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D A Fell

Publications and source records attributed to D A Fell.

At least 37 records · Page 2Linked to original sources
PubMed1997

Finite change analysis of glycolytic intermediates in tuber tissue of lines of transgenic potato (Solanum tuberosum) overexpressing phosphofructokinase.

Genetically engineered organisms overexpressing phosphofructokinase (PFK), a supposed 'regulatory' step of glycolysis, often show little or no measurable change in glycolytic or respiratory flux, although the concentrations of glycolytic intermediates may change. We have used the finite change theory of Metabolic Control Analysis (MCA) to analyse the concentrations of glycolytic metabolites in aged disks of tuber tissue from four lines of transgenic potatoes expressing different amounts of PFK that, under aerobic conditions, showed statistically indistinguishable rates of respiration. The constancy of the metabolites' concentration deviation indices for different increases in PFK expression indicated that the metabolite changes from a graded series, excluding the possibility of anomalous behaviour that might be observed in a single transgenic line. Consequently we were able to use the finite change method to validate the results of an MCA model of tuber glycolysis [Thomas, Mooney, Burrell and Fell (1997) Biochem. J. 322, 119-127]. Furthermore the metabolite changes with PFK activity are evidence that near-equilibrium steps do not transmit increased substrate concentrations down the pathway without attenuation. Our results support the view that flux increase by activation of a single enzyme early in the pathway will, contrary to expectations, be of limited effectiveness in achieving flux increases.

Energy Metabolism↗
PubMed1997

Metabolic Control Analysis of glycolysis in tuber tissue of potato (Solanum tuberosum): explanation for the low control coefficient of phosphofructokinase over respiratory flux.

We have applied Metabolic Control Analysis (MCA) in an attempt to determine the distribution of glycolytic flux control between the steps of glycolysis in aged disks of potato tuber under aerobic conditions, using concentrations of glycolytic metabolites in tuber tissue from a range of transgenic potato plants and published enzyme kinetic data. We modelled the substrate and effector kinetics of potato tuber phosphofructokinase (PFK) by reanalysing published results. Despite the scarcity of reliable kinetic data, our results are in agreement with experimental findings namely that, under the conditions described, PFK has little control over glycolytic flux. Furthermore our analysis predicts that under these conditions far more control lies in the dephosphorylation of phosphoenolpyruvate and/or in the steps beyond. We have validated the results of our analysis in two ways. First, predictions based on calculated concentration control coefficients from the analysis show generally good agreement with observed metabolite deviation indices discussed in the preceding paper [Thomas, Mooney, Burrell, and Fell (1997) Biochem. J. 322, 111-117]. Second, sensitivity analysis of our results shows that the calculated control coefficients are robust to errors in the elasticities used in the analysis, of which relatively few need to be known accurately. Experimental and control analysis results agree with previous predictions of MCA that strong co-operative feedback inhibition of enzymes serves to move flux control downstream of the inhibiting metabolite. We conclude that MCA can successfully model the outcome of experiments in the genetic manipulation of enzyme amounts.

Electron Transport↗
PubMed1996

Design of metabolic control for large flux changes.

Metabolic Control Analysis has invalidated many traditional biochemical concepts of control, in particular the rate-limiting step. However, it has not been used to question the mechanisms by which pathway flux is thought to be controlled, such as the action of allosteric effectors or of covalent modification mechanisms. Here we use Control Analysis and computer simulation to examine the response of pathway segments to change in flux imposed by action on an enzyme outside the segment. Whether these segments contain near-equilibrium enzyme-catalysed reactions, cooperative enzymes, feedforward activation loops or feedback inhibition loops, their responses are significantly different from those observed in vivo. In particular, they do not exhibit the remarkable degrees of metabolite homoeostasis during large flux changes that have frequently been observed experimentally. On the other hand, near-constant levels of metabolites in spite of large changes of flux are consistent with our recent proposal that multi-site modulation--simultaneous activation of many pathway steps-is the normal method by which metabolism is controlled.

Animals↗
PubMed1995

Physiological control of metabolic flux: the requirement for multisite modulation.

Biochemists have long assumed that the flux through a metabolic pathway can be controlled by the activity of a key regulatory enzyme near the beginning of the pathway. We present the accumulating evidence that every step in this assumption is flawed. Instead, effective physiological control of metabolism is shown to involve simultaneous multisite modulation through action on a number of enzymes.

