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A Mathematical Model for Trehalose Uptake in the Thermophilic Bacterium Rhodothermus marinus

A Mathematical Model for Trehalose Uptake in the Thermophilic Bacterium Rhodothermus marinus

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HENDEKOVIC, Irena, 2006. A Mathematical Model for Trehalose Uptake in the Thermophilic Bacterium Rhodothermus marinus [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Hendekovic2006Mathe-7557, title={A Mathematical Model for Trehalose Uptake in the Thermophilic Bacterium Rhodothermus marinus}, year={2006}, author={Hendekovic, Irena}, address={Konstanz}, school={Universität Konstanz} }

2011-03-24T17:35:22Z eng The aim of this work was to elucidate the mechanism of trehalose<br />uptake in the thermophilic halophilic bacterium Rhodothermus marinus.<br />Mathematical modeling was employed to translate chemical networks describing<br />subsystems of the postulated trehalose uptake mechanism into coupled<br />differential equations. Analytical functions resulting from these differential<br />equations were used for a multidimensional least-squares fit to experimental<br />data, thus yielding values for fit parameters and serving as a test to<br />decide between two models. The data used were kindly provided by Carla Jorge<br />from the laboratory of<br />Prof. Dr. Helena Santos at the Instituto de Tecnologia Quimica e Biologica (ITQB)<br />in Oeiras, Portugal.<br /><br />Several data sets contributed information to resolve the issue: the experimental<br />data consist of measurements of the uptake rate for radioactive<br />glucose analyzed in chapter 2, measurements of glucose release into the medium<br />due to in vivo trehalase activity<br />dealt with in chapter 3, uptake data for radioactive trehalose modeled in<br />chapters 4 and 5, and cross-inhibition experiments with glucose and trehalose<br />simulated in chapter 6. In addition to these, in vitro inhibition<br />experiments with purified trehalase have shown that glucose inhibits trehalase<br />activity with a K_i of 12 mM.<br /><br />We have proposed two models for trehalose uptake: model 1 consists of a diffusion<br />channel for glucose and trehalose in the outer membrane, a periplasmic trehalase<br />cleaving incoming trehalose into two units of glucose, and a glucose transporter<br />in the cytoplasmic membrane. Model 2 contains an additional trehalose transporter<br />in the inner membrane.<br />Since the glucose transport system is a functionally independent part of both<br />trehalose uptake models, the parameters obtained by fitting the glucose uptake<br />data alone can be used for the fit of the trehalose uptake<br />and enzymatic cleavage data. To describe the in vivo trehalase data four<br />additional parameters are needed. The same four parameters are used in model 1<br />for trehalose uptake; model 2 contains two more. Both experimental curves have to<br />be fitted together, since the same set of parameters has to describe both<br />measurements.<br /><br />One of the remaining parameters, z_2, can be<br />calculated from the K_i for the in vitro glucose inhibition of trehalase<br />mentioned above and the K_m {gt} of<br />the glucose transporter in the cytoplasmic membrane found in chapter 2.8.<br />The theories for enzymatic cleavage and trehalose uptake provide restrictions<br />coupling the remaining fit parameters to the kinetic parameters<br />K_m e and V_{max} e from the enzyme curve and K_m {tr} and V_{max} {tr}<br />(model 1) or K_m {su} and V_{max}{su} (model 2) from the trehalose uptake<br />curve, for which we obtain an approximation<br />by fitting a Michaelis-Menten function through the data. These restrictions<br />together suffice to calculate all fit parameters in model 1, for model two an<br />additional scan over z_6 is needed.<br /><br />By optimizing the four kinetic parameters<br />using model 2 we have obtained a fit that describes all measurements. Model 1<br />can either fit the cleavage data or the high concentration trehalose uptake data,<br />but not both, and is unable to describe the trehalose uptake measurements at low<br />substrate concentrations. The inability of model one to coherently describe<br />all experimental findings shows the necessity of the additional threhalose<br />transporter distinguishing model 1 from model 2.<br /><br />In order to further validate the model we have used the parameters from the<br />glucose fit and from model 2 to simulate an<br />inhibition experiment with a constant concentration of the substrate trehalose<br />and increasing concentrations of the inhibitor glucose. The simulation<br />shows that in model 2 glucose inhibits the uptake of trehalose considerably in<br />the range of high trehalose concentrations, but at low trehalose concentrations<br />even a very high concentration of glucose causes only a slight inhibiting effect.<br />This observation is due to the fact that the dominant uptake pathway at<br />low trehalose concentrations is the direct trehalose transporter which is not<br />inhibited by glucose in our model.<br /><br />The result of the simulation is in qualitative agreement with experimental<br />findings, but those show a tenfold inhibition of<br />trehalose uptake already at the low concentration of 1 µM. To be able to<br />theoretically reproduce this experimental result quantitatively, an extension of<br />the model network by a glucose mediated inhibition of the trehalose transporter<br />in the cytoplasmic membrane should be adressed in further investigations. Hendekovic, Irena 2011-03-24T17:35:22Z A Mathematical Model for Trehalose Uptake in the Thermophilic Bacterium Rhodothermus marinus Hendekovic, Irena Ein mathematisches Modell für Trehaloseaufnahme im thermophilen Bakterium Rhodothermus marinus terms-of-use application/pdf 2006

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