Enzymes combination with chemical catalyst

Subsequently, selected apphcations of biocatalysts will be examined, used as either isolated enzymes or enzymes that operate in immobilized or permeabihzed cells. Synthesis routes in which one or all of the steps are biocatalytic have advanced dramatically in recent years. Increasingly, biocatalysts are combined with chemical catalysts or utilized in a network of reactions in a whole cell. It can be pointed out, that biocatalysts do not operate by different scientific principles from usual catalysts. All enzyme actions can be explained by rational chemical and physical principles. However, enzymes can create imusual and superior reaction conditions such as extremely low p/fa values or a high positive potential for a redox metal ion. 

Certain constituents when added to the reaction mixture, slow down the rate of reaction. This phenomena is called inhibition and constituent called inhibitor. Such an effect is similar to the negative catalysis. But the constituent usually undergoes chemical change, inhibition is the preferred term. Inhibition may occur in chain reactions, enzyme catalysed reactions, surface reactions or many reversible or irreversible reactions. A trace amount of an inhibitor may cause a marked decrease in the rate of reaction. The inhibitor sometimes combines with a catalyst and prevents it from catalyzing the reaction. 

The superfamily of P450 cytochrome enzymes is one of the most sophisticated catalysts of drug biotransformation reactions. It represents up to 25% of the total microsomal proteins, and over 50 cytochromes P450 are expressed by human beings. Cytochromes P450 catalyze a ivide variety of oxidative and reductive reactions, and react with chemically diverse substrates. Despite the large amount of information on the functional role of these enzymes combined with the knowledge of their three-dimensional structure, elucidation of cytochrome inhibition, induction, isoform selectivity, rate and position of metabolism all still remain incomplete [6]. 

Biocatalytic membrane reactors combine selective mass transport with chemical reactions and the selective removal of products from the reaction site increases the conversion of product-inhibited or thermodynamically unfavorable reactions. Membrane reactors using biological catalysts can be used in production, processing and treatment operations. Recent advances towards environmentally friendly technologies make these membrane reactors pai ticulaiiy attractive because they do not require additives, are able to function at moderate temperatures and pressrue, and reduce the formation of by-products. The catalytic action of enzymes is extremely efficient and selective compared with chemical catalysts. Uiese enzymes demonstrate higher reaction rates, milder reaction conditions and greater stereospecificity. 

In vitro synthesis of polyesters using isolated enzymes as catalyst via non-biosynthetic pathways is reviewed. In most cases, lipase was used as catalyst and various monomer combinations, typically oxyacids or their esters, dicarboxylic acids or their derivatives/glycols, and lactones, afforded the polyesters. The enzymatic polymerization often proceeded under mild reaction conditions in comparison with chemical processes. By utilizing characteristic properties of lipases, regio- and enantioselective polymerizations proceeded to give functional polymers, most of which are difficult to synthesize by conventional methodologies. 

In the absence of an enzyme, the reaction rate v is proportional to the concentration of substance A (top). The constant k is the rate constant of the uncatalyzed reaction. Like all catalysts, the enzyme E (total concentration [E]t) creates a new reaction pathway, initially, A is bound to E (partial reaction 1, left), if this reaction is in chemical equilibrium, then with the help of the law of mass action—and taking into account the fact that [E]t = [E] + [EA]—one can express the concentration [EA] of the enzyme-substrate complex as a function of [A] (left). The Michaelis constant lknow that kcat > k—in other words, enzyme-bound substrate reacts to B much faster than A alone (partial reaction 2, right), kcat. the enzyme s turnover number, corresponds to the number of substrate molecules converted by one enzyme molecule per second. Like the conversion A B, the formation of B from EA is a first-order reaction—i. e., V = k [EA] applies. When this equation is combined with the expression already derived for EA, the result is the Michaelis-Menten equation. 

This review covers the recent literature (2002-2007) in which either multienzyme systems or, alternatively, true chemoenzymatic processes (i.e. those in which an enzyme is combined with a chemical catalysts/reagent in a key step) are employed for the synthesis of chiral amino acids and amines. Not included are those papers in which the term chemoenzymatic refers to the fact that the substrate for the biotransformation has simply been prepared via chemical synthesis. 

Enzymes are catalysts, substances that play a role in specific chemical reactions, but which are not changed by the reaction. Thus, as a result of the enzyme-substrate reaction, the enzyme stays the same but the substrate is changed. Its shape can be different, or it can be broken down into products or combined with another molecule to make something new. 

Resolution of cheap racemic mixtures with enzymes is a common route to enantiomerically pure chemicals on an industrial scale. However, the yield with a classical resolution is limited to 50%. An in situ racemization of the undesired enantiomer, combined with the enzymatic kinetic resolution, gives rise to a dynamic kinetic resolution (DKR) that should in principle lead to a 100% yield in the desired isomer. In spite of several Ru and Pd homogeneous systems successfully combined with enzymes and successfully applied on industrial scale in DKR [71, 72], few metal-based heterogeneous catalysts active for alcohol racemization have been reported [19, 73, 74]. 

Enzymes are proteins employed by Mother Nature to catalyze the chemical reactions necessary to sustain life in plants and animals. As catalysts, enzymes may influence the rates and/or the directions of chemical reactions involving an enormous range of substrates (reactants). Enzymes function by combining with substrates to form enzyme-substrate complexes (reaction intermediates) that subsequently react further to yield products while regenerating the free enzyme. 

In chemical synthesis, particularly for pharmaceutical applications, they give highly stereo- and regioselective syntheses that often are impossible using dassical catalysts. Enzymatic reactions combined with enantio-resolution can generate en-antiopure materials with high yields [150]. Enzymes can be extremely enantioselec-... 

Biocatalysis is a key route to both natural and non-natural polysaccharide structures. Research in this area is particularly rich and generally involves at least one of the following three synthetic approaches 1) isolated enzyme, 2) whole-cell, and 3) some combination of chemical and enzymatic catalysts (i.e. chemoenzymatic methods) (87-90). Two elegant examples that used cell-fi-ee enzymatic catalysts were described by Makino and Kobayashi (25) and van der Vlist and Loos (27). Indeed, for many years, Kobayashi has pioneered the use of glycosidic hydrolases as catalysts for polymerizations to prepare polysaccharides (88,91). In their paper, Makino and Kobayashi (25) made new monomers and synthesized unnatural hybrid polysaccharides with regio- and stereochemical-control. Van der Vlist and Loos (27) made use of tandem reactions catalyzed by two different enzymes in order to prepare branched amylose. One enzyme catalyzed the synthesis of linear structures (amylose) where the second enzyme introduced branches. In this way, artificial starch can be prepared with controlled quantities of branched regions. 

Peptide catalysts, peptides that catalyze chemical reactions such as aldol, retro-aldol, and Michael reactions. In contrast to enzymes or catalytic antibodies ( abzymes), small peptides often display limited catalytic activity and substrate specificity. Combinatorial methods combined with reaction-based or catalysis-based high-throughput selection approaches are suited for catalyst optimization [E. Tanaka, Chem. Record 2005, 5, 276]. 

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