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J F Hervagault

Publications and source records attributed to J F Hervagault.

At least 19 recordsLinked to original sources
PubMed2000

alpha-Amylase production by free and immobilized Bacillus subtilis.

The effect of glucose on the alpha-amylase production by Bacillus subtilis ATCC-21556 was studied. Initial glucose concentrations up to 20 g/L were found to be directly proportional to the specific alpha-amylase production in an immobilized-cell batch system, whereas a free-cell batch system presented an inversely proportional relationship with the initial glucose concentration. This might be owing to the alpha-amylase repression by the glucose present in the culture medium. Three hundred eighty-five percent of the specific alpha-amylase production with the free-cell system was produced by the immobilized-cell batch culture.

Bacillus subtilis↗
PubMed1997

Irreversible metabolic transitions: the glucose 6-phosphate metabolism in yeast cell-free extracts.

The steady-state and dynamic behavior of a partial glycolytic reaction sequence are investigated in cell-free extracts of yeast. Pyruvate kinase, adenylate kinase and glucose 6-phosphate isomerase cooperate to a multienzyme system centered around the 6-phosphofructokinase (6-PFK) and fructose 1,6-bisphosphatase (FBPase) cycle. The reaction system operates under thermodynamically open conditions maintained by a continuous supply of substrates, i.e., glucose 6-phosphate (Glc6P), ATP and phosphoenolpyruvate (PPrv) in a flow-through reaction chamber. Appropriate conditions lead to the occurrence of (two) coexisting and markedly different time-independent states in the metabolite concentrations and fluxes. For particular experimental conditions, changes in the influx adenylic energy charge, [AEC]IN, may cause transitions between these alternative steady states which are either reversible as it occurs in classical hysteresis phenomena, or, more importantly, irreversible (irreversible transitions, IT) where the system is not able to switch back to its previous state even when the perturbation is reverted. The emergence of these irreversible transitions do not result from artificial or non-realistic experimental constraints, but are a potential intrinsic property of any non-linear dynamic system exhibiting bi- or multistability. These one-way transitions may well have important biological implications with respect to switching, adaptation and memory phenomena.

Adenosine Triphosphate↗
PubMed1994

An approach to the in vitro study of the UTP/UDPglucose/UDP moiety-conserved cycle.

The kinetic behavior of a moiety-conserved ternary cycle is tested experimentally. This system contains the enzymes UDPglucose pyrophosphorylase, glycogen synthase and nucleoside diphosphokinase, converting respectively UTP into UDPglucose, then into UDP and back to UTP in a cyclic manner. The UDPGlc P2ase and NDPK steps are made irreversible by addition of inorganic pyrophosphatase and phosphocreatine kinase, respectively. In order to predict both the evolution and the steady-state values of the various substrates, a model is derived, which takes into account the actual enzyme rate expressions and parameter values, as determined under our experimental conditions. In that model, the UTP, UDPglucose and UDP are taken as the variables, whereas the total concentration of the substrate pool and the four enzyme maximal activities are chosen as the control parameters. Depending upon the various parameter values, monostability, reversible bistability and irreversible transitions may theoretically occur. However, it turns out that some of these values for which multistability might occur, are not accessible experimentally. Under conditions of monostability, the evolutions of the three substrates as experimentally measured are shown to be in good qualitative and quantitative agreement with the model predictions. The relaxation times between two consecutive steady states when a parameter is varied, are shown to be long-lasting processes (several hours). That such an experimental ternary substrate cycle actually exhibits a low sensitivity to any perturbation, addresses the issue to knowing if the same property is likely to occur in vivo, or, in other words, do large moiety-conserved cycles act as metabolic buffers?

Animals↗
PubMed1994

Various vectorial behaviours of a spatially structured substrate cycle.

