Enzyme , as catalysts

Enzymes are the body s catalysts. Without them, the cell s chemical reactions would be too slow to be useful, and many would not occur at all. A catalyst is an agent which speeds up a chemical reaction without being changed itself. 

An example of a catalyst used frequently in the laboratory is palladium on activated charcoal (Fig. 4.1) 

Note that the above reaction is shown as an equilibrium. It is therefore more correct to describe a catalyst as an agent which can speed up the approach to equilibrium. In an equilibrium reaction, a catalyst will speed up the reverse reaction just as efficiently as the forward reaction. Consequently, the final equilibrium concentrations of the starting materials and products are unaffected by a catalyst. 

A catalyst acts to reduce the activation energy by helping to stabilize the transition state. The energy of the starting material and products are unaffected, and therefore the equilibrium ratio of starting material to product is unaffected. 

We can relate energy, and the rate and equilibrium constants with the following equations  

Enzymes function as biocatalysts and, as such, are involved in all metabolic reactions. Characteristics of enzymes are their high efficiency, high specificity, extreme stereoselectivity, and their ability to be regulated. Analogously to chemical catalysts, enzymes do not alter the equilibrium of a reaction, but only accelerate the establishment of that equilibrium. 

For the tight binding of the transition state, the binding surface of the enzyme must be complementary to the structure of the transition state, so that optimal interactions between the enzyme and the transition state are possible. This demand implies that 

The body is the most efficient set of complementary reactions known, mainly due to the effects of special catalysts called enzymes. Most reactions occurring in the body - and indeed in all living cells - are catalysed by enz3Tnes. Enzymes are very specific catalysts each enzyme affects only one reaction and a single cell may contain many different enzymes. 

When glucose releases energy in the muscles, a small amount of hydrogen peroxide is sometimes formed as a by-product, which can damage the body. The blood contains an enzyme called catalase which effectively speeds up the decomposition of the hydrogen peroxide into water and oxygen, rendering it harmless  

Catalase is a more effective catalyst on this reaction than manganese(I V) oxide. This can be shown by adding a piece of liver or liquidised celery (which both contain catalase) to a solution of hydrogen peroxide and observing that oxygen gas is evolved instantly and dramatically. 

People have made use of enzymes in reactions for thousands of years, for instance in fermentation and cheese-making processes, but it is only recently that their identity and the importance of their role has been recognised and understood. 

Enzymes are long-chain proteins that have a complicated structure and normally have only one particular site at which a reaction can take place. X-Ray diffraction results have shown that this region, called the active site, has a definite shape for each enzyme, and therefore only molecules with a similar complementary shape will fit into it - rather like keys in a lock. For example, only hydrogen peroxide molecules will fit the active site of catalase. Those molecules that do fit the active site are called substrate molecules. Thus, enzymes are much more specific catalysts than inorganic substances such as manganese(IV) oxide. 

Because of their remarkable catalytic activity, a given number of enzyme molecules convert an enormous number of substrate molecules to products within a short time. Therefore, the appearance of increased amounts of enzymes in the blood stream is easily detected, although the amount of enzyme protein released from damaged cells is small compared with the total level of nonenzymatic proteins in bloods Thus a particular enzyme is recognized by its characteristic effect on a given chemical reaction despite the presence of a vast excess of other proteins. 

Like other catalysts, an enzyme changes only the rate at which equilibrium is established between reactants and products it does not alter the equilibrium constant of the reaction. In a reaction in which only one set of products is-chemically possible, the catalyst cannot effect any change in the nature of the products. But when several different possible pathways exist, the enzyme directs the reaction along only one pathway. 

With the exception of enzymes such as proteases, nucleases, and amylases, which act on macromolecular substrates, enzyme molecules are considerably larger than the molecules of their substrates. Consideration of the structure of an enzyme s active site and its relationship to the structures of the enzyme s substrate(s) in its ground and transition states is necessary to understand the rate enhancement and specificity of the chemical reactions performed by the enzyme, 

The active site of an enzyme will vary between enzymes but 

The active site of an enzyme is relatively small compared with the total volume of the enzyme molecule because its structure may involve less than 5% of the total amino acids in the molecule. 

