Reactions bioreactions

Most nonconventional media used for bioreaction consist of more than one distinct phase. The organic phase can be used as nonpolar fluid, which acts as a reservoir for one or many reactants. It is generally a continuous phase. The other phase contains the biocatalyst and is generally the site for the reaction. This relatively polar phase is continuous or dispersed in the organic phase. 

Many interesting biocatalytic reactions involve organic components that are poorly water-soluble. When using organic-aqueous biphasic bioreactor, availability of poorly water-soluble reactants to cells and enzymes is improved, and product extraction can be coupled to the bioreaction. Many applications in two-phase media can use the existing standard-type bioreactors, such as stirred-tank, fluidized-bed, and column reactors with minor adjustments. 

Optical biosensors can be designed when a selective and fast bioreaction produces chemical species that can be determined by an optical sensor. Like the electrochemical sensors, enzymatic reactions that produce oxygen, ammonia, hydrogen peroxide, and protons can be utilized to fabricate optical sensors. 

In this chapter chromatographic bioreactors are considered as chromatographic reactors where the reaction is catalyzed by an enzyme or enzyme system, which can be present in pure form or as a cell component. The enzyme can be immobilized on the matrix or it can be dissolved in a liquid phase. Therefore, the reaction can take place in either phase. Several different bioreactions were performed in chromatographic reactors of different types. In the following part some pertinent examples are presented according to their type of reaction. 

A relatively short but interesting paper by Arnold et al.55 suggests different calibrations for different segments of a bioreaction. Using the cultivation of S. fradiae and monitoring the oils and tylosin, the reaction was followed for 150 h. It was determined that the extensive matrix changes over the entire reaction time would make calibration difficult. The process was broken into (1) 0 to 50 h, (2) 50 to 100 h, and (3) 100+ h. 

Semibatch reactors are especially important for bioreactions, where one wants to add an enzyme continuously, and for multiple-reaction systems, where one wants to maximize the selectivity to a specific product. For these processes we may want to place one reactant (say, A) in the reactor initially and add another reactant (say, B) continuously. This makes Ca large at all times but keeps Cg small. We will see the value of these concentrations on selectivity and yield in multiple-reaction systems in the next chapter. 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

In general, bioreactions can occur in either a homogeneous hquid phase or in heterogeneous phases including gas, hquid, and/or sohd. Reactions with particles of catalysts, or of immobilized enzymes and aerobic fermentation with oxygen supply, represent examples of reactions in heterogeneous phases. 

In this chapter we provide the fundamental concepts of chemical and biochemical kinetics that are important for understanding the mechanisms of bioreactions and also for the design and operation of bioreactors. First, we shall discuss general chemical kinetics in a homogeneous phase and then apply its principles to enzymatic reactions in homogeneous and heterogeneous systems. 

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) ... 

Several special terms are used to describe traditional reaction engineering concepts. Examples include yield coefficients for the generally fermentation environment-dependent stoichiometric coefficients, metabolic network for reaction network, substrate for feed, metabolite for secreted bioreaction products, biomass for cells, broth for the fermenter medium, aeration rate for the rate of air addition, vvm for volumetric airflow rate per broth volume, OUR for 02 uptake rate per broth volume, and CER for C02 evolution rate per broth volume. For continuous fermentation, dilution rate stands for feed or effluent rate (equal at steady state), washout for a condition where the feed rate exceeds the cell growth rate, resulting in washout of cells from the reactor. Section 7 discusses a simple model of a CSTR reactor (called a chemostat) using empirical kinetics. 

Bioreactions are exothermic. The net heat released during growth represents the sum of the many enzymatic reactions involved. Reasonably, this measure depends on both the biomass concentration and the metabolic state of the cells. Its general use in biotechnology has been reviewed by von Stockar and Marison [415]. A theoretical thermodynamic derivation for aerobic growth gives a prediction for the heat yield coefficient YQ/0 of 460 kj (mol 02) 1 and it was ex-... 

