion and its relation to similar alterations caused by oxidized glutathione and hemin deprivation.

The impairment of peptide chain initiation in lysates from ATP-depleted rabbit reticulocytes is accompanied by a loss of their ability to form the 40 S methionyl-tRNAfMet complex with a resultant failure to promote the AUG codon-dependent combination of the complex with the 60 S ribosomal subunit. These partial initiation reactions, as well as the overall protein-synthetic activity of the defective lysates could be restored by addition of a 0.5 M KC1 ribosomal extract or normal postribosomal supernatant or a 40-70% (NH4)2-SO4 fraction derived from it. Alternatively, reactivation of the impaired lysates could be achieved by supplementation with millimolar amounts of cyclic AMP or certain purine derivatives. The same subcellular fractions, as well as cyclic AMP or purine derivatives were also capable of overcoming the inhibition caused by incubating reticulocyte lysates in the presence of oxidized glutathione or in the absence of hemin. Severe intracellular ATP deprivation resulted in accumulation of a soluble translational inhibitor in the postribosomal fraction, thus resembling the parallel phenomenon described in hemin-deprived lysates. The striking similarities between the three kinds of inhibition studied by us point to an identical site of the underlying biochemical lesion, despite the different mechanisms mediating their induction.

Adenine↗