Enzymatic reactions selectivities

Besides enantioselectivity, regioselectivity (Chapter 12, Section 12.6) and to a lesser extent, chemoselectivity (Chapter 7, Section 7.3.2) are also important issues of enzymatic reaction selectivity. 

To try to circumvent this limitation, Jestin et al. [68] have designed and tested a selection scheme involving two chemically independent reactions the reaction catalyzed by the enzyme and a chemical reaction leading to phage labeling by the substrate or product of the enzymatic reaction. Selection of labeled phages with a product-specific binder (e.g. a monoclonal antibody) should allow recovery of the product-labeled phages only (Fig. 5.15). 

Retey, J., 1990, Enzymatic-reaction selectivity by negative catalysis or how do enzymes deal with highly reactive intermediates. Angew. Chem. Inti. Ed. Engl. 29 3559361. 

The regio selectivity observed in the acylation of underivatized pyranoses in principle parallels that recorded for the classical chemical introduction of acyl groups into carbohydrates. However, if the 6-OH groups are protected first or deoxygenated, in the corresponding enzymatic reactions selectivities are observed which can not be realized with classical chemical methods. By careful choice of solvent and lipase, it is possible to rnodifiy selectively a number of C6 protected pyranoses at the secondary hydroxy groups (Table 18-2). 

The reactant is referred to as a substrate. Alternatively it may be a nutrient for the growth of cells or its main function may require being transformed into some desirable chemical. The cells select reactants that will be combined and molecules that may be decomposed by using enzymes. These are produced only by living organisms, and commercial enzymes are produced by bacteria. Enzymes operate under mild conditions of temperature and pH. A database of the various types of enzymes and functions can be assessed from the following web site http //www.expasy.ch/enzyme/. This site also provides information about enzymatic reactions. 

Attempts to realize enzymatic reactions have been reported over the past four decades in the context of host-guest chemistry, presently a well-established research field. In the field of CPOs, much attention has been paid to identical research objectives. The host-guest chemistry based on CPOs holds a special position, because specific selectivity and reactivity will be achieved using the coordination-bond-forming reactions between the substrate and the incorporated metals in the porphyrins, as well as the redox reaction associated with the porphyrin s rr-electron system. 

Monitoring enzyme catalyzed reactions by voltammetry and amperometry is an extremely active area of bioelectrochemical interest. Whereas liquid chromatography provides selectivity, the use of enzymes to generate electroactive products provides specificity to electroanalytical techniques. In essence, enzymes are used as a derivatiz-ing agent to convert a nonelectroactive species into an electroactive species. Alternatively, electrochemistry has been used as a sensitive method to follow enzymatic reactions and to determine enzyme activity. Enzyme-linked immunoassays with electrochemical detection have been reported to provide even greater specificity and sensitivity than other enzyme linked electrochemical techniques. 

The demand for monitoring common metabolites of diagnostic utility such as glucose, urea and creatinine continue to provide the impetus for a staggering research effort towards more perfect enzyme electrodes. The inherent specificity of an enzyme for a given substrate, coupled with the ability to electrochemically detect many of the products of enzymatic reactions initiated the search for molecule-selective electrodes. 

However, the active site is only a conceptual tool and the assignment of the active-site atoms is more or less arbitrary. It is not possible to know beforehand which residues and protein interactions that will turn out to be important for the studied reaction. Hybrid QM/MM methods have been used to extend the active site only models by incorporating larger parts of the protein matrix in studies of enzymatic reactions [19-22], The problem to select active-site residues appears both for active-site and QM/MM models, but in the latter, explicit effects of the surrounding protein (i.e. atoms outside the active-site selection) can at least be approximately evaluated. As this and several other contributions in this volume show, this is in many cases highly desirable. 

Naturally selectivity in a several-component system is primarily influenced by rather strong effects such as the presence or absence of strong H-bonding, but possibly also by much weaker interactions (e.g. of C—H... O type). In this regard, it is interesting to note the similarity between the selectivity exerted by such simple inclusion hosts, e.g. /, and chiral recognition 103). In both cases, weak interactions are of decisive importance in the final outcome of the experiments. Entropic effects have been demonstrated to play a fundamental role in enzymatic reactions 102,107 >. Conceptual similarity of inclusion compounds to more complicated associates is underlined thereby. 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

This report surveys new routes for the synthesis of oligosaccharides, with emphasis on enzymatic reactions, since they offer unique properties, proceeding highly regio-and stereo-selectively in water solution, and generally afford high yields. Summarized... 

Develop novel transformations that perform with the selectivities typical of enzymatic reactions, so that geometric factors are more important than the intrinsic reactivity of a molecule. 

On the other hand, the most severe constraint of CL analyses is their relatively low selectivity. One major goal of CL methodologies is thus to improve selectivity, which can be accomplished in three main ways (1) by coupling the CL reaction to a previous, highly selective biochemical process such as an immunochemical and/or enzymatic reaction (2) by using a prior continuous separation technique such as liquid chromatography or capillary electrophoresis or (3) by mathematical discrimination of the combined CL signals. This last approach is discussed in Sec. 4. 

FIA has also found wide application in pharmaceutical analysis.214,215 Direct UV detection of active ingredients is the most popular pharmaceutical analysis application of FIA. For single component analysis of samples with little matrix interference such as dissolution and content uniformity of conventional dosage forms, many pharmaceutical chemists simply replace a column with suitable tubing between the injector and the detector to run FIA on standard HPLC instrumentation. When direct UV detection offers inadequate selectivity, simple online reaction schemes with more specific reagents including chemical, photochemical, and enzymatic reactions of derivatization are applied for flow injection determination of pharmaceuticals.216... 

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