Catalysis nonenzymic

A. Comparison Between Enzymic and Nonenzymic Asymmetric Catalysis.88... 

Many attempts have been made during the past 30 years to imitate enzymes. Studies in the preparation of artificial enzymes (6) and model enzymes abound. While it is undoubtedly true that most enzymes can achieve the transformation of an achiral substrate to a chiral one more rapidly and with higher specificity than can be achieved using nonenzymic catalysts, the many limitations to which enzymic catalysis is subject should be properly evaluated. These are as follows ... 

In his consideration of the nature of catalysis Berzelius had assumed the catalyst played no part in the actual reaction. Studies on nonenzyme catalysis, and especially the roles of finely divided metals, such as platinum, seemed to substantiate this—a view apparently consistent with the concept of the adsorption isotherm introduced by Langmuir (1916). 

P Dunnill, A Wisemann, N Blakebrough, Enzymic and Nonenzymic Catalysis, Ellis Horwood, Chichester, UK, 1980... 

At the present time, commercial isomerization processes based on enzymic catalysis are predominant, so only brief mention will be made of some of the nonenzymic processes that have been considered for commercialization in the past. Probably the major reasons for the current commercial use of enzymic rather than nonenzymic systems are that the nonenzymic systems so far developed result in products having one or more of the following defects too much ash, color, acid, off-flavor, a content of D-mannose or D-psicose, and high ratios of D-glucose to D-fructose. Probably, further advances in our understanding of the isomerization reaction and the mechanisms of the catalysis will lead to more efficient, nonenzymic processes that could replace the enzymic-isomerization systems now used commercially. 

Rhee, K. S. (1988). Enzymic and nonenzymic catalysis of lipid oxidation in muscle foods. Food Technol. (June), pp. 127-132. 

J. B. Jones, in P. Dunnill, A. Wiseman and N. Blakeborough (Eds), Enzymes and Nonenzymic Catalysis, Horwood/Wiley, Chichester/New York,1980. 

A mechanistic update on enzymic and nonenzymic catalysis of decarboxylation. Chem. Rev. 87, 863—876. 

It is interesting that, in contrast to nearly all nucleotidyltransferase reactions, most nonenzymic reactions that are used for synthesizing phosphoanhydrides and phosphodiesters are best carried out in nucleophilic solvents or in reaction media containing nucleophilic catalysts. In one case stereochemical evidence has been advanced as evidence for nucleophilic catalysis by pyridine used as the solvent in the synthesis of a thiophosphoanhydride (60). While many of these nonen-... 

In the case of Fig. IB the enzyme has equal binding affinity for S, TS, and P as indicated by the equal free energy differences between the bound and free forms of each species. The energy barrier for conversion of S to TS is the same as that for conversion of E-S to E-TS, and equals the rate constant, k , for the nonenzymic process. This situation as well does not produce catalysis since the net rate for conversion of S to P is not increased by addition of E as shown by... 

The effect of hydroxylamine treatment shows that the bond linking the synthetase to the proposed carrier molecules is susceptible to nonenzymic base catalysis (Fig. 5A). Compared to the result obtained with in vitro conjugates however, there were again qualitative differences. Whereas hydroxylamine removed the majority of the poly(ADP-ribose) residues from... 

This is illustrated in Fig. 6 for the reaction of succinyl CoA acetoacetate coenzyme A transferase with its specific substrate succinyl CoA, compared with the corresponding nonenzymic reaction through the transition state [34]. Both reactions involve thiol interchange to form a new thiol ester of coenzyme A. Catalysis by the enzyme increases the reaction rate by a factor of 5 x 10, which corresponds to a stabilization of the transition state by 18.7 kcal moP Most of this stabilization arises from the utilization of noncovalent binding interactions between the coenzyme A moiety of the substrate and the enzyme. A short chain thiol ester of succinate with methyl mercaptopropionate has the same chemical properties as succinyl CoA but lacks the specific binding groups of the normal substrate this compound reacts with the enzyme some 10 more slowly than succinyl CoA, only an order of... 

Kluger, R., and Tittmann, K., 2008. Thiamin diphosphate catalysis enzymic and nonenzymic covalent intermediates. Chemical Reviews. 108 1797-1833. 

In the first place, in an enzymic catalysis, the ratio of the reaction velocities leading to the two antipodal forms of the new dissymmetric molecule is very high, often of the order of 1000 1 or more. In nonenzymic asymmetric catalysis, on the other hand—for example, in the work of Bredig and Fiske— the ratio is very much less, generally of the order of only 2 1. Correspondingly the equilibrium constant is greater, and the optical stability of the product higher. 

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