Nonenzymic

Porschke, D., Eigen, M. Cooperative nonenzymic base recognition. HI. Kinetics of the helix-coil transition of the oligoribouridylic oligoriboadenylic acid system and of oligoriboadenylic acid alone at acidic pH. J. Mol. Biol. 62 (1971) 361-381... 

The new Uquid laundry detergents, with no phosphates, have developed a use for alkan olamines. In nonenzyme formulations, they contribute alkalinity, pH control, and enhanced product stabiUty. In enzyme products, alkan olamines contribute to the stabiUty of the enzyme in water solutions (107). 

Coffer, M.T., Shaw, C.F., Hermann, A.L., Mirabelli, C.K. and Crooke, S.T. (1987) Thiol competition for EtjPAuS-albumin a nonenzymic mechanism for EtjPO formation. Journal of Inorganic Biochemistry, 30, 177-187. 

The initial oxidation of the flavanol components of fresh leaf to quinone structures through the mediation of tea polyphenol oxidase is the essential driving force in the production of black tea. While each of the catechins is oxidizable by this route, epigallocatechin and its galloyl ester are preferentially oxidized.68 Subsequent reactions of the flavonoid substances are largely nonenzymic. 

O Brien RD (1975) Nonenzymic effects of pesticides on membranes, pp 331-342. In Haque and Freed (1975)... 

Popov IN and Lewin G. 1994. Photochemiluminescent detection of antiradical activity. II. Testing of nonenzymic water-soluble antioxidants. Free Radic Biol Med 17(3) 267—271. 

Chapter 20 includes an extensive review of the CL sensors for determination of analytes in air, vapors, or liquids using different immobilization modes, its application to different analytes, based on enzyme or nonenzyme reactions, as well as CL immunosensors and DNA sensors. 

The reaction partners for antiradical substances are products of the first reaction. The SOD reacts selectively with 02" the nonenzymic antioxidants can react with both superoxide and luminol radicals. Theoretically, the carbonate radicals can also be involved in the PCL [25, 26],... 

CL sensors based on immobilization of nonenzyme reagents have been extensively studied in recent years. Nakagama et al. [63] developed a CL sensor for monitoring free chlorine in tap water. This sensor consisted of a Pyrex tube, packed with the uranine (fluoresceine disodium) complex immobilized on IRA-93 anion-exchange resin, and a PMT placed close to the Pyrex tube. It was used for monitoring the concentration of free chlorine (as HCIO) in tap water, up to 1 mmol/L, with a detection limit of 2 nmol/L. The coefficient of variation (n =... 

Kluger and Brandi (1986b) also studied the decarboxylation and base-catalysed elimination reactions of lactylthiamin, the adduct of pyruvate and thiamin (Scheme 2). These reactions are nonenzymic models for reactions of the intermediates formed during the reaction catalysed by the enzyme pyruvate decarboxylase. The secondary j3-deuterium KIE for the decarboxylation was found to be 1.09 at pH 3.8 in 0.5 mol dm-3 sodium acetate at 25°C. In the less polar medium, 38% ethanolic aqueous sodium acetate, chosen to mimic the nonpolar reactive site in the enzyme, the reaction is significantly faster but the KIE was, within experimental error, identical to the KIE found in water. This clearly demonstrates that the stabilization of the transition state by hyperconjugation is unaffected by the change in solvent. 

The reactive intermediate oxidation states of this class of compounds, namely, the A-hydroxyarylamines and nitrosoarenes, are in rapid metabolic equilibrium. The nitrosoarenes are readily reduced to the corresponding A-hydroxyarylamines, both enzymically and nonenzymically, and the A-hydroxyarylamines are quickly re-oxidized by autoxidation and, particularly effective, by oxyhemoglobin17,19,34,100. 

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 ... 

Many enzymes need cofactors. Here again, a nonenzymic chiral catalyst functioning without a cofactor would offer an advantage. 

A major advantage that nonenzymic chiral catalysts might have over enzymes, then, is their potential ability to accept substrates of different structures by contrast, an enzyme will select only its substrate from a mixture. Striking examples are the chiral phosphine-rhodium catalysts, which catalyze die hydrogenation of double bonds to produce chiral amino acids (10-12), and the titanium isopropoxide-tartrate complex of Sharpless (11,13,14), which catalyzes the epoxidation of numerous allylic alcohols. Since the enantiomeric purities of the products from these reactions are exceedingly high (>90%), we might conclude... 

Perhaps the only distinct advantage of enzymic catalysts is their (occasionally) very high turnover rate in situ. Thus, the molar activity (formerly called the turnover number) of some enzymes approaches 36,000,000/min/molecule (7). This latter number pertains to carbonic anhydrase C, the enzyme that converts C02 to HC03 . However, chemists do not need enzymes to convert COz to HCO3-, as long as we are not considering in vivo reactions. Since many enzymes have molar activities as low as 1150/min/molecule, we need not consider molar activities of 100 to 500 (for nonenzymic catalysts) as a severe handicap. It is evident that enzymes and nonenzymic chiral catalysts, rather than being competitors, complement one another. 

The case in favor of nonenzymic chiral catalysts is summarized in Table 1. 

Since the reaction has been reviewed recently (12) only a few additional facts will be mentioned. Many optically active cyanohydrins can be prepared (33) with e.e. s of 84 to 100% by the use of the flavopnotein D-oxynitrilase adsorbed on special (34) cellulose ion-exchange resins. Although the enzyme is stable, permitting the use of a continuously operating column, naturally only one enantiomer, usually the R isomer, is produced in excess. This (reversible) enzyme-catalyzed reaction is very rapid (34). Nonenzymic catalysts, such as the cinchona alkaloids, permit either enantiomer to be prepared in excess. 

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). 

Enzyme and Nonenzyme Catalysts By nature, enzymes themselves are chiral and they catalyze a variety of chemical reactions with stereoselectivity. These reactions include oxidation, reduction, and hydration. Examples of enzymes are oxidases, dehydrogenases, lipases, and proteases. Metoprolol, an adrenoceptor-blocking drug, is produced using an enzyme-catalyzed method. 

Models for biochemical switches, logic gates, and information-processing devices that are also based on enzymic reactions but do not use the cyclic enzyme system were also introduced [76,115,117-122]. Examples of these models are presented in Table 1.3. It should also be mentioned that in other studies [108,112-114,116], models of chemical neurons and chemical neural networks based on nonenzymic chemical reactions were also introduced. 

Stafford HA, Lester HH (1982) Enzymic and nonenzymic reduction of (+)-dihydroquercetin to its 3,4,-diol. Plant Physiol 70(3) 695-698... 

Any chemical process in which one reactant removes an atom (neutral or charged) from the other reacting entity. An example is the generation of a free radical by the action of an initiator on another molecule. If abstraction takes place at a chiral carbon, racemization is almost always observed in nonenzymic processes. On the other hand, enzymes frequently abstract and reattach atoms or groups of atoms in a fashion that maintains stereochemistry. 

This enzyme [EC 1.4.1.12] catalyzes the reaction of 2,4-diaminopentanoate with NAD(P)+ and water to produce 2-amino-4-oxopentanoate, ammonia, and NAD(P)H. The enzyme can also utilize 2,5-diaminohexanoate as a substrate (although not as effectively as the substrate mentioned above) forming 2-amino-5-oxohexanoate, which then cyclizes nonenzymically to form 1-pyrroline-2-methyl-5-carboxylate. 

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

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