Biomedical subjects

W Schellenberger

Publications and source records attributed to W Schellenberger.

At least 37 records · Page 2Linked to original sources

PubMed1987

The appearance of a critical input concentration of fructose 6-phosphate in the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase cycle.

Stationary states of the fructose 6-phosphate/fructose 2,6-bisphosphate cycle were investigated in relation to the input concentration of fructose 6-phosphate. Below a critical input concentration of fructose 6-phosphate very low levels of fructose 2,6-bisphosphate were obtained. Above this point the fructose 2,6-bisphosphate changes in direct proportion to the input of fructose 6-phosphate. Phosphorylation of the enzyme causes an increase of the critical input concentration of fructose 6-phosphate. The control coefficients for fructose 2,6-bisphosphate have their maximum at the critical input concentration of fructose 6-phosphate.

Fructosephosphates↗

PubMed1986

Kinetic effects of fructose-1,6-bisphosphate on yeast phosphofructokinase.

Yeast phosphofructokinase is known to be effectively activated by fructose-2,6-bisphosphate and AMP. In the absence of the two effectors, fructose-1,6-bisphosphate activates or inhibits the enzyme according to the concentrations of the substrates and of inorganic phosphate. At cellular concentrations of the substrates, however, the effects of fructose-1,6-bisphosphate are negligible. Whereas the activation of the enzyme by AMP is not affected by fructose-1,6-bisphosphate, the latter was found to diminish strongly the activity of the fructose-2,6-bisphosphate-activated enzyme. Inorganic phosphate amplifies the activating effect of fructose-2,6-bisphosphate and augments also the deactivation of the fructose-2,6-bisphosphate-activated enzyme. The deactivating action of fructose-1,6-bisphosphate with respect to fructose-2,6-bisphosphate dominates at low concentrations of fructose-6-phosphate and high levels of ATP and might be of regulatory significance.

Enzyme Activation↗

PubMed1986

Binding of fructose-1,6-bisphosphate to yeast phosphofructokinase.

Binding of fructose-1,6-bisphosphate to yeast phosphofructokinase (EC 2.7.1.11) was measured in a concentration range of 5 to 200 microM of fructose-1,6-bisphosphate with the ultrafiltration technique. At saturation two molecules of fructose-1,6-bisphosphate are bound per subunit of the octameric enzyme. Two distinct types of binding sites have been observed. The high affinity sites (KH = 32.7 +/- 5 microM) exhibit a hyperbolic response in respect to the binding of fructose-1,6-bisphosphate, the low affinity sites (KL = 57.2 +/- 6 microM) show significant positive cooperativity.

Fructosediphosphates↗

PubMed1986

Inhibition of fructose 1,6-bisphosphatase from pig liver by fructose 2,6-bisphosphate.

The inhibition of pig liver fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate has been investigated over a wide range of substrate concentration by measuring the release of labelled inorganic phosphate from [1-32P]fructose 1,6-bisphosphate. The activity of the enzyme can be inhibited completely by fructose 2,6-bisphosphate. The inhibiting effect is most pronounced at low substrate concentrations. The results have been analyzed in terms of a mathematical model assuming a competitive interaction of fructose 1,6-bisphosphate and fructose 2,6-bisphosphate at the catalytic site as well as a synergistic cooperation of the two hexose bisphosphates at an inhibiting allosteric site of the enzyme.

Animals↗

PubMed1985

Effects of fructose 1,6-bisphosphate on the activation of yeast phosphofructokinase by fructose 2,6-bisphosphate and AMP.

Fructose 1,6-bisphosphate decreases the activation of yeast 6-phosphofructokinase (ATP:fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.11) by fructose 2,6-bisphosphate, especially at cellular substrate concentrations. AMP activation of the enzyme is not influenced by fructose 1,6-bisphosphate. Inorganic phosphate increases the activation by fructose 2,6-bisphosphate and augments the deactivation of the fructose 2,6-bisphosphate activated enzyme by fructose 1,6-bisphosphate. Because various states of yeast glucose metabolism differ in the levels of the two fructose bisphosphates, the observed interactions might be of regulatory significance.

Adenosine Monophosphate↗

PubMed1985

Quasi-stationary concentrations of fructose-2,6-bisphosphate in the phosphofructokinase-2/fructose-2,6-bisphosphatase cycle.

