Team:Bielefeld-Germany/Modell

Model of a fixed-bed reactor

In our project we plan to construct a fixed-bed reactor, where immobilized laccases degrade synthetic estrogen and other harmful substances. As a small selection we plan to characterize our different laccases for three estrogens, three analgesics, four PAHs, one insecticide and three possible redox mediators. If we could model the degradation of one substrate by one laccase, we could easily replace the specific K_cat K_M^-1 quotient of other laccases and the amounts of the other substrates.

We ignore the possible cofactors ABTS, syringaldazine and viuloric acid, because on the one hand using these cofactors would increase the costs, and on the other hand they are harmful substances. As shown by Team Substrate Analytic TVEL0 degrades ethinyl estradiol without a redox mediator. So the reaction should follow the Michaelis Menten kinetics.

The reaction

The normal Michaelis Menten kinetic (1) can be adapted for low substrate concentrations. In order to do this formula (2) replaces [ES] in formula (3). The result is formula (4). The velocity of this reaction describes the cange of the substrate concentration over time (5). Formula (6) and (7) solve the integral. Equation (8) is the resulting formula to determine the time dependent substrate concentration by a given startconcentration A.

This transformation of the Michaelis Menten kinetic can be found in "Stryer biochemie". The resulting formula is suitable for very low substrate concentrations. In this case we can estimate degradation of initial substrate concentrations below 0.1 µg L^-1.

Because we can not find any information about the K_cat K_M^-1 quotient for the degradation of estradiol in the enzyme database [http://www.brenda-enzymes.info Brenda] we decided to use the K_cat K_M^-1 quotients for the degradation of [http://www.sigmaaldrich.com/catalog/product/sigma/a1888?lang=de&region=DE ABTS], a redox mediator of the laccase. We use ABTS in this case only as placehoder, until we get suitable data for the substrates. In the following picture we show a few possible reactions of our laccase.

Our Model with different K_cat K_M^-1 values. The span differs from 0.07 seconds to 1500 seconds.

We use K_cat K_M^-1 values from 11 to 643500 that result in degradation time points from 0.07s to 1500s. Furthermore we try to integrate different temperatures to our model. In a span from 10°C to 30°C and a K_cat K_M^-1 value of 1, the degradation differs from 40s to 100s.

Extend the model with sewageplant data

Date taken from the sewage plant located in Schloss Holte.. Temperature minimum: 8.1 °C, maximum: 20,8 °C and average temperature 14.4 °C; pH minimum: 6.6, maximum: 7.2 and average 6.9; flow minimum: 39.2 L s^-1, maximum 232.8 L s^-1 and avarage 76.8 L s^-1

The sewageplant in Schloss Holte provided information about the discharge water, particularly water temperature, pH value and flowrate. The temperature and pH value have a direct influence on the enzymatic activity. Dependent on the enzymatic activity we want to calculate the time our fixed-bed reactor will need to degrdate 80 % of the substrates. This will be the residence time. Combined with the actual flowrate we can determine the reactorsize. To estimate the feasibility we want to know how much enzyme has to be produced for a sewage plant. A 3D model of the required enzme amount dependent of K_cat K_M^-1 values and flowrate.

The required enzyme amount (z axis) for a sewageplant is dependent on K_cat K_M^-1 value (x axis) influenced by temperature and pH value and the flow rate (y axis). Opimal we will need 40 Kg laccase. In the worst case we will need 140 Kg

Outlook

The next step will be, to add more substrates and different laccases to our model. Hence we will need more data about laccases. K_cat K_M^-1 values have to be determined as well as the specifity of the laccases. We have the opportunity to work with a lab-scale water treatment system. So we plan to test our model in defined conditions.