Enzyme catalysed processes

This is the first example of a reaction for which the presence of a chelating ligand was observed to facilitate rather than retard metal-catalysed epoxidation (Gao et al., 1987). It was found that the use of molecular sieves greatly improves this process by removing minute amounts of water present in the reaction medium. Water was found to deactivate the catalyst. All these developments led to an improved catalytic version that allows a five-fold increased substrate concentration relative to the stoichiometric method. Sensitive water-soluble, optically active glycidols can be prepared in an efficient manner by an in situ derivatisation. This epoxidation method appears to be competitive with enzyme-catalysed processes and was applied in 1981 for the commercial production of the gypsy moth pheromone, (-1-) disparlure, used for insect control (Eqn. (25)). 

Thummel, K. E., Kunze, K. L., Shen, D. D., Enzyme catalysed processes of first-pass hepatic and intestinal drug extraction, Adv. Drug Del. Rev. 1997,... 

B. Nidetzky, K. Schmidt, W. Neuhau-SER, et al, Application of charged ultrafil-tration membranes in continuous, enzyme-catalysed processes with coenzyme regeneration, in Separations for Biotechnology 3 (ed. D. L. Pyle) Royal Society of Chemistry, 1994, 351. 

Enzymes provide catalytic pathways which are often superior to non-enzymic routes. The advantage of enzymes can overcome their disadvantages as relatively unstable species which workbest in aqueous solution. In synthetic processes, the selectivity of the enzyme-catalysed process is not approached by chemical catalysis particularly regarding the enantioselective synthesis of chiral molecules. Furthermore, protective group chemistry is often not required in enzyme-catalysed reactions. 

Biodegradation is an enzyme-catalysed process, and typically occurs in two stages. In the first of them, the enzyme binds to the... 

Enantiomerically pure cyclopentenones are accessible via enzyme-catalysed processes. By photochemical oxidation and acetylation, c s-l,4-diacetoxycyclo-pent-2-ene is obtained. Taking advantage of the meso-trick cf. section 5.1.3) with an enzyme from Pseudomonasfluorescens, the latter can be converted upon recrystallisation and oxidation into (R)-acetoxycyclopentenone. (The (S)-enan-tiomer is obtainable by hydrolysis with pig Ever esterase.) [221,222]... 

Both photosynthesis occurring in plants, and the oxidation of carbohydrate during food metabolism are complex multistage enzyme-catalysed processes which utilise phosphate energy-carrying molecules. 

Earlier, one of the perceived difficulties in the area of biotransformations was that of scale-up. Thus, while it seemed that test-tube experiments probably would be entirely feasible, the use of enzyme-catalysed processes on a kilogram scale or a tonne scale seemed a daunting task. It seems that, in many cases, those fears were unfounded, and the final chapter deals with some of the large-scale processes that have been industrial successes in terms of engineering and commerce. Because previously this topic has not been reviewed as extensively as the material described in Chapters 3-5, more detailed descriptions of the processes have been included. The latter part of the final chapter describes some special techniques of increasing importance in the field of biotransformations. 

It is well known that various parameters (e.g. solvent, pH, immobilization, chemical modification and temperature) can have an effect on the enantioselectivity of enzyme-catalysed processes. Most studies in this respect have been carried out on hydrolytic enzymes, especially lipases, esterases and proteases [28]. Recent reports, especially those involving non-hydrolytic enzymes, are discussed below. 

Compounds (9) and (10) have also been synthesized by an enzyme-catalysed process as indicated in Scheme 5 1,4-dideoxy-l,4-imino-D-arabinitol was also made from the same azido-hydroxypropanal using... 

However, although microbes are always included in the degradation process, parts of the entire degradation mechanism can also be of abiotic nature. One example is the degradation of the aliphatic polyester PLA, which is hydrolysed under natural conditions by a non-enzymically catalysed process into short oligomers and monomers. These chemically produced intermediates then are metabolised by microorganisms to form products that become part of natural cycles. 

In this chapter, we will illustrate the state of the art of the computational methods devoted to the study of chemical reactivity in enzyme catalysed processes. It will be shown how the use of computational sophisticated tools are required to describe such complex systems. The adequate computational protocol to explore enzyme reactivity wiU be dictated by the fact that enzymes are huge macromolecules, with noticeable flexibility required to acquire the different steps of the fiiU catalytic process from binding of reactants to the product release, and also by the electronic and quantum complexity of the reactions catalysed in their actives sites. 

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