Biological catalysts, selectivity

In a sense, speciation will prompt utilization of more specific or selective reactions , compared to typical chemical transformations. Thus, biological catalysts find utility here... 

In metal-containing biological catalysts it is the protein matrix surrounding the metal centres that provides the unique environment for the Fe and Ni atoms which allows hydrogenases to function properly, selectively and effectively. Therefore, a major goal of hydrogenase basic research is to understand the protein-metal interaction. 

In order to circumvent both shortcomings, achieving adequate catalytic selectivity with a non-biological catalyst entails isolating both the substrate and the catalyst. On the other hand, non-enzyme catalysts are usually much... 

By far the most efficient catalysts are enzymes, which regulate most biological reactions. Biological catalysts are without question the most important catalysts (to us) because without them life would be impossible. Enzymes are proteins that may be either isolated molecules in solution (homogeneous) or molecules bound to large macromolecules or to a cell wall (heterogeneous). We have not yet learned how to create catalysts with nearly the efficiency and selectivity of nature s enzyme catalysts. We will consider biological reactors at the end of this chapter as the example of the most efficient chemical reactor possible. 

Enzymes are biological catalysts. Without their presence in a cell, most biochemical reactions would not proceed at the required rate. The physicochemical and biological properties of enzymes have been investigated since the early 1800s. The unrelenting interest in enzymes is due to several factors— their dynamic and essential role in the cell, their extraordinary catalytic power, and their selectivity. Two of these dynamic characteristics will be evaluated in this experiment, namely a kinetic description of enzyme activity and molecular selectivity. 

Biocatalysts often feature much better selectivity than non-biological catalysts, so they are developed for use because of their selectivity, be it enantioselectivity, chemo- or regioselectivity. 

Biological processes are highly sophisticated but are not driven by unknown and mysterious powers. They occur due to complicated combinations of known chemical reactions. These chemical reactions are conducted by enzymes (biological catalysts), which encourage desirable reactions to occur with high selectivity and efficiency. The application of naturally occurring enzymes to... 

In addition to chemicals, biological catalysts such as enzymes can be used to catalyze reactions in SC CO2. Since the first attempt to operate reactions in supercritical fluids published by Randolph et al. [34], various type of enzymes were studied lipase, oxidase, decarboxylase, dehydrogenase, proteinase, etc. [33,35-37]. The effect of different parameters was extensively reported by Ballesteros et al. [35]. Enzyme activity and stability in supercritical conditions as well as the benefits of using supercritical fluids for enzymatic reactions (improved reaction rates, control of selectivity, etc.) have been demonstrated [36]. 

Membrane reactors using biological catalysts can be used in enantioselective processes. Methodologies for the preparation of emulsions (sub-micron) of oil in water have been developed and such emulsions have been used for kinetic resolutions in heterogeneous reactions catalyzed by enantioselective enzyme (Figure 43.4). A catalytic reactor containing membrane immobilized lipase has been realized. In this reactor, the substrate has been fed as emulsion [18]. The distribution of the water organic interface at the level of the immobUized enzyme has remarkably improved the property of transport, kinetic, and selectivity of the immobilized biocatalyst. 

Homogeneons catalysts are present in the same phase as the reaction being catalyzed whereas heterogeneous catalysts are present in a different phase. Enzymes are biological catalysts that bind substrates with exquisite selectivity, position reactants at optimal locations and stabilize transition states, all of which leads to lower activation barriers than those in the uncatalyzed reaction. 

In most instances, bioselective electrodes are prepared by immobilizing biocatalysts, e.g., enzymes, intact cells, or organelles, at the surftice of conventional ISE devices. Selectivity of the final bioprobe is dependent on the selectivity of the biological catalyst and on the innate selectivity of the ISE transducer used to construct the electrode. Thus, in order to avoid ionic interferences, gas-sensing devices are often used to fiibricate such electrodes. 

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