As will be shown later, some ceramic membranes have been used to immobilize some biocatalysts such as enzymes for increasing the reaction rate of bioreactions. Membrane pores when mostly used as catalyst carriers are advantageous over the conventional catalyst carriers in the pellet or bead form in having less mass transfer resistance and more efficient contact of the reactant(s) with the catalysL In a strict sense, the membrane material when used in this mode is not a membrane which is defined as a permselective medium. 

In this chapter we discuss four topics the pseudo-steady-state hypothesis, polymerization, enzymes, and bioreactors. The pseudo-steady-state hypothesis (PSSH) plays an important role in developing nonelementary rate laws. Consequently, we will first discuss the fundamentals of the PSSH, followed by its use of polymerization reactions and enzymatic reactions. Because enzymes are involved in all living organisms, we close the chapter with a discussion on bioreactions and reactors. 

In the last 15 years, the use of monoliths has been extended to include applications for performing multiphase reactions. Particular interest has been focused on the application of monolith reactors in three-phase catalytic reactions, such as hydrogenations, oxidations, and bioreactions. There is also growing interest in the chemical industries to find new applications for monoliths as catalyst support in three-phase catalytic reactions. 

Fig. 4 General approach of the bioreactive MALDI mass spectrometer probe tips. Goid piated probe tips are activated through the covalent attachment of enzymes (the general terminology of Au/enzyme is used to indicate the nature of the activated surfaces). The probe tips are then used for protein characterization by direct appiication of the analyte and time given for digestion. The digestions are stopped with the addition of a MALDI matrix, the reaction product-matrix mixture aiiowed to dry, and the probe tips are inserted into the mass spectrometer for MALDI-TOF analysis.

Biochemical pathway synthesis is the construction of consistent sets of enzyme-catalyzed bioreactions meeting certain specifications. One seeks to construct pathways which produce certain target bioproducts, under partial constraints on the available substrates (reactants), allowed byproducts, desired yield, productivity, etc. The pathway must include all reactions needed to convert initial substrates supplied to the bioprocess into final... 

The set M contains a one-step pathway for each individual enzymatic reaction available in the database, which has been compiled from known biochemistry. The set N contains the metabolites present in the enzymatic reactions of the set M. Naturally, these include the substrates (raw materials) that can be used, the desired products, and a large number of other compounds that occur in the bioreactions but will not serve as raw materials or desired products. This last set of compounds will carry the restrictions of excluded reactants and excluded products. 

Mechanism and kinetics in biochemical systems describe the cellular reactions that occur in living cells. Biochemical reactions involve two or three phases. For example, aerobic fermentation involves gas (air), liquid (water and dissolved nutrients), and solid (cells), as described in the Biocatalysis subsection above. Bioreactions convert feeds called substrates into more cells or biomass (cell growth), proteins, and metabolic products. Any of these can be the desired product in a commercial fermentation. For instance, methane is converted to biomass in a commercial process to supply fish meal to the fish farming industry. Ethanol, a metabolic product used in transportation fuels, is obtained by fermentation of corn-based or sugar-cane-based sugars. There is a substantial effort to develop genetically modified biocatalysts that produce a desired metabolite at high yield. 

Unstructured models view the cell as a single component interacting with the fermentation medium, and each bioreaction is considered to be a global reaction, with a corresponding empirical rate expression. 

Structured models include information on individual reactions or groups of reactions occurring in the cell, and cell components such as DNA, RNA, and proteins are included in addition to the primary metabolites and substrates (see, e.g., the active cell model of Nielsen and Villadsen, Bioreaction Engineering Principles, 2d ed., Kluwer Academic/Plenum Press, 2003). 

The products of bioreactions can be reduced or oxidized, and all feasible pathways have to be redox neutral. There are several cofactors that transfer redox power in a pathway or between pathways, each equivalent to the reducing power of a molecule of H2, e.g., nicotinamide adenine dinucleotide (NADH), and these have to be included in the stoichiometric balances as H equivalents through redox balancing. For instance, for the reaction of glucose to glycerol (CHs/30), j NADH equivalent is consumed ... 

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