Biocatalyst enzymes

True Separation of the biocatalyst (enzyme or cells) from the product stream is not required and relatively few other substances are present. 

Despite the still growing number of available methods for the preparation of enantiopure compounds by the use ofasymmetric catalysis, kinetic resolution (KR) is still the most employed method in the industry [4], and in most cases biocatalysts (enzymes) are used. 

Special reactors are required to conduct biochemical reactions for the transformation and production of chemical and biological substances involving the use of biocatalysts (enzymes, immobilised enzymes, microorganisms, plant and animal cells). These bioreactors have to be designed so that the enzymes or living organisms can be used under defined, optimal conditions. The bioreactors which are mainly used on laboratory scale and industrially are roller bottles, shake flasks, stirred tanks and bubble columns (see Table 1). 

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

Biocatalysts Enzymes Microbial, plant, animal cells Sub-cellular organelles... 

Ballesteros, A., van Beynum, G., Bomd, O. and Buchholz, K., Guidelines for the characterization of immobilized biocatalysts. Enzyme Microb. TechnoL, 1983, 5, 304-307. 

Lastly, cost is a crucial issue in the commercialization of fuel cells, particularly as performance and lifetimes have improved to the threshold of practicability. The major costs associated with these systems are materials-related, with separator and catalyst materials at the top of the list. It is envisioned that the cost of separator materials will decrease with increased production and competition and as alternative materials are perfected. However, the cost of conventional noble metal catalysts, particularly platinum, is expected only to increase with increased production and demand. Therefore, the cost issue could perhaps be addressed by employing alternative catalysts, including biocatalysts. Enzymes are de-... 

Bioelectrocatalysis involves the coupling of redox enzymes with electrochemical reactions [44]. Thus, oxidizing enzymes can be incorporated into redox systems applied in bioreactors, biosensors and biofuel cells. While biosensors and enzyme electrodes are not synthetic systems, they are, essentially, biocatalytic in nature (Scheme 3.5) and are therefore worthy of mention here. Oxidases are frequently used as the biological agent in biosensors, in combinations designed to detect specific target molecules. Enzyme electrodes are possibly one of the more common applications of oxidase biocatalysts. Enzymes such as glucose oxidase or cholesterol oxidase can be combined with a peroxidase such as horseradish peroxidase. 

The commercial availability of enzymes or whole cell biocatalysts for a desired biotransformation is freqnently a limiting factor for commercial application of biocatalysts. Enzymes that are cheaply available are typically used in detergents, processing of food, feed and textiles, as well as in waste management applications. Most of these are hydrolytic enzymes, bnt also isomerases (e.g. glucose isomerase) and oxidorednctases are used on indnstrial scale (Table 5.1). 

Cheetam, P.S. (1987) Screening for novel biocatalysts. Enzyme Microb. Technol., 9, 194-213. 

The performance of immobilized biocatalyst (enzyme) reactors is influenced by enzyme inactivation during operation, mainly due to thermal denaturation, desorption of the biocatalyst from the solid support, disintegration or solubilisation of the support and microbial attack. 

Section 3.3 is devoted to biochemical enzyme systems in which the biocatalyst enzyme does not change during the progressing reaction. Biological systems whose biocatalysts change with time are presented later in the book. 

The third method used in the resolution of racemates is the kinetic resolution. The success of this method is depending on the fact that the two enantiomers react at different rates with a chiral entity. The chiral entity should be present in catalytic amounts it may be a biocatalyst (enzyme or a microorganism) or a chemocatalyst (chiral acid or base or even a chiral metal complex). Kinetic resolution of racemic compounds is by far the most common transformation catalyzed by lipases, in which, the enzyme discriminate between the two enantiomers of racemic mixture, so that one enantiomer is readily transferred to the product faster than the other.1"18 (cf. fig 3)... 

R Bahulekar, NR Ayangar, S Ponrathnam. Polyethyleneimine in immobilization of biocatalysts. Enzyme Microb Technol 13 858-868, 1991. 

Figure 14.2 Membrane bioreactor with immobilized biocatalysts (enzyme or micro-organism).

The catalytic action of biocatalysts (enzymes, abzymes, antibodies, cells) is extremely efficient and selective compared to conventional chemical catalysts. They demonstrate higher reaction rates, milder reaction conditions and greater stereospecificity. Most of these properties come from the high molecular flexibility biocatalysts exhibit. On the other hand, this is also the origin of their major limit that holds back their application at the large scale, that is, the molecular stability, and then the catalyst lifetime. 

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

Aoun S, Chebli C, Baboulene M (1998) Noncovalent immobilization of chloroperoxidase onto talc catalytic properties of a new biocatalyst. Enzyme Microb Technol 23 380-385... 

Gekas VC (1986) Artificial membranes as carriers for the immobilisation of biocatalysts. Enzyme Microb Technol 8 450 -61... 

This section deals with carbide compounds and hexacarbonyl. Organotungsten compoimds are treated separately under in Section 10.5. For information about tungsten-containing biocatalysts (enzymes), see Section 14.2. 

Multiple authors (1983) The Working Party on Immobilized Biocatalysts Enzyme Microb Technol 5 304... 

Buchholz, K., Kasche, V. and Bornscheuer, U.T. (2005) Biocatalysts Enzyme Technology, Wiley-VCH Verlag, Weinheim. 

During the last ten years enzyme technology has moved mainly towards the development of new immobilization techniques and the improvement of those already existing. In turn, the attention of applied research has been focused on the engineering of systems based on immobilized biocatalysts. Enzymes involved in this development were enzymes catalyzing simple reactions that normally require no cofactors. A number of drawbacks affected the use of immobilized enzymatic preparations. An often dramatic reduction of initial enzyme activity due to the binding process, and the existence of diffusional resistances limits this approach with low activity enzymes, with macromolecular substrates and in general with enzymes whose cata-... 

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