Binding Sites↗
PubMed1994

Control of glucose utilization in working perfused rat heart.

Metabolic control analyses of glucose utilization were performed for four groups of working rat hearts perfused with Krebs-Henseleit buffer containing 10 mM glucose only, or with the addition of 4 mM D-beta-hydroxybutyrate/1 mM acetoacetate, 100 nM insulin (0.05 unit/ml), or both. Net glycogen breakdown occurred in the glucose group only and was converted to net glycogen synthesis in the presence of all additions. The flux of [2-3H]glucose through P-glucoisomerase (EC 5.3.1.9) was reduced with ketones, elevated with insulin, and unchanged with the combination. Net glycolytic flux was reduced in the presence of ketones and the combination. The flux control coefficients were determined for the portion of the pathway involving glucose transport to the branches of glycogen synthesis and glycolysis. Major control was divided between the glucose transporter and hexokinase (EC 2.7.1.1) in the glucose group. The distribution of the control was slightly shifted to hexokinase with ketones, and control at the glucose transport step was abolished in the presence of insulin. Analysis of the pathway from 3-P-glycerate to pyruvate determined that the major control was shared by enolase (EC 4.2.1.1) and pyruvate kinase (EC 2.7.1.40) in the glucose group. Addition of ketones, insulin, or the combination shifted the control to P-glycerate mutase (EC 5.4.2.1) and pyruvate kinase. These results illustrate that the control of the metabolic flux in glucose metabolism of rat heart is not exerted by a single enzyme but variably distributed among enzymes depending upon substrate availability, hormonal stimulation, or other changes of conditions.

3-Hydroxybutyric Acid↗
PubMed1993

A computer program for the algebraic determination of control coefficients in Metabolic Control Analysis.

A computer program (MetaCon) is described for the evaluation of flux control, concentration control and branch-point distribution control coefficients of a metabolic pathway. Requiring only the reaction scheme as input, the program produces algebraic expressions for the control coefficients in terms of elasticity coefficients, metabolite concentrations and pathway fluxes. Any of these variables can be substituted by numeric or simple algebraic expressions; the expressions will then be automatically rearranged in terms of the remaining unknown variables. When all variables have been substituted, numeric values will be obtained for the control coefficients. The program is a computerized implementation of the matrix method for the determination of control coefficients. The features of MetaCon are compared with those of other programs available to workers in Metabolic Control Analysis. Potential benefits of, and methods of using, MetaCon are discussed. The mathematical background and validity of the matrix method rules are discussed, and the algorithm used by MetaCon is described. The matrix method is shown to be a specific case of a previously described general formalism for calculating control coefficients.

Metabolism↗
PubMed1990

Metabolic control analysis. The effects of high enzyme concentrations.

Differing views have been given in the literature as to whether the presence in a pathway of an enzyme at a concentration comparable to that of its substrate affects the values of control coefficients and the theorems of metabolic control analysis. Here we argue in favour of one of those views: that there is no effect unless the enzyme sequesters a substrate that contains a conserved moiety. In this particular case, we derive both a general criterion for estimating whether such an effect will be of a significant magnitude, and equations for determining the changes in the flux control coefficients. The nature of the phenomenom and the application of the equations are illustrated with a numerical simulation.

Enzymes↗
PubMed1990

Covalent modification and metabolic control analysis. Modification to the theorems and their application to metabolic systems containing covalently modifiable enzymes.

A study of the sensitivity properties of metabolic systems containing covalently modifiable enzymes and cascades has been carried out with the aid of metabolic control analysis. We have considered how the theorems of metabolic control analysis must be modified to take into account covalently modifiable enzymes, and have used these results to investigate the effects of increasing the total amount of modifiable enzyme. The sensitivity of system variables to an effector acting through a covalent-modification cycle has also been investigated.

Energy Metabolism↗
PubMed1990

Metabolic control analysis. Sensitivity of control coefficients to elasticities.

This paper illustrates a method to calculate the sensitivities of control coefficients to the elasticities which determine their values and it is shown that these sensitivities are systemic properties. We show, both theoretically and with a practical example, how they can be used to investigate: (a) the relative importance of a particular elasticity in the determination of the value of a control coefficient; (b) the effect of experimental error on the values of the control coefficients and (c) the construction of confidence limits around the values of the control coefficients.

Animals↗