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Enzymatic reactions characteristics

The response characteristics of enzyme electrodes depend on many variables, and an understanding of the theoretical basis of their function would help to improve their performance. Enzymatic reactions involving a single substrate can be formulated in a general way as... [Pg.174]

The characteristic times on which catalytic events occur vary more or less in parallel with the different length scales discussed above. The activation and breaking of a chemical bond inside a molecule occurs in the picosecond regime, completion of an entire reaction cycle from complexation between catalyst and reactants through separation from the product may take anywhere between microseconds for the fastest enzymatic reactions to minutes for complicated reactions on surfaces. On the mesoscopic level, diffusion in and outside pores, and through shaped catalyst particles may take between seconds and minutes, and the residence times of molecules inside entire reactors may be from seconds to, effectively, infinity if the reactants end up in unwanted byproducts such as coke, which stay on the catalyst. [Pg.18]

Since many years, pectolytic enzymes have been widely used in industrial beverage processing to improve either the quality and the yields in fruit juice extraction or the characteristics of the final product [1,2]. To this purpose, complex enzymatic mixtures, containing several pectolytic enzymes and often also cellulose, hemicellulose and ligninolytic activities, are usually employed in the free form. The interactions among enzymes, substrates and other components of fruit juice make the system very difficult to be investigated and only few publications are devoted to the study of enzymatic pools [3-5], An effective alternative way to carry out the depectinisation process is represented by the use of immobilized enzymes. This approach allows for a facile and efficient enzymatic reaction control to be achieved. In fact, it is possible to avoid or at least to reduce the level of extraneous substances originating from the raw pectinases in the final product. In addition, continuous processes can be set up. [Pg.971]

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... [Pg.7]

Another possibility is to immobilise enzymes either on the sensor element itself or in the vicinity of the sensing element. The operation principle is in most cases a semi-continuous spectral difference measurement in combination with a kinetic data evaluation. A sample containing the analyte of interest is recorded by the sensor immediately after contact with the sample and again after a certain time. Provided that no other changes in the composition of the sample occur over time, the spectral differences between the two measurements are characteristic for the analyte (and the metabolic products of the enzymatic reaction) and can quantitatively evaluated. Provided that suitable enzymes are available that can be immobilised, this may be a viable option to build a sensor, in particular when the enzymatic reaction can not (easily) be monitored otherwise, e.g. by production or consumption of oxygen or a change of pH. In any case, the specific properties and stumbling blocks related to enzymatic systems must be observed (see chapter 16). [Pg.141]

In order to understand the mechanisms of the enzymatic process and also predict the reaction characteristics, one needs to understand the kinetics of the reaction. The important factor that effects the enzyme reaction is the availability and concentration of the substrates. An important model that gives a mathematical relationship is the Michaelis-Menten and Hill equation. The equation is denoted as... [Pg.82]

In MET, a low-molecular-weight, redox-active species, referred to as a mediator, is introduced to shuttle electrons between the enzyme active site and the electrode.In this case, the enzyme catalyzes the oxidation or reduction of the redox mediator. The reverse transformation (regeneration) of the mediator occurs on the electrode surface. The major characteristics of mediator-assisted electron transfer are that (i) the mediator acts as a cosubstrate for the enzymatic reaction and (ii) the electrochemical transformation of the mediator on the electrode has to be reversible. In these systems, the catalytic process involves enzymatic transformations of both the first substrate (fuel or oxidant) and the second substrate (mediator). The mediator is regenerated at the electrode surface, preferably at low overvoltage. The enzymatic reaction and the electrode reaction can be considered as separate yet coupled. [Pg.633]

All the proteins described in this chapter and that catalyze very different reactions involving 02 exhibit the following common characteristics (1) the presence of an iron porphyrin cofactor, (2) an axial iron ligand coming from the protein that is in most cases a histidine imidazole (except for cytochrome P-450), and (3) the intermediate formation of high-valent iron-oxo species, formally equivalent to Fe(IV)=0 or Fe(V)=0, which are key intermediates in the enzymatic reactions. The different reactions performed by these iron-oxo species and the different uses of 02 by the hemoproteins are due to the very different environments of the heme in these hemoproteins and to a more or less rapid electron transfer. As a function of its environment, the iron-oxo species may either be reduced into H20 if electrons are easily transferred to the heme or oxidize a substrate or a protein amino acid that could be present in close proximity in the active site (Figure 14). [Pg.347]

Further advantages of biocatalysis over chemical catalysis include shorter synthesis routes and milder reaction conditions. Enzymatic reactions are not confined to in vivo systems - many enzymes are also available as isolated compounds which catalyze reactions in water and even in organic solvents [28]. Despite these advantages, the activity and stability of most wild-type enzymes do not meet the demands of industrial processes. Fortunately, modern protein engineering methods can be used to change enzyme properties and optimize desired characteristics. In Chapter 5 we will outline these optimization methods, including site-directed mutagenesis and directed evolution. [Pg.17]

Pteridines are widely distributed in nature and function as pigments, biological markers, and cofactors of enzymatic reactions. The oxidized heteroaromatic forms show a characteristic fluorescence which allows easy detection even in low concentrations. However, the more active 5,6,7,8-tetrahydro derivatives are nonfluorescing and oxidizable and create experimental problems in handling this type of compound. So far, all naturally occurring pteridines have turned out to be derivatives of pterin (2) and lumazine (3) which are modified by different substituents and functional groups in the 6- and/or 7-position. [Pg.729]


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See also in sourсe #XX -- [ Pg.71 ]




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