The dynamic properties of the glucose/glucose-6-phosphate cycle are studied under conditions where the phosphatase and kinase interconverting enzymes are spatially distributed. A semi-artificial membrane made of compacted plant cell walls bearing active phosphatase in its natural state, separates two compartments, one of these compartments containing soluble hexokinase. Depending only upon the two enzyme activity levels and the initial distribution of the substrates, numerous asymmetrical and vectorial behaviours can be observed, such as facilitated glucose 6-phosphate diffusion, active transport of either glucose or glucose 6-phosphate and sequential (alternative) transport between glucose and glucose 6-phosphate. A diffusion-partition reaction coupling can account for these oriented mass transfers. The possibility of such a coupling in this model system is clearly dictated by the global analog of the Curie principle. These results may provide new insight on (a) the still obscure role played by the cell-wall phosphatase activities, particularly their involvement in the transport of exogenous phosphomonoesters, and (b) the actual in vivo operation of substrate and protein cycles in view of the heterogeneity and anisotropy of the cellular milieu.

Biological Transport↗
PubMed1993

Combined effects of diffusional hindrances, electrostatic repulsion and product inhibition on the kinetic properties of a bound acid phosphatase.

The kinetic properties of a soybean cell wall phosphatase were studied and compared under different environmental conditions. The native enzyme isolated from the wall exhibits classical Michaelis-Menten kinetics. When buried into the cell wall, its natural environment, the enzyme mimics an apparent negative cooperativity. This deviation from hyperbolic dependence of the activity vs. the substrate concentration may result from an heterogeneous distribution of the enzyme molecules between the surface and the inside of the wall, where an electrostatic partition effect takes place. Cell wall fragments compacted on a Millipore-type membrane allow the study of the dual effects of diffusional hindrances and electrostatic interactions on the global kinetic behavior. By increasing the ionic strength, partition effects can be suppressed and therefore diffusional effects alone can be taken into account. On the one hand, diffusion and partition act individually and synergetically to decrease the apparent global affinity of phosphatase with respect to glucose 6-phosphate. On the other hand, product inhibition by inorganic phosphate is subjected to dual effects from diffusion through accumulation and partition through repulsion. A simple diffusion-partition reaction model accounts qualitatively and quantitatively for the experimental observations.

Acid Phosphatase↗
PubMed1993

Physiological behaviour of encapsulated somatic embryos.

Somatic embryos are characterised by the absence of any protection, a very low reserve level and a high water content. The effects on the respiration and the radicular elongation of somatic embryos of a non toxic and easy to use hydrogel, such as alginate, have been studied. Respiration or germination rates decreased with an increase in alginate concentration. When the encapsulated somatic embryos were placed in a liquid medium, there was very little difference between the germination rates observed at different alginate concentrations, either with or without an additional PEI layer. The effect of capsule concentration on germination rate was significatively different when the artificial seeds were growing on solid media. Besides the anoxic treatment upon encapsulated somatic embryos, storage for one month also decreased the germination rate.

Alginates↗
PubMed1992

Patterns of spatiotemporal organization in an "ambiquitous" enzyme model.

Many enzymes in pathways such as glycolysis associate reversibly with cellular substructures. The spatiotemporal behavior of a "limit-cycle" oscillation model is studied under the condition that the "ambiquitous" oscillophor, phosphofructokinase, is partitioned between "bulk-phase" and "bound" forms in a heterogeneous system. Computer simulation demonstrates the occurrence of sustained, wave-like spatiotemporal patterns of chemical concentration in the bulk medium. Kinetic dissimilarity among the localized populations of bound enzyme leads to a "polarity" effect in the wave phenomenon. It is suggested that a key physiological role of the limit-cycle regime is to engender a rapid, site-to-site, signal-transmission modality in large eukaryotic (e.g., mammalian) cells.

Adenine Nucleotides↗
PubMed1992

Application of the metabolic control theory to the study of the dynamics of substrate cycles.