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High yields of optically active cyanohydrins have been prepared from hydrogen cyanide and carbonyl compounds using an enzyme as catalyst. Reduction of these optically active cyanohydrins with lithium aluminum hydride in ether affords the corresponding substituted, optically active ethanolamine (5) (see Alkanolamines). 

D. Hoppe, Synthesis of Enantiomerically Pure Unnutural Compounds via Nonhiomimetic Ho-moaldol Reactions in Enzymes as Catalysts in Organic Synthesis, M. P. Schneider, Ed., pp 177-198, D. Reidcl, Dordrecht 1986. 

Enzymes are generally classified into six groups. Table 1 shows typical polymers produced with catalysis by respective enzymes. The target macromolecules for the enzymatic polymerization have been polysaccharides, poly(amino acid)s, polyesters, polycarbonates, phenolic polymers, poly(aniline)s, vinyl polymers, etc. In the standpoint of potential industrial applications, this chapter deals with recent topics on enzymatic synthesis of polyesters and phenolic polymers by using enzymes as catalyst. 

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. 

Fig. 1. Typical routes of polyester production using an isolated enzyme as catalyst...

In vitro polyester syntheses using an isolated enzyme as catalyst via non-bio-synthetic pathways is reviewed. These enzymatic routes for production of biodegradable polyesters possess several advances in comparison with fermentation and chemical processes ... 

There is another approach that is increasingly part of synthesis the use of enzymes as catalysts. This approach is strengthened by the new ability of chemists and molecular biologists to modify enzymes and change their properties. There is also interest in the use of artificial enzymes for this purpose, either those that are enzyme-like but are not proteins, or those that are proteins but based on antibodies. Catalytic antibodies and nonprotein enzyme mimics have shown some of the attractive features of enzymes in processes for which natural enzymes are not suitable. 

Green Oxidation of Alcohols using Biominetic Cu Complexes and Cu Enzymes as Catalysts Isabel W.C.E Arends, Patrick Gamez and Roger A. Sheldon... 

Organic surface treatments, on titanium dioxide pigments, 25 26 Organic syntheses, 13 412—113 acid and base catalysts for, 24 182 advantages of fermentation over, 11 6 enzymes as catalysts for, 10 307 high pressure in, 13 438—139 ozone use in, 17 810-811 silylation in, 22 695-696 Organic tellurium compounds, 24 414-415, 422... 

Green Oxidation of Alcohols using Biominetic Cu Complexes and Cu Enzymes as Catalysts... 

Berti G (1986) In Schneider MP (ed) Enzymes as catalysts in organic synthesis. Reidel, Dordrecht, NATO ASl Series C 178 349... 

A major advantage of enzymes as catalysts is that they are capable of inducing very high degrees of enantioselectivity and, consequently, they are particularly useful in the synthesis of enanhomerically pure compounds. In cases where the enanh-oselectivity is less than optimum it can generally be improved using protein engineering techniques such as in vitro evolution [10]. Hence, in this chapter we shall be mainly concerned with the application of enzymatic cascade processes to the... 

Directed Evolution of Enantioselective Enzymes as Catalysts for Organic Synthesis... 

The actions of enzymes as catalysts depend on their three-dimensional conformation, or how the protein is folded into a three-dimensional object. A protein is said to be denatured if its three-dimensional conformation is altered, such as by heat or mechanical stirring, and is no longer biochemically active as a catalyst. 

Another of the most important properties of enzymes as catalysts is that they are not changed during the reactions they catalyze, which allows a single enzyme to catalyze a reaction many times. 

Yamada, H. and Shimizu, S. (1985) Microbial Enzymes as catalysts for the synthesis of biologically useful compounds. In Biocatalysts in Organic Synthesis, edited by J.Tramper, H.C.vanderPlas andP.Linko, pp. 19-40. Amsterdam Elsevier. 

M. Muller, G. A. Sprenger, Thiamine-dependent enzymes as catalysts of C-C-bonding reactions The role of orphan enzymes, in Thiamine Catalytic Mechanisms and Role in Normal and Disease States, Marcel Dekker, New York, 2004, pp. 77-91. 

Nature uses enzymes as catalysts, yet the number of po lymerio catalysts in me in preparative chemistry is small. Surely if man could prepare catalysts with the high speed ficity and activity of most enzymes, they would replace the less specific, less active synthetic catalysts in use today. 

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