The cooperation of phosphofructokinase-2 and fructose-2,6-bisphosphatase is investigated. Experimentally derived rate laws of the kinase and bisphosphatase activities introduced into the respective differential equations permitted to describe the time evolution of fructose-2,6-bisphosphate to quasi-stationary levels. The two enzyme activities were found to exert strong temperature dependence. The quasi-stationary levels of fructose-2,6-bisphosphate, however, are independent on temperature.

Adenosine Triphosphate↗

PubMed1985

Influence of fructose 2,6-bisphosphate on the phosphofructokinase/fructose 1,6-bisphosphatase cycle.

In a reconstituted enzyme system multiple stationary states and oscillatory motions of the substrate cycle catalyzed by phosphofructokinase and fructose 1,6-bisphosphatase are significantly influenced by fructose 2,6-bisphosphate. Depending on the initial conditions, fructose 2,6-bisphosphate was found either to generate or to extinguish oscillatory motions between glycolytic and gluconeogenic states. In general, stable glycolytic modes are favored because of the efficient activation of phosphofructokinase by this effector. The complex effect of fructose 2,6-bisphosphate on the rate of substrate cycling correlates with its synergistic cooperation with AMP in the activation of phosphofructokinase and inhibition of fructose 1,6-bisphosphatase.

Fructose-Bisphosphatase↗

PubMed1985

Temporal organization of the phosphofructokinase/fructose-1,6-biphosphatase cycle.

The dynamic and functional organization of the fructose-6-phosphate/fructose-1,6-bisphosphate cycle has been investigated in an open and homogeneous reconstituted enzyme system containing phosphofructokinase, fructose-1,6-biphosphatase, pyruvate kinase, adenylate kinase and glucose 6-phosphate isomerase. The properties of this system were analyzed by a model based on the kinetic properties of the individual enzymes. It could be shown that in a broad parameter region sustained oscillations arise. At low maximum activities of phosphofructokinase a domain of multiple stationary states occurs, in which stable stationary states can coexist with a stable oscillatory or with an alternate stable stationary state. The occurrence of oscillations and the emergence of alternate stationary motions are caused mainly by the reciprocal effect of the allosteric effectors AMP and fructose-2,6-bisphosphatase must be involved in the reaction network. The study of bisphosphatase. The attained states can either be glycolytic or gluconeogenic, their metabolic efficiencies depend mainly on the maximum activities of phosphofructokinase and fructose-1,6-bisphosphatase as well as on the supply of fructose-6-phosphate and fructose-1,6-bisphosphate. Efficient metabolic states arise only when both the enzyme concentrations and the rates of substrate supply favor either the glycolytic or the gluconeogenic mode of action. At medium maximum concentrations of the enzymes oscillations occur, in which glycolytic and gluconeogenic states are consecutively passed. A high rate of substrate cycling is observed only at the transitions between the functionally antagonistic phases of the periodicities. By this temporal organization the mean efficiency of the states is increased. The integration of fructose-2,6-bisphosphate as very sensitively acting activator of phosphofructokinase and inhibitor of fructose-1,6-bisphosphatase gives rise either to emergence of oscillations or of their extinction. Generally, the glycolytic mode is favored by this effector because of its stimulatory action on the phosphofructokinase activity.

Fructose-Bisphosphatase↗

PubMed1985

Interaction of ADP and fructose-2,6-bisphosphate with phosphofructokinase-1 from yeast.

ADP was found to activate or, depending on the experimental conditions, to inhibit yeast phosphofructokinase-1. In the absence of AMP and fructose-2,6-bisphosphate ADP increases the apparent affinity of the enzyme to fructose-6-phosphate. At low ATP concentrations the maximum activity with respect to fructose-6-phosphate decreases in the presence of ADP, while at high ATP a significant increase of the maximum activity by ADP is observed. In the presence of fructose-2,6-bisphosphate and AMP only the inhibiting effect of ADP persists. The data may be interpreted in terms of a hyperbolic inhibition mechanism.

Adenosine Diphosphate↗

PubMed1985

Inorganic phosphate amplifies the effects of AMP and fructose-2,6-bisphosphate on yeast phosphofructokinase.

Inorganic phosphate is an important regulator of yeast phosphofructokinase activity. In the absence of AMP and fructose-2,6-bisphosphate the dependence of enzyme activity on the concentration of inorganic phosphate is sigmoidal. AMP and fructose-2,6-bisphosphate increase the affinity of phosphofructokinase to inorganic phosphate. At low fructose-6-phosphate concentrations inorganic phosphate amplifies the activating effect of AMP and fructose-2,6-bisphosphate. Yeast phosphofructokinase is more sensitive to ATP inhibition in the absence of inorganic phosphate than in its presence. While in the absence of inorganic phosphate a definite ATP inhibition prevails even at high levels of AMP or fructose-2,6-bisphosphate, the ATP inhibition can be relieved by the cooperation of inorganic phosphate and fructose-2,6-bisphosphate. These effects of inorganic phosphate provide an explanation for the stimulation of glycolysis under anaerobic conditions by inorganic phosphate at unchanged concentrations of AMP and fructose-2,6-bisphosphate (Lagunas and Gancedo, Eur. J. Biochem. 137, 479-483 (1983)).

Adenosine Monophosphate↗

PubMed1985

Glycolytic and gluconeogenic states in an enzyme system reconstituted from phosphofructokinase and fructose 1,6-bisphosphatase.

Transitions between glycolytic and gluconeogenic states have been investigated in an open and homogeneous enzyme system containing phosphofructokinase, fructose 1,6-bisphosphatase, pyruvate kinase, adenylate kinase and glucose 6-phosphate isomerase. The direction of substrate flow was found to depend on the maximum activities of phosphofructokinase and fructose 1,6-bisphosphatase as well as on the influx concentrations of fructose 6-phosphate and fructose 1,6-bisphosphate. At high and low maximum activities of phosphofructokinase and fructose 1,6-bisphosphatase unique and stable stationary states occur, whereas at intermediate enzyme concentrations sustained oscillations emerge. Stationary states with a low rate of substrate cycling demand both appropriate enzyme concentrations and an adequate substrate supply. Accordingly, transitions between efficient glycolytic and gluconeogenic states require changes of the enzyme concentrations and of the supply of substrates. Such transitions exhibit a transient oscillatory response. The sustained oscillations generated at intermediate activities of phosphofructokinase and fructose 1,6-bisphosphatase lead to a significant diminution of the rate of substrate cycling when compared with the respective steady state values. During the oscillations glycolytic and gluconeogenic states are consecutively passed through. Because of this a temporal organization of the antagonistic reactions is achieved. In our system the kinetic organization of the two opposite reactions is mainly brought about by the reciprocal allosteric effects of AMP on the activities of the two enzymes.

Adenosine Triphosphate↗

PubMed1985

Dynamic structures of the fructose 6-phosphate/fructose 1,6-bisphosphate cycle in a reconstituted enzyme system.

The dynamics of the fructose 6-phosphate/fructose 1,6-bisphosphate cycle was investigated in an open and homogeneous system reconstituted from purified enzymes. In addition to phosphofructokinase and fructose 1,6-bisphosphatase, pyruvate kinase, adenylate kinase and glucose 6-phosphate isomerases are involved. The time evolution of the metabolite concentrations is governed by a set of differential equations which take into account flow processes and enzymic conversions of metabolites. Depending on the experimental parameters stable attractors, multiple states and sustained oscillations occur. The main source of the nonlinear dynamics is the reciprocal effect of AMP on the activities of phosphofructokinase and fructose 1,6-bisphosphatase. States are characterized by the net flow rates of substrates and by the rate of futile substrate cycling. For efficient glycolytic states high ratios between the influx rates of fructose 6-phosphate and fructose 1,6-bisphosphate and between the maximum activities of phosphofructokinase and fructose 1,6-bisphosphatase must be maintained, while for an efficient gluconeogenic mode the reverse must hold. Fructose 2,6-bisphosphate exerts reciprocal effects on the activities of phosphofructokinase and fructose 1,6-bisphosphatase. In dependence on the experimental conditions fructose 2,6-bisphosphate was found either to generate or to extinguish oscillations.

Animals↗

PubMed1984

Inhibition of fructose-1,6-bisphosphatase from pig liver by fructose-6-phosphate.

The inhibition of the neutral form of fructose-1,6-bisphosphatase from pig liver by fructose-6-phosphate was investigated in the substrate concentration range of 0.05-500 microM by determination of the rate of formation of labelled inorganic phosphate from [1-32P]fructose-1,6-bisphosphate. The inhibition of the enzyme by fructose-6-phosphate is biphasic, the extent of inhibition decreases with increasing substrate concentrations. Even at high concentrations of fructose-6-phosphate the enzyme is not inhibited completely. The results were interpreted in terms of the model of MONOD, WYMAN and CHANGEUX [10] by assuming a weak competition of fructose-6-phosphate and fructose-1,6-bisphosphate at the catalytic site and a cooperation of the two ligands at the same allosteric site.

Animals↗

PubMed1984

Interaction of Cibacron blue F3G-A with yeast phosphofructokinase.

The binding of Cibacron blue F3G-A to yeast phosphofructokinase was investigated by means of ultracentrifugation. Four moles of Cibacron blue are tightly bound per subunit of phosphofructokinase (dissociation constant = 0.26 microM). This stoichiometry does not correspond to the stoichiometry of ATP binding to yeast phosphofructokinase (two moles of ATP per subunit). Moreover, 32 moles of the dye are bound per subunit of phosphofructokinase to a second class of binding sites with low affinity (dissociation constant = = 53 microM). The action of Cibacron blue on yeast phosphofructokinase cannot be explained completely in terms of its function as ATP analogue.

Adenosine Triphosphate↗

PubMed1984

Cooperation of fructose-2,6-bisphosphate and AMP in the activation of yeast phosphofructokinase.

Yeast phosphofructokinase is effectively activated by AMP and fructose-2,6-bisphosphate. Both effectors influence the sensitivity of the enzyme with respect to fructose-6-phosphate and increase the respective maximum activities. The dependence of phosphofructokinase activity on the concentration of fructose-2,6-bisphosphate was measured at different AMP concentrations and vice versa. By AMP the half activation constant for fructose-2,6-bisphosphate is decreased by one order of magnitude. The affinity to AMP is significantly increased by fructose-2,6-bisphosphate. AMP increases the maximum activity of the enzyme with respect to fructose-2,6-bisphosphate only slightly, while the maximum activity with respect to AMP is drastically increased by fructose-2,6-bisphosphate. The interaction of the two activators is most pronounced at low levels of fructose-6-phosphate and at high concentrations of ATP.

Adenosine Monophosphate↗

PubMed1984

Influence of inorganic phosphate on the kinetic properties of yeast phosphofructokinase.

Yeast phosphofructokinase is effectively activated by inorganic phosphate. In the absence of other allosteric stimulators, inorganic phosphate increases the maximum activity of the enzyme only. In the presence of the activators AMP and fructose 2,6-bisphosphate inorganic phosphate causes changes in the maximum activity and the enzyme affinity to fructose 6-phosphate. Inorganic phosphate augments the sensitivity of phosphofructokinase to the activators AMP and fructose 2,6-bisphosphate and increases the respective maximum activities. The extent of activation of the enzyme by inorganic phosphate prevails at low levels of fructose 6-phosphate and high ATP concentrations.

Adenosine Monophosphate↗

PubMed1984

Effect of enzyme concentrations on sustained oscillations in the fructose 6-phosphate/fructose 1,6-bisphosphate-cycle.

Sustained oscillations were investigated in an open and homogeneous enzyme system reconstituted from phosphofructokinase, fructose 1,6- biphosphatase , pyruvate kinase, adenylate kinase, and glucose 6-phosphate isomerase. The generation of oscillations if mainly due to antagonistic kinetic effects of AMP on phosphofructokinase and fructose 1,6- biphosphatase under the experimental conditions applied. Sustained oscillations were obtained in a broad range of maximum activities of phosphofructokinase or fructose 1,6- biphosphatase are increased. At low maximum activities of the two enzymes oscillations arise, which form a folded limit cycle, while by increasing concentrations of phosphofructokinase or fructose 1,6-bisphosphatase sustained oscillations are introduced, which are restricted to two dimensions.

Adenylate Kinase↗

PubMed1983

Sustained oscillations in a reconstituted enzyme system containing phosphofructokinase and fructose 1,6-bisphosphatase.

In a reconstituted open and homogeneous enzyme system containing phosphofructokinase, fructose 1,6-bisphosphatase, pyruvate kinase, adenylate kinase, and glucose-6-phosphate isomerase sustained oscillations could experimentally be generated. The approach is based on a stirred flow-through reaction chamber. The periodic motions of the reactants are mainly caused by the antagonistic allosteric effects of the adenine nucleotides on the activities of the phosphofructokinase and fructose 1,6-bisphosphatase.

Allosteric Regulation↗