Substrate cycles are ubiquitous structures of the cellular metabolism (e.g. Krebs cycle, fatty acids beta-oxydation cycles, etc...). Moiety-conserved cycles (e.g. adenine nucleotides and NADH/NAD, etc...) are also important. The role played by such cycles in the metabolism and its regulation is not clearly understood so far. However, it was shown that these cycles can generate multistationarity (bistability), irreversible transitions, enhancement of sensitivity, temporal oscillations and chaotic motions (Hervagault & Canu, 1987; Hervagault & Cimino, 1989; Reich & Sel'kov, 1981; Ricard & Soulié, 1982). [formula: see text] Fig. 1: Scheme of the open binary substrate cycle under study. The substrate S is converted into P with a net rate v2. Substrate P is converted in turn into S with a net rate v3. Step v2 is inhibited by excess of the substrate, S. In addition, the cycle operates under open conditions, that is zero-order input of S at rates alpha 0(v1) and first order outputs of S and P at rates alpha S and alpha P(v4), respectively. The metabolic control theory (see also Fell, 1990), which shows how a metabolic network reacts to small perturbations in the vicinity of a steady state, and is formulated with the so-called "control coefficients", was applied to such a cycle in order to get a better knowledge on the importance of each step at the regulatory point of view. The behaviour of a binary substrate cycle (fig. 1) in which one of the enzymes may be subjected to inhibition by excess of its substrate (v2) was studied theoretically.(ABSTRACT TRUNCATED AT 250 WORDS)

Models, Biological↗
PubMed1992

Steady-state properties of a model ternary substrate cycle: theoretical predictions.

Numerous ternary substrate cycles are metabolically operative in vivo. The relative concentrations of the interconverted substrates are generally correlated with different physiological states. These cycles often include reversible and/or substrate-inhibited enzymic steps. The switch between one steady state (metabolic state) and another may be the consequence of either the effect of an exogeneous metabolite or signal, or the alteration of a cycle internal parameter. The interpretation of results obtained with currently designed experiments on substrate cycles seldom take into account the very dynamic and regulatory properties inherent in the cyclic and often autocatalytic nature of the pathway. In the present report, the various dynamic properties of a model ternary substrate cycle, bounded by moiety conservation, are investigated. Three situations with increasing complexity are considered: (i) the three enzymes are michaelian and catalyse irreversible steps; (ii) one of the enzymic steps is reversible; and (iii) one step is subjected to a destabilizing factor, i.e. inhibition by excess of substrate. The behavior(s) of the whole cycle is mainly controlled by four parameters, that is, ST, the total concentration of the substrate pool, and the three enzyme maximal velocities, VMi (i = 1,2,3). As ST (= S1 + S2 + S3) is constant, the Si steady-state concentrations (stable or not) can be represented in barycentric coordinates in a triangle (simplex). This convenient representation allows us to predict the different states of the system when one enzyme maximal activity is varied. The steady-state concentration dependencies as a function of one or several parameters may be either monostable (possibility of zero-order ultrasensitivity) or bistable (with or without reversible transitions). The physiological and experimental relevances of these observations are emphasized.

Enzyme Stability↗
PubMed1991

A plausible model for reversal of neoplastic transformations in plants based on multiple steady states.

We offer a plausible interpretation of some experiments on the reversal of neoplastic transformations in plants. We suggest that normal cells and tumorous cells represent multiple stable-steady states corresponding to a reaction feedback mechanism. The (autocatalytic) feedback loop is constructed from observations on the role played by myo-inositol: it increases the permeability of ions through the membrane and the biosynthetic pathway to myo-inositol is activated by ions. Provided that the permeabilities of nutrients (sugars and salts) are a product-enhanced function of myo-inositol, then we have a (oversimplified) model that can exhibit multiple stationary stable states, one or two depending on the exogenous nutrients and myo-inositol concentrations, and reversible and irreversible transitions from one of these states to the other are possible. From this model, straightforward simple experiments are suggested. We also propose that recent models dealing with the intracellular calcium regulation by hormones, where one key step requires the hydrolysis of inositol phospholipids, take into account free myo-inositol and endogenous hormone concentrations (e.g., auxins).

Calcium↗
PubMed1991

Irreversible transitions in the 6-phosphofructokinase/fructose 1,6-bisphosphatase cycle.

The dynamics of the fructose 6-phosphate fructose-1,6-bisphosphate cycle operating in an open and homogeneous system reconstituted from purified enzymes was extensively studied. In addition to 6-phosphofructokinase and fructose-1,6-bisphosphatase, pyruvate kinase, adenylate kinae and glucose-6-phosphate isomerase were involved. In that multi-enzyme system, the main source of non-linearity is the reciprocal effect of AMP on the activities of 6-phosphofructokinase and fructose-1,6-bisphosphatase. Depending upon the experimental parameter values, stable attractors, various types of multiple states and sustained oscillations were shown to occur. In the present report we show that irreversible transitions are also likely to occur for realistic operating conditions. Two parameters of the system, that is the adenylate energy charge of the influx and the fructose-1,6-bisphosphatase maximal activity, are potential candidates to provoke such irreversible transitions from one steady state to the other: (a) when varying the maximal activity of fructose-1,6-bisphosphatase, the system can jump irreversibly from a low to a high stable steady state, and (b) when the adenylate energy charge of the influx is the changing parameter, irreversible transitions occur from a high stable steady state to a stable oscillatory state (limit cycle motion). This behavior can be predicted by constructing the loci of limit points and Hopf bifurcation points.

Fructose-Bisphosphatase↗
PubMed1990

Irreversible transitions in a model substrate cycle. An experimental illustration.

In a previous article [(1987) J. Theor. Biol, 127, 439-449], the dynamic behavior of a simple substrate cycle, bounded by moiety conservation, and in which one of the two antagonist enzymes is subjected to a destabilizing factor, was investigated. Depending upon the control parameter chosen, that is, the total interconverted substrate concentration and the ratio of the interconverting enzyme maximal activities, monostability, reversible (hysteresis) and/or irreversible transitions could be observed. In the present work, we report experiments dealing with the moiety ATP/ADP interconverted by enzymes phosphofructokinase (PFK) and pyruvate kinase (PK). The cycle operates under conditions where (1) PFK is inhibited by excess of its substrate, ATP, and (2) both enzymes are working under zero-order kinetics for their respective cosubstrates F6P and PEP. Under conditions where the PK maximal activity is lower than the PFK optimal activity, irreversible transitions from a high ATP (resp. low ADP) steady-state concentration to a lower (resp. higher) one, are observed when varying the total moiety (ATP + ADP) concentration. A graphical interpretation of the observed behavior is given. Plausible biochemical consequences of this phenomenon are also emphasized.

Adenosine Diphosphate↗
PubMed1989

pH-induced bistable dynamic behaviour in the reaction catalysed by glucose-6-phosphate dehydrogenase and conformational hysteresis of the enzyme.

1. Bistable (multiple stationary states) dynamic behaviour in the activity of glucose-6-phosphate dehydrogenase that was subjected to successive pH change was demonstrated in an open continuously stirred tank reactor. Although the enzyme under study did not exhibit an autocatalytic effect and was homogeneously distributed, bistability was shown to occur. 2. The successive pH changes of the enzyme solution corresponded to a pH transition (8.3 in equilibrium 2), i.e. an acidification (forward direction) and an alkalinization (reverse direction). By use of intrinsic protein fluorescence methods, a glucose-6-phosphate dehydrogenase conformational hysteresis was shown to exist concomitant with the pH transition before and after enzyme injection into the reactor. 3. The results obtained suggest that the enzyme behaves, conformationally, as a memory device that stores information about its pH history (i.e. the enzyme records information in its structure about the environment to which it was previously exposed) and transduces it in a non-linear dynamic fashion, producing the bistable behaviour observed in the open reactor.

Catalysis↗
PubMed1989

Spatial patterns in a photobiochemical system.

Illumination of a system consisting of a vertical tube containing an unstirred homogeneous suspension of thylakoids in an imposed linear concentration gradient of an electron acceptor, 2,6-dichloroindophenol, gave rise to a one-dimensional banded pattern of the acceptor that evolved in a time-dependent manner. The spatial pattern was obtained only within certain parameters of the dichloroindophenol gradient and with certain concentrations of thylakoids. Various numbers of bands were obtained by varying the gradient parameters, the thylakoid concentration, or both. Pattern formation was reaction-dependent since inhibition of the water-splitting system, either by a physical (heating) or by a chemical (methylamine) method, resulted in abolition of the spatial periodicity. Obtaining the self-organized redox transition of dichloroindophenol in a gelled medium suggests that pattern formation in this system could be explained mainly by a reaction-diffusion mechanism.

Journal Article↗
PubMed1989

Predictions of thermodynamic efficiency in a pumped biochemical reaction.

We propose and analyze a possible experimental system for the investigation of the thermodynamic efficiency of generating biochemical gradients. We investigate the efficiency of a model pump that uses 6-phosphofructokinase (EC 2.7.1.11, an enzyme that exhibits highly nonlinear kinetics), chromatophores from Rhodobacter sphaeroides, and light to generate a biochemical gradient. We analyze the experimental system and an equivalent alternative configuration and show that the establishment and maintenance of a concentration gradient across a membrane is thermodynamically equivalent to the establishment and maintenance of a stationary state in a single-phase, isothermal, open, homogeneous reaction system. With a constant input of light, the system can exist in a stable node (a stable steady state), a stable focus (upon perturbation from its steady state, the system returns to its steady state with an oscillatory component), and a stable limit cycle (at steady state the system exhibits stable oscillations). We investigate the efficiency of the system with both steady and oscillatory light input and observe efficiency changes that depend upon the autonomous state of the system and the frequency and amplitude of the periodic light input. When the system is in a stable focus, an efficiency maximum is seen when the system is perturbed at its resonant frequency. When the system is in a stable limit cycle, efficiency increases are seen near the 1:3, 1:2, and 2:1 entrainment regions and an efficiency decrease is seen near the 1:1 entrainment region. We further calculate various contributions to the efficiency: the phase shift of the force and flux, the magnitude of the response to the perturbation, and changes in the average values of the force and flux during a perturbation. We show that all three changes contribute to the overall changes in efficiency, but increases and decreases in the average force make the largest contributions.

Kinetics↗
PubMed1987

Experimental evidence for a zero-order ultrasensitivity in a simple substrate cycle.

It was shown [Goldbeter & Koshland (1981), Proc. Natl. Acad. Sci. USA, 78, 6840-6844] that amplified sensitivity may arise in reversible covalent modification systems, when the converter enzymes operate in their zero-order region. We show that "zero-order ultrasensitivity" may also occur in simple substrate cycles. The experimental study deals with the Formate/Lactic dehydrogenases model cycle, interconverting the reduced and oxidized forms of NAD. For NAD(H) concentrations high enough (with respect to the enzyme KM's), abrupt changes in the steady-state substrate concentrations may result from small variations in the ratio of maximal enzyme activities. The amplification factors are measured. Implications in metabolic regulation are also taken up.

Aldehyde Oxidoreductases↗
PubMed1987

Bistability and irreversible transitions in a simple substrate cycle.

The dynamic properties of a simple substrate cycle involving two antagonist enzymes are investigated. One of these enzymes exhibits a non-linearity through inhibition by excess substrate. Depending either on the interconverted substrate pool concentration or the maximal activity of the non-inhibited enzyme, monostability, bistability and irreversible transitions may occur. A reversible bistable cycle is shown to present interesting features for regulatory purposes as it can respond to external (and/or internal) modulations in two different ways: A buffering effect by efficient stabilization of the steady-states, or, an increase in sensitivity by switching the system from one regime to the opposite one. The plausible biochemical and biological implications of irreversible transitions are discussed and emphasized in terms of "metabolic transitions".

Enzymes↗