Bioreactors immobilized enzyme particles

Consider an idealized simple case of a Michaelis-Menten type bioreaction taking place in a vertical cylindrical packed-bed bioreactor containing immobilized enzyme particles. The effects of mass transfer within and outside the enzyme particles are assumed to be negligible. The reaction rate per dilfcrential packed height (m) and per unit horizontal cross-sectional area of the bed (m ) is given as (cf. Equation 3.28) ... 

As most biochemical reactions occur in the liquid phase, bioreactors usually handle liquids. Processes in bioreactors often also involve a gas phase, as in cases of aerobic fermentors. Some bioreactors must handle particles, such as immobilized enzymes or cells, either suspended or fixed in a liquid phase. With regard to mass transfer, microbial or biological cells may be regarded as minute particles. 

In the design and operation of various bioreactors, a practical knowledge of physical transfer processes - that is, mass and heat transfer, as described in the relevant previous chapters - are often also required in addition to knowledge of the kinetics of biochemical reactions and of cell kinetics. Some basic concepts on the effects of diffusion inside the particles of catalysts, or of immobilized enzymes or cells, is provided in the following section. 

Bubble columns in which gas is bubbled through suspensions of solid particles in liquids are known as slurry bubble columns . These are widely used as reactors for a variety of chemical reactions, and also as bioreactors with suspensions of microbial cells or particles of immobilized enzymes. 

The diversity of compounds that can be analyzed by HPLC currently preclude the use of any one HPLC/MS technique to specifically detect trace quantities. Thermospray is one of the most popular HPLC/MS techniques due to its ability to ionize nonvolatile and thermally labile compounds with minimal compromises on a HPLC separation or MS operation. Ion evaporation ionization (no filament or discharge) is suitable for numerous compounds, but often results in spectra with insufficient structural information (i.e. one ion spectra), with sensitivities varying drastically between compound classes. In cases where thermospray specificity or sensitivity is not sufficient, complementary HPLC/MS approaches need to be employed. The use of thermospray HPLC/MS and complementary techniques (i.e. immobilized enzyme bioreactors, chemical degradation and particle beam HPLC/MS) have been evaluated for the specific analysis of three major classes of compounds--peptides, pharmaceuticals, and pesticides. 

The packed-bed reactor is a cylindrical, usually vertical, reaction vessel into which particles containing the catalyst or enzyme are packed. The reaction proceeds while the fluid containing reactants is passed through the packed bed. In the case of a packed-bed bioreactor, a liquid containing the substrate is passed through a bed of particles of immobilized enzyme or cells. 

Immobilized enzyme and cell particles may be used in packed bed bioreactors, or the particles may be suspended in stirred tanks, bubble columns, airlift bioreactors, and fluidized beds, as discussed in an earlier section of this... 

The correlations for as discussed above are for homogeneous liquids. Bubbling gas-liquid reactors are sometimes used for suspensions, and bioreactors of this type must often handle suspensions of microorganisms, cells, or immobilized cells or enzymes. Occasionally, suspensions of nonbiological particles, to which organisms are attached, are handled. Consequently, it is often necessary to predict how the values for suspensions will be affected by the system properties and operating conditions. In fermentation with a hydrocarbon substrate, the substrate is usually dispersed as droplets in an aqueous culture medium. Details of... 

Mass transfer limitations can be relevant in heterogeneous biocatalysis. If the enzyme is immobilized in the surface or inside a solid matrix, external (EDR) or internal (IDR) diffusional restrictions may be significant and have to be considered for proper bioreactor design. As shown in Fig. 3.1, this effect can be conveniently incorporated into the model that describes enzyme reactor operation in terms of the effectiveness factor, defined as the ratio between the effective (or observed) and inherent (in the absence of diffusional restrictions) reaction rates. Expressions for the effectiveness factor (rj), in the case of EDR, and the global effectiveness factor (t ) for different particle geometries, in the case of IDR, were developed in sections 4.4.1 and 4.4.2 (see Eqs. 4.39-4.42,4.53,4.54,4.71 and 4.72). Such functions can be generically written as ... 

The preparation of an easily replaceable protease microreactor for microchip application was described by Bilkova and colleagues [83]. The most important advantage of microsphere-packed microchip bioreactors is that the enzyme-immobilized microspheres in the channel can be replaced. Magnetic particles coated with... 

The nanoparticle assemblies can also be used to enhance the chemical reactivity of biomolecules. Au nanoparticles assembled on polyurethane nucrospheres are used as permeable high-surface supports for the immobilization of enzymes such as pepsin to provide easy access of the substrate molecules to the enzyme active centers in the multilayer enzyme assembly. Proteins immobilized in this way exhibit biocataly tic activity higher than that of the free enzyme in solution and significantly enhanced temperature and pH stability [108]. In another approach, the layer-by-layer deposition of enzymes and magnetic particles is applied to prepare a bioreactor, which allows the biocatalytic layer to be stripped out with an external magnet when it is deactivated, so that the surface could be reloaded with a new active biocatalyst layer [109